The ability to establish rheological and fluid‑transport laws in the lithosphere for use in geomechanics and geodynamics models depends on laboratory experiments validated by field observations. In experiments, a key challenge is reproducing the pressure, temperature, and fluid‑chemistry conditions found at depth while acquiring sufficient information inside rock samples to understand and generalize the detailed mechanisms of rock deformation and chemical evolution. Over the past fifteen years, breakthroughs in rock physics have been enabled by experiments conducted at large user facilities such as synchrotron and neutron sources. From shallow subsurface fluid–rock interactions to slow and fast rupture and down to the brittle–ductile transition at the base of the seismogenic zone and deep earthquakes, it is now possible to image geological processes in 4D (3D + time) with unprecedented spatial and temporal resolution in samples large enough to be representative of lithospheric processes.

Recent experiments demonstrate how a porous rock can become clogged and store carbon dioxide, including direct imaging of fluid mixing and precipitate formation, how porosity can be generated at the brittle–ductile transition, altering our view of fluid transfer at the base of the seismogenic zone, and how damage nucleates before and during earthquakes. These findings highlight the importance of dynamic porosity — which controls fluid transport and deformation — and call for integrating more widely this property into large‑scale models of Earth’s crust dynamics.

How to cite: Renard, F.: A revolution in rock physics: 4D imaging, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3917, https://doi.org/10.5194/egusphere-egu26-3917, 2026.

Rocks can be described as heterogeneous random materials, in the sense that they are composed of multiple domains of different phases. At the scale of a representative elementary volume (REV) — defined as the smallest volume over which measurements yield values representative of the entire rock sample — such heterogeneous materials can be treated as homogeneous and characterized by macroscopic or effective properties. Determining these effective properties is essential for describing geological processes occurring in reservoirs, aquifers, fault zones, and volcanic environments, as well as the structural changes they induce. However, complex interactions among the constituent phases result in a strong dependence of effective properties on nontrivial aspects of the microstructure, in particular for rocks with more than two or three different phases.

Over the past decades, a fruitful approach to investigating the relationship between rock properties and microstructure has involved predicting effective properties directly from microstructural information. This framework enables quantitative links to be established between microstructural evolution and changes in macroscopic properties. Following this rationale, laboratory-prepared synthetic rocks with specifically designed microstructural attributes have proven particularly valuable. Such materials have provided key insights into the influence of microstructure on mechanical properties, using relatively simple single- or two-phase rock analogs first, and synthetic materials with progressively more numerous distinct phases.

Here, I will summarize results we have collected in the past years by systematically investigating how specific microstructural attributes influence the mechanical behaviour of rocks. Compression experiments conducted on monodisperse sintered glass beads samples show that the stress required to reach inelastic deformation decreases when porosity or grain size alone increase. Using bidisperse and polydisperse sintered glass beads samples, we observe that this stress decreases when the degree of polydispersivity increases. In addition, under high-pressure triaxial compression, an increase in the degree of polydispersity alone leads to a transition in damage evolution from localized to more spatially distributed deformation. These results echo observations from shear experiments performed on heterogeneous fault gouges, where the spatial arrangement of weak and strong mineral phases, in addition to their relative proportions, exerts control on frictional properties and damage evolution during shearing.

More recently, we have employed nanoindentation to resolve spatial variations in elastic properties directly at the grain and crystal scale in natural rocks, allowing for the mechanical characterization of individual phases and quantification of mechanical heterogeneity at the bulk sample scale. These measurements will hopefully provide input parameters for the development and calibration of increasingly realistic synthetic rocks.

How to cite: Carbillet, L.: Determining the mechanical properties of heterogeneous rocks from a knowledge of microstructure, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21775, https://doi.org/10.5194/egusphere-egu26-21775, 2026.

In the Chundi–Malakonda–Ayyavaripalle region of the Nellore Schist Belt, part of the Dharwar Craton, six north-south (N–S) trending, elongated Banded Magnetite Quartzite (BMQ) bands have been identified. These bands are exposed within meta-rhyolite and quartzite formations and are associated with biotite–muscovite schist. The BMQs vary in width from 20 to 40 meters and extend in length from 1.5 to 4.5 kilometres. ​ High-resolution aeromagnetic surveys with a terrain clearance of 80 meters have revealed significant magnetic anomalies over the study area (source: https://geodataindia.gov.in). These anomalies range from –3,900 to +5,000 nanoteslas (nT), indicating a high concentration of magnetic minerals within the exposed BMQs, designated as Bands 1 to 6. In addition to these exposed bands, a concealed, parallel, N–S trending BMQ band has been identified through detailed analysis of aeromagnetic data. 2D and 3D Interpretation of the magnetic anomalies suggests that meta-rhyolites exist up to an average depth of 250 m from the surface and might be associated with BIF bands at depth. This depth extent highlights the substantial vertical continuity of the magnetite-rich formations in the region. The integration of geological mapping and aeromagnetic data provides a comprehensive understanding of subsurface geology, highlighting the potential for significant mineralization within the Nellore Schist Belt.

How to cite: Dharavathu, S., Kosuri, S. K., Vappangi, P. K., and Kumar, P.: Unveiling concealed Banded Magnetite Quartzites (BMQs) through high-resolution aeromagnetic surveys: New insights from the Nellore Schist belt of Eastern Dharwar Craton, India, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2063, https://doi.org/10.5194/egusphere-egu26-2063, 2026.

Within the comprehensive framework of the Sun–Earth system, plasma environments exhibit an exceptionally wide range of physical conditions. These encompass the ultra-high-temperature, high-pressure, and high-density liquid metallic outer core of Earth, which generates the geomagnetic field through the geodynamo process; the tenuous, partially ionized ionosphere; and the magnetosphere, which provides essential shielding against energetic cosmic and solar radiation while exerting substantial influence on human technological systems, most notably microwave communication infrastructure. In addition, transient ultra-high-temperature plasmas generated by solar flares and coronal mass ejections (CMEs) represent the primary drivers of disturbed space electromagnetic environments, as they propagate through interplanetary space and subsequently interact with Earth's magnetosphere.Although prior research has extensively employed the first-principles quantum Monte Carlo method coupled with the lattice Boltzmann approach (FPQM-LBM) to address various theoretical and computational aspects of plasma behavior in this context, no existing modeling framework has successfully integrated — within a single consistent methodology — the extreme conditions of the Earth's outer core plasma, the low-density ionospheric plasma, the magnetospheric plasma, and the highly energetic, transient flare/CME plasmas. As a result, a unified and comprehensive understanding of particle transport mechanisms and internal structural properties across the full spectrum of plasma regimes in the Sun–Earth system remains elusive.The present study aims to address this critical gap by developing novel theoretical frameworks and advanced computational methodologies for elucidating the particle migration mechanisms and structural characteristics of space electromagnetic plasmas throughout the panoramic Sun–Earth system. To this end, we will enhance the first-principles quantum Monte Carlo–lattice Boltzmann method (FPQM-LBM) to establish robust techniques capable of modeling particle transport under the complex electromagnetic conditions prevailing in space environments. The improved FPQM-LBM framework will be systematically applied to simulate particle dynamics across the aforementioned plasma regimes — namely, the ultra-high-temperature/pressure/density outer core plasma, the low-density ionosphere, the magnetosphere, and transient flare/CME plasmas — with particular emphasis on ionic characteristics, microstructural evolution, fine-scale particle transport processes, internal structural transformations, and the response of plasma properties to external electromagnetic perturbations. The anticipated results are expected to furnish a solid theoretical foundation and valuable predictive capabilities for advancing solar–terrestrial space physics and enhancing electromagnetic monitoring and forecasting in space weather research.

How to cite: Zhu, B.: Theoretical Foundations and Methodological Developments in the Study of Particle Transport Mechanisms and Microstructural Evolution Employing the Hybrid Quantum Monte Carlo–Boltzmann Transport Model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3254, https://doi.org/10.5194/egusphere-egu26-3254, 2026.

Space weather can affect the operation of high voltage AC transformers in power grids by applying an offset DC current during periods of heightened geomagnetic activity. Modelling GIC requires knowledge of magnetic field variation, the response of the local subsurface geoelectric field (related to conductivity) and a representation of the connections between transformers and various resistance parameters of the power network. Presently, in Britain, the largest uncertainty in this chain applies to the resistance parameters of the network, as these values come from open-source data which are known to have many approximations.

Recent work with a transmission network operator in the UK has provided us with an improved dataset of resistance parameters of transformers, power lines and substation grounding. The grounding resistance at electrical substations has not been known before and so historically was set at 0.5 Ω in our models. The new dataset of 110 sites around central Scotland reveals substation grounding resistance varies from 0.04 Ω to 11.7 Ω with a mean of 0.54 Ω but a median of 0.2 Ω. Combined with line and transformer resistance information, we have created an improved representation of the power grid in Scotland.

Using GIC measurements from three sites (Torness, Strathaven and Neilston) for the largest geomagnetic storms in the past 25 years (October 2003, September 2017 and May 2024), we are able to validate the new model, demonstrating its improved accuracy.

The new model demonstrates that our previous assumptions of grounding resistance were too high but our estimate of line resistance was too low, thus balancing out the overall GIC magnitude on average. However, in detail, some locations show large differences in GIC compared to the original model. This highlights the importance of using accurate resistance information to correctly capture GIC.

How to cite: Beggan, C., Richardson, G., and Lawrence, E.: Improving GIC modelling and validation with high-quality information on power network parameters, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5611, https://doi.org/10.5194/egusphere-egu26-5611, 2026.

During intense solar atmospheric activity—such as major solar flares and geomagnetic storms—magnetic energy is converted into plasma kinetic and thermal energy through three-dimensional turbulent magnetic reconnection within large-scale, extended current sheets. This process releases enormous amounts of stored energy, often accompanied by the rapid ejection of high-energy particles into interplanetary space. These high-energy particles include electrons, protons, helium nuclei, and heavier ions, forming a complex multi-component, multi-abundance, and multi-isotopic population. Their energies span from ~100 keV to ~100 MeV and even into the GeV range, making them a primary driver of space weather hazards. Understanding the sources and acceleration mechanisms of these particles remains one of the most critical challenges in space weather research. Previous studies have shown that high-energy particle acceleration is highly complex, involving multiple species, a variety of mechanisms, and interactions across scales. It remains an open and challenging problem in solar and plasma physics. This paper provides a systematic review and forward-looking perspective on recent advances in high-energy particle acceleration during the fine-scale evolution of large-scale current sheets. The discussion is organized around three key pillars: theory, observations, and numerical simulations. First, we summarize the turbulence-fractal model as it applies to typical solar atmospheric events. We focus on acceleration mechanisms in turbulent magnetic reconnection within large spatiotemporal current sheets, with particular emphasis on: Turbulent (second-order) Fermi acceleration, Turbulent shock acceleration, and Turbulent wave-particle resonant acceleration. These mechanisms operate synergistically in the turbulent environment generated by reconnection, enabling efficient energy transfer to particles. Second, we review recent progress in coupling macroscopic (hydrodynamic and magnetohydrodynamic) dynamics to microscopic kinetic processes in high-energy particle acceleration. This includes multi-scale modeling of turbulence, reconnection, and particle transport. Finally, we outline promising future research directions, including improved multi-spacecraft observations, higher-resolution simulations that incorporate kinetic effects, and integrated models that bridge MHD turbulence and particle-in-cell approaches. We also highlight several urgent unresolved issues, such as the relative contributions of different mechanisms across energy regimes, the role of fractal structures in particle trapping and escape, and the origin of observed abundance enhancements in heavy ions. This review synthesizes recent theoretical, observational, and computational developments to provide a comprehensive framework for understanding high-energy particle acceleration in large-scale turbulent current sheets, with implications for solar flares, space weather forecasting, and broader astrophysical plasma processes.

How to cite: Zhu, B.: Research Progress on SEPs on the Fine Structures of the Large Temporal-spatial Current Sheets in Solar Flares/CMEs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6969, https://doi.org/10.5194/egusphere-egu26-6969, 2026.

Lago del Desierto (49°02′S, 72°51′W) is located in a climatically sensitive sector near the Southern Patagonian Ice Field (Argentina). Three sediment cores collected from two sites in the lake were analyzed using a multi-proxy approach to reconstruct past environmental variability (Kastner et al., 2010). Numerous turbidites were identified in the sedimentological record. After excluding these event layers, a new age–depth model was developed for the first 3 sections of the core DES05-3 (289 cm), and paleosecular variations (PSV) were reconstructed for the interval between ~1000 and 3500 cal. BP.

Standard paleomagnetic measurements (alternating-field demagnetization) were performed on 125 samples from the same core. In addition, rock-magnetic measurements, including Anhysteretic Remanent Magnetization (ARM, 100 mT peak AF, 0.05 mT DC field), Isothermal Remanent Magnetization (IRM, acquisition up to 1.5 T and backfield curves), hysteresis loops and thermomagnetic analyses, were applied to extract complementary paleoenvironmental information from the sediment cores.

Rock-magnetic measurements indicate that the magnetic mineralogy is dominated by a low-coercivity component (magnetite-type), accompanied by a secondary high-coercivity fraction (hematite/goethite-type). This downcore distribution mirrors the paleoenvironmental shift described by Kastner et al. (2010): the lower part of the sequence shows an enhanced contribution of high-coercivity Fe oxides, consistent with more stable and chemically weathered catchment conditions. In contrast, the upper part shows an increasing dominance of detrital magnetite, indicating strengthened minerogenic supply and enhanced erosion, matching the onset of warmer conditions and glacier retreat during the Medieval Climate Anomaly as inferred from geochemical and lithological proxies. This agreement between magnetic and non-magnetic sediment parameters suggests coherent changes in the provenience of the sediment and in catchment dynamics over the last millennia. As expected from the catchment instability and the numerous turbidites in the upper part of the sequence, this interval could not be used for PSV reconstruction due to its discontinuous directional record. In contrast, samples from the lower part (~1000–3500 cal. BP) provided a continuous sequence suitable for paleosecular variation analysis. Although samples from this unit were not completely demagnetized at 100 mT, due to the presence of a high-coercivity component, magnetization directions consistently decayed toward the origin with high precision. Characteristic remanent magnetization (ChRM) directions were determined using principal component analysis, with maximum angular deviation (MAD) values below 2.5° for all non-turbidite samples. The resulting PSV record compares well with geomagnetic field models and other Patagonian paleomagnetic reconstructions. Inclination values range from −40° to −70°, displaying coherent directional variability over the last ~3500 years.

References:
Kastner, S., Enters, D., Ohlendorf, C., Haberzettl, T., Kuhn, G., Lücke, A., Mayr, C., Reyss, J.-L., Wastegård, S., & Zolitschka, B. (2010). Reconstructing 2000   years of hydrological variation derived from laminated proglacial sediments of Lago del Desierto at the eastern margin of the South Patagonian Ice Field, Argentina. Global and Planetary Change, 72(3), 201-214. https://doi.org/10.1016/j.gloplacha.2010.04.007

How to cite: Achaga, R. V., Gogorza, C. S. G., Irurzun, M. A., Ohlendorf, C., Haberzettl, T., and Zolitschka, B.: A Late Holocene Paleomagnetic Record from Lago del Desierto, Southern Patagonia (Argentina), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8141, https://doi.org/10.5194/egusphere-egu26-8141, 2026.

The source of compressional Pc3 magnetic pulsations has been considered to be the plasma process in the solar wind or in the magnetosphere. However, we found some strong inland earthquakes that occur on the dayside also excite Pc3s. The Lamb waves generated by the January 2022 Tongan undersea volcanic eruption are also believed to have excited large amplitude and short period compressional Pc3 geomagnetic pulsations in the dayside plasmasphere (Iyemori et al., 2025). Increases in power spectral density (PSD) in the Pc3 frequency band were observed 10-30 minutes after the origin time of large inland earthquakes (M>6.5) during the daytime (10-14 LT). During these large earthquakes, seismic waves with period of 10-30 seconds propagate far away (even more than several thousand km), causing slight fluctuations in the orientation of magnetometer sensors, resulting in apparent Pc3-like fluctuations. To avoid such sensor tremor effect, we analyzed the total force of magnetic field, or analyzed comparing with seismometer data. We also used the Swarm satellite observation. The PSD of Pc3s caused by earthquakes or by Lamb wave show many spectral peaks having interval of 3-5 mHz, and this is similar with the characteristic reported by, for example, Samson et al. (1995) for normal, i.e., solar wind origin Pc3s. In this paper, we will also show the commonality between the Pc3s caused by earthquakes or Lamb waves and those originated from the solar wind and discuss what the commonality means.

How to cite: Iyemori, T., Aoyama, T., and Yokoyama, Y.: Pc3 geomagnetic pulsations excited by earthquakes and their commonality with solar wind-originated Pc3, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8566, https://doi.org/10.5194/egusphere-egu26-8566, 2026.

Continental sedimentary sequences of alternating loess and palaeosol horizons preserve detailed records of past global climate changes during the Pleistocene. Obtaining deeper and genuine knowledge on the history of past climates using proxy data depends on interdisciplinary approaches, novel techniques and thinking “out-of-the-box”. The LOEs-CLIMBE team members gather around this concept and present here the first pilot magnetic data from the Kolobar loess-palaeosol section in NE Bulgaria. The 25 m thick section is exposed in an active quarry and was sampled at 2-cm-resolution, covering the Holocene soil, seven palaeosol units and loess horizons L1 to L7 of varying thicknesses. New high resolution magnetic susceptibility data, delineates palaeosol horizons with high values of mass specific magnetic susceptibility except the special case of fourth palaeosol S4, showing no magnetic enhancement as compared to the underlying thin loess. Such depletion of pedogenic magnetic enhancement in paleosol units from the Lower Danube area is rarely reported. This phenomenon will be further examined by detailed magnetic and colorimetric methods. The strongest pedogenic magnetic signal is observed in the three youngest palaeosol units S1, S2 and S3, tentatively related to the interglacial stages MIS 5, MIS 7 and MIS 9. The weakest magnetic susceptibility is typical for the younger part of the loess unit L2, punctuated by the signal of a tephra layer, which is a widespread chronostratigraphic marker in the region.  This research is carried out and financed within the framework of the second Swiss Contribution MAPS, LOEs-CLIMBE project № IZ11Z0_230102.

How to cite: Jordanova, D., Georgieva – Ishlyamska, B., Veres, D., Baykal, Y., Robu, M., Jordanova, N., Hambach, U., Ishlyamski, D., Dimov, D., Trott, A., and Wiesenberg, G.: High-resolution magnetic record of environmental changes during Middle – Late Pleistocene from a loess-palaeosol sequence in NE Bulgaria – pilot data from the LOEs-CLIMBE project, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8852, https://doi.org/10.5194/egusphere-egu26-8852, 2026.

The Earth's magnetic field is divided into internal and external sources, with the internal field including the main magnetic field, the crustal magnetic field, and the induced magnetic field. Among these, the main magnetic field accounts for approximately 95% of the Earth's total magnetic field. Data from paleomagnetic records in rocks, various geomagnetic observatories, and satellites indicate that the main magnetic field exhibits westward drift, polarity reversals, intensity decay, and brief geomagnetic excursions at the core-mantle boundary. To explain these phenomena, several models have been proposed in previous studies. The prevailing view is that the outer core is composed of liquid metal, and the Earth's main magnetic field is generated by the turbulent fluid motion of this liquid metal, influenced primarily by factors such as its composition and properties, thermal convection, Lorentz force, and Coriolis force. Considering the strong Coulomb forces between electrons and ions, previous research has usually treated the electrons and ions in the outer core's metallic fluid as a unified component, greatly simplifying the study and achieving satisfactory results. However, existing studies have not taken into account the differences in motion between ions and electrons under the dynamics of the outer core, the resulting spatial distribution differences of electrons and ions in the outer core, or the impact of these differential distributions on the Earth's main magnetic field. In view of this, This paper studies the effect of the factors generating outer core dynamics on the distribution of electrons and ions in the outer core. It examines the distribution of electrons and ions in the outer core space under equilibrium conditions and estimates their contribution to Earth's main magnetic field. Then, by changing parameter conditions (such as temperature gradients) and adding convective terms (non-equilibrium state), the calculations are redone. These results are used to explain changes in Earth's magnetic field.

How to cite: Wang, S., Li, Y., Zhu, B., Zhao, Y., Wang, Q., and Liu, H.: The influence of the heterogeneity (stratification) of the outer core fluid on the variation of the geomagnetic field, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9301, https://doi.org/10.5194/egusphere-egu26-9301, 2026.

The stability of critical infrastructure in Northwest Bulgaria (Western Moesian Platform) could be compromised by ground instability within Neogene sediments that cover the region. This is evidenced by the collapse of the I-1 national road near Dimovo town in 2006, which involved vertical displacements of 3–4 meters. The purpose of this study is to identify the underlying geological drivers of this failure and to evaluate the specific hazard in the area resulting from the interaction between the sediments and the local environmental conditions. We hypothesize that the instability is not merely a result of conventional failure mechanisms but is governed by an anomalous mineralogical composition, specifically by the presence of aragonite and gypsum layers, which could create a dual hazard.

To elucidate geological drivers, we employed a methodology that integrates field mapping and sampling with laboratory analyses. Samples from the Neogene sediments in the area of 2006 damage underwent mineralogical analyses using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) to determine phase composition and morphology. These analyses were coupled with X-ray fluorescence (XRF) for chemical profiling and standard geotechnical testing to determine grain size distribution, Atterberg limits, and Activity index according to Skempton’s classification.

The analysis reveals a heterogeneous sediment succession with the presence of inorganic clays with high plasticity. The XRD and SEM results identified a mineralogical anomaly where the high concentrations of metastable, acicular aragonite coexist with active swelling phyllosilicates (smectite/illite). Furthermore, various amounts of gypsum were detected in some of the samples, indicating an evaporitic paleoenvironment. Geotechnically, these materials exhibit extreme reactivity. Liquid limits range from 34.85% to 67.88%, and plasticity indices reach up to 47.39%. The Activity index peaks at 2.00, categorizing the sediments as "highly active" and prone to volume change driven by moisture variations.

The study concludes that ground failure is a direct consequence of a synergistic hydro-chemo-mechanical mechanism driven by the sediments' mineralogy. The specific aragonite fabric allows rapid water infiltration, triggering the hydration of smectites that could lead to loss of shear strength. Simultaneously, gypsum dissolution could create secondary porosity, reduce effective stress, and release sulfate ions, which could pose a potential chemical hazard to concrete foundations through sulfate attack. Furthermore, the high silt content facilitates internal erosion and possible piping through fracture networks, which could explain the sudden loss of support and large vertical displacements observed in the 2006 case. These findings imply that standard geotechnical data alone are insufficient for risk assessment in this region. Effective mitigation strategies must integrate mineralogical analysis to address both the physical swelling and the chemical durability risks.

Acknowledgements: This research was funded by the European Union-NextGenerationEU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, Project No BG-RRP-2.004-0008-C01.

How to cite: Dotseva, Z., Vangelov, D., and Stanimirova, T.: Mineralogical Drivers of Ground Failure in Neogene Sediments: a Case Study from Northwest Bulgaria, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10911, https://doi.org/10.5194/egusphere-egu26-10911, 2026.

How to cite: Girgis, K., Arthus Schanner, M., Panovska, S., and Yoshikawa, A.: Effects of the Historical Geomagnetic Field on Earth's Energetic Particle Environment: Magnetic Anomalies and Auroral Regions , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14934, https://doi.org/10.5194/egusphere-egu26-14934, 2026.

The Swarm Data Fusion (SwarmDF) toolbox is designed as an easy-to-use Python module for analysing the local electrodynamics of the high-latitude ionosphere by combining measurements from ESA’s Swarm satellites with additional ionospheric and thermospheric datasets. Given a Swarm satellite ID, a regional grid, and a time interval, the toolbox automatically retrieves and combines available observations from SuperDARN, SuperMAG, Iridium/AMPERE, and Swarm electromagnetic field measurements. SwarmDF uses the local mapping of polar ionospheric electrodynamics (Lompe) technique to reconstruct two-dimensional maps of key electrodynamic parameters in the vicinity of the Swarm satellite tracks. To assess and quantify reconstruction performance, SwarmDF integrates the LompeOSSE Python module, which generates controlled synthetic electrodynamics datasets based on Gamera simulations and enables systematic comparisons with the toolbox outputs under different data availability and configuration scenarios. Featuring a user-friendly graphical interface, SwarmDF simplifies data handling and model setup for high-latitude ionospheric electrodynamic studies using Swarm observations.

How to cite: Decotte, M., Laundal, K. M., and Kebede, F. T.: SwarmDF: A toolbox for analysing high-latitude ionospheric electrodynamics, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18706, https://doi.org/10.5194/egusphere-egu26-18706, 2026.

The Iberian Pyrite Belt (IPB) hosts one of the largest concentrations of massive sulfide deposits in Europe, yet its lithosphere architecture remains incompletely understood. In this study, we employ magnetotelluric (MT) impedance tensor data to investigate the dimensionality and structural characteristics of the IPB crust. The analysis combines two complementary approaches: WAL invariants, computed from the MT impedance tensor using the Waldim code (Martí et al., 2013), which are scalar, rotation invariant quantities providing a robust, frequency dependent measure of three-dimensionality and highlighting anisotropic features in the conductivity distribution; and the Phase Tensor, following the methodology of Caldwell et al. (2004), which offers distortion free insights into the orientation and geometry of regional conductive structures. Integrating these methods enables a systematic dimensional analysis of the impedance tensor, revealing lateral heterogeneities, preferred orientations of conductive features, and depth dependent variations in lithospheric responses.

The results demonstrate that WAL invariants and Phase Tensor analysis together allow the separation of near surface distortions from deeper geoelectric structures, providing a robust framework for characterizing the lithospheric architecture of the IPB. This study highlights the enhanced resolution and robustness achieved by complementing the tensor based analysis of MT data with invariant derived quantities that provide rotationally independent measures of three-dimensionality and anisotropy.

Consequently, this dimensional and structural assessment constitutes a critical prerequisite for subsequent MT data inversion, as it provides essential constraints on model dimensionality, structural orientation, and the treatment of near surface distortion. By supporting the choice between 2D and 3D inversion strategies, the proposed framework enhances the stability of the inversion process, increases the reliability of the conductivity distributions, and ensures greater geological consistency of the resulting models.

Acknowledgment

This work is supported by FCT, I.P./MCTES through national funds (PIDDAC): LA/P/0068/2020, https://doi.org/10.54499/LA/P/0068/2020,UID/50019/2025,  https://doi.org/10.54499/UID/PRR/50019/2025, UID/PRR2/50019/2025

References

Caldwell, T.G., Bibby, H.M. and Brown, C. (2004). The magnetotelluric phase tensor. Geophysical Journal International, 158: 457-469. https://doi.org/10.1111/j.1365-246X.2004.02281.x

Castro, C., Hering, P., Junge, A. (2020). FFMT: a MATLAB-based toolbox for Magnetotellurics (MT). 10.13140/RG.2.2.12465.92007.

F. E. M. Lilley. (1998). Magnetotelluric tensor decomposition; Part, Theory for a basic procedure. Geophysics; 63 (6): 1885–1897. doi: https://doi.org/10.1190/1.1444481.

Martí, A., Queralt, P., Ledo, J., Farquharson, C. (2010). Dimensionality imprint of electrical anisotropy in magnetotelluric responses, Physics of the Earth and Planetary Interiors, Volume 182, Issues 3–4, 2010, Pages 139-151, ISSN 0031-9201. https://doi.org/10.1016/j.pepi.2010.07.007.

Martí, A., Queralt, P., Ledo, J. (2013). WALDIM: A code for the dimensionality analysis of magnetotelluric data using the rotational invariants of the magnetotelluric tensor. Computers & Geosciences. 2295-2303. 10.1016/j.cageo.2009.03.004

Miensopust, M. P. (2017). Application of 3-D electromagnetic inversion in practice: Challenges, pitfalls and solution approaches. Surveys in Geophysics, 38(5), 869–933. https://doi.org/10.1007/s10712-017-9435-1.

Vozoff, K. (1991). The magnetotelluric method: Electromagnetic methods. In M. N. Nabighian (Ed.), Applied Geophysics (pp. 641–712).

Kelbert, A., Meqbel, N., Egbert, G. D., & Tandon, K. (2014). ModEM: A modular system for inversion of electromagnetic geophysical data. Computers & Geosciences, 66, 40–53. https://doi.org/10.1016/j.cageo.2014.01.010.

How to cite: Baltazar-Soares, P., Martinéz-Moreno, F. J., Gonzaléz-Castillo, L., Galindo-Zaldívar, J., Monteiro Santos, F., Mateus, A., and Matias, L.: Dimensionality Analysis of the Iberian Pyrite Belt Lithosphere derived from the Magnetotelluric Impedance Tensor., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21147, https://doi.org/10.5194/egusphere-egu26-21147, 2026.

This study investigates the deep structure of the Ouarzazate Basin, located between the Central High Atlas and the Anti-Atlas, using gravity data analysis. Gravimetric methods were applied to map subsurface structural lineaments beneath the sedimentary cover. The resulting structural map reveals that the basin is mainly controlled by ENE-WSW oriented faults, with subordinate E-W and NE-SW trends related to Variscan deformation, Triassic-Jurassic rifting, and Atlas tectonic inversion. Positive gravity anomalies show preferential NE-SW and E-W orientations and are linked to the structural configuration of the Central High Atlas, which acted as a major source area for the basin. The identified fault systems and compressional structures in the Central High Atlas and Anti-Atlas are consistent with the regional geodynamic evolution. These results highlight the strong tectonic connection between the Ouarzazate Basin and adjacent Atlas basins, particularly the Central High Atlas, and provide new insights into the basin’s geodynamic development.

How to cite: Bouali, B., Tabayaoui, F.-Z., Sahbi, H., Manar, A., Nouayti, A., Boujamaoui, M., and Berkat, N. E.: Subsurface structural mapping using high-resolution gravity data and advanced processing techniques in the Ouarzazate Basin, Southern Morocco., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3508, https://doi.org/10.5194/egusphere-egu26-3508, 2026.

The Zahedan fault zone in Iran constitutes an active tectonic zone characterised by a complex network of strike-slip faults that dominate the local deformation pattern. This area is located within a large-scale transpressional shear zone accommodating relative motion between the Central East Iranian block and the Afghan Helmand block. The region provides a natural laboratory for investigating the relationship between strike-slip faulting and tectonic blocks rotated around vertical axes.

We present herein, based on high-resolution 2025 Airbus satellite imagery and cartographic and geophysical data, a new strike-slip fault pattern that facilitated the development of the rotated tectonic blocks.

Our observations show that the major strike-slip fault zones are accompanied by dense networks of second-order faults, including single sets of antithetic and synthetic strike-slip faults, conjugate strike-slip fault sets, restraining and releasing stepovers, and thrust faults. In several sectors along the major faults occur the zones of deformation consisting of the rotated tectonic blocks. The scale, orientation, and spatial organisation of the mapped structures indicate that block rotation is controlled by the interaction between major strike-slip faults and subsidiary fault networks.

The individual second-order antithetic faults display that these faults commonly accommodate small displacements, but the faults play a critical role in allowing internal deformation within blocks and facilitate the progressive block rotation. The sense of movements along the major fault and the antithetic strike-slip faults bounding the tectonic blocks allows us to consider the structures as the blocks rotated around vertical axes in a domino-like orientation. Recognised examples of structures show that some rotating blocks are rigid, with no evidence of significant internal deformation, while other rotating blocks exhibit strong internal deformation.

Understanding these spectra of behaviours and the determination of the relationships between them will improve our knowledge of fault interaction processes in eastern Iran and related patterns of seismicity, and it also has implications for seismic hazard assessment in active transpressional settings.

How to cite: Paktarmani, Z., Konon, A., and Mikołajczak, M.: Rotation of tectonic blocks controlled by strike-slip component along the Zahedan fault, Iran, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5809, https://doi.org/10.5194/egusphere-egu26-5809, 2026.

Charnockite is generally regarded as a product of high-temperature melting; however, its specific origin, generation, and preservation mechanisms, as well as its relationship to high-grade metamorphism or deep crustal reworking, remain poorly constrained. During the early Paleozoic, the South China Block underwent intense orogeny that resulted in significant crustal shortening and thickening, subsequently inducing widespread anatexis and extensive S-type granites. This study identifies ~431 Ma charnockites containing granulitic enclaves that were exposed in the Yunkai massif, providing key insights into the early Paleozoic crustal reworking and deep crustal melting behaviors in South China. The body displays A-type characteristics with crustal reworking zircon isotopic features (δ¹⁸O = 8.0–9.8 ‰; ε_Hf(t) = - 11.5 to - 3.4). The charnockite and its enclaves show identical mineral assemblages and comparable orthopyroxene chemical compositions. The two anhydrous minerals of orthopyroxene and garnet are identified as of peritectic and magmatic origins given their textural features and geochemical compositions. Moreover, petrographic observations and bulk geochemical data argue that the peritectic minerals were derived from the entrainment of their granulitic sources. Crystallization phase modeling indicates orthopyroxene would have been completely hydrated and formed biotite when water contents exceed ∼0.3 wt.% near the solidus. Water-in-zircon analysis and thermodynamic modeling indicate low magma water contents (∼0.15 wt.%; 135 ppm, zircon water medians) for the Gaozhou charnockite from early crystallization to final solidification. CO₂‐rich fluids flushed the charnockite reservoir further contributing to the stabilization of the orthopyroxene. Accordingly, the Yunkai charnockite reveals deep crustal melting processes involving anhydrous minerals entrained in a low-water environment. This low-water environment correlates with high-temperature melting of granulite-facies rocks in the lower crust and the presence of CO₂-rich fluids within the system. Regional magmatic-metamorphic-tectonic data indicate that the formation of the Yunkai A-type charnockite occurred within a post-orogenic extension regime, representing the peak of intracrustal reworking in South China.

How to cite: yang, H. and Yao, J.: Formation of A-type charnockite and constraints on deep crustal anatexis in early Paleozoic orogen, South China, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8520, https://doi.org/10.5194/egusphere-egu26-8520, 2026.

Unravelling the tectonic evolution of collisional orogens and the forces driving the exhumation of high-grade metamorphic rocks requires constraining the kinematics and timing of major fault systems. The Main Mantle Thrust (MMT) in the Swat Valley (northern Pakistan) marks the Eocene suture between the Indian Plate and the Kohistan–Ladakh Island Arc. Its later reactivation is critical yet debated, with models ranging from dominantly dextral strike-slip faulting with minor normal offset (typically less than a few meters) to significant normal faulting facilitating regional exhumation of high-grade metamorphic rocks. This research integrates structural geology with a new, multi-method low-temperature thermochronology dataset—including zircon and apatite (U–Th)/He and apatite fission-track ages—to contribute to this debate. Structural analysis of over 150 kinematic indicators collected along the MMT and its footwall shows that the MMT experienced semi-ductile to brittle reactivation dominated by top-to-the-north normal faulting. Structures formed under greenschist- to sub-greenschist facies conditions (e.g., C′ shear bands) in the footwall record progressive exhumation, with purely brittle cataclastic deformation marking the final stages. Our new thermochronometer dateset includes zircon (U–Th)/He (ZHe) single-grain ages (14.9 ± 1.2 to 24.4 ± 2.0 Ma), apatite (U–Th)/He (AHe) single-grain ages (7.6 ± 0.4 to 15.9 ± 1.0 Ma), and apatite fission-track (AFT) central ages (15.3 ± 2.4 to 16.4 ± 3.2 Ma). This dataset, alongside its thermal history modelling, reveals a consistent cooling signal across the Swat Valley. Following the Eocene collision and peak metamorphism, a major tectonic shift occurred in the Early Miocene. Our data indicate the onset of rapid cooling at ~20 Ma, with a slightly later initiation at ~18 Ma in the eastern Loe Sar Dome. This distinct phase of rapid cooling records the top-to-the-north normal reactivation of the MMT, which lasted until ~15–14 Ma. Collectively, our results provide structural and timing constraints supporting a model of protracted, normal-sense reactivation of the MMT between ~20 and 15 Ma. This event facilitated the final unroofing of high-grade metamorphic rocks in the Swat Valley. 

How to cite: Hernández Chaparro, D. R., Faccena, C., Olivetti, V., Fellin, G., and Ghani, H.: Constraining Early Miocene Reactivation of the Main Mantle Thrust (Swat Valley, Pakistan) through Integrated Structural and Thermochronologic Analysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12696, https://doi.org/10.5194/egusphere-egu26-12696, 2026.

The Kadavur Anorthosite Complex represents a distinctive structural domain within a folded high-grade crystalline terrain, where a massif-type anorthosite body occupies the core of a regional-scale fold and is surrounded by folded quartzite ridges. This study examines the relationship between lineament development and the pre-existing ductile fold architecture through integrated DEM–SRTM data analysis and quantitative lineament network characterisation using FracPaQ. The objective is to assess how fold geometry and lithological contrasts influence the spatial distribution and mechanical behaviour of brittle structures. DEM analysis reveals a coherent folded morpho-structural architecture characterised by a well-defined core, axial-plane domains, and limbs expressed as quartzite ridges. FracPaQ-derived results show that lineaments are non-randomly distributed and define multiple dominant orientation sets, reflecting systematic structural control rather than random patterns. Spatial variations in lineament density and lineament intensity show pronounced localisation within and adjacent to the fold core, whereas lineament attributes vary systematically between the anorthosite-dominated core and the surrounding folded quartzite limbs.

Slip tendency analysis indicates that brittle deformation is predominantly shear-controlled across the study area, while dilation tendency values are generally low to moderate, suggesting a subordinate role for opening-mode fracturing. Lineaments within the anorthosite core are comparatively longer, less densely spaced, and display lower orientation dispersion, reflecting brittle stress accommodation within a mechanically competent lithology. In contrast, lineaments developed in the folded quartzite ridges are shorter, more closely spaced, and strongly influenced by lithological layering and fold-related bending stresses.

Although comparable lineament orientation patterns occur across the fold core, axial planes, and limbs, their geometric characteristics, spatial distribution, and inferred mechanical roles differ significantly, indicating that brittle deformation was modulated by local fold geometry and lithological contrasts. The results indicate a structural association between ductile folding and later brittle deformation; however, the tectonic conditions responsible for anorthosite exhumation cannot be uniquely constrained from the present dataset. This study highlights the importance of domain-specific lineament analysis in folded crystalline terrains and emphasizes the role of inherited ductile architecture in controlling later brittle deformation.

How to cite: Prathapachandran, A., Manilal Girija, A., and Kumar, R. S.: Quantitative lineament network analysis of a folded crystalline terrain using FracPaQ: The Kadavur Anorthosite Complex, Southern Granulite Terrane, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16222, https://doi.org/10.5194/egusphere-egu26-16222, 2026.

With the continuous advancement of geological research, quantitative analysis of fracture networks has become a crucial research direction in geological exploration and resource development. To overcome the limitations of traditional methods in quantitatively analyzing fault data under complex geological conditions, we adopt a quantitative fracture characterization method based on geometric and topological theories. This study focuses on the overlay analysis of multi-period and multi-layer faults in a typical area of the western Junggar Basin, aiming to reveal their significant role in hydrocarbon exploration. By means of this method, we achieve multi-dimensional automatic quantification of geometric features (including fracture network length, orientation, and Pxy system), as well as node/branch types and topological parameters. Through the construction of a fracture topological network, we can quantitatively analyze the connectivity characteristics of each period and the vertically favorable conduction zones across multiple periods, thereby providing valuable guidance for hydrocarbon migration path prediction.

How to cite: Tao, Y., Cui, L., Niu, Y., Huang, Y., Bai, S., Zhao, G., Piluolan, P., and liu, Y.: Quantitative Characterization of Fracture Networks Based on Geometric-Topological Integration and Its Application in Hydrocarbon Migration Prediction in the Western Junggar Basin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18367, https://doi.org/10.5194/egusphere-egu26-18367, 2026.

The Achankovil Shear zone (AKSZ), juxtaposing the Trivandrum Granulite Block (TB) and the Madurai Granulite Block (MB) of the Southern Granulite Terrane (SGT), represents a paleo-suture zone related to the late Neoproterozoic to early Cambrian Gondwana assembly (Rajesh et al., 1998; Praharaj et al., 2021). The polyphase deformational history of the Achankovil Shear Zone (AKSZ) reveals progressive transition from a ductile to brittle deformation regime concomitant with high-grade granulite facies metamorphism and subsequent cooling–exhumation of the granulites. Progressive evolution of the state of stress and the variation of crustal dynamics during the ductile to brittle deformation regime transition, and the genetic link between these two contrasting episodes, if any, have been evaluated from statistical analysis of solid-state fabric and kinematic analysis of brittle fractures.

Three ductile deformation phases, D₁, D₂, and D₃, and associated solid-state fabrics, i.e., S₁, S₂, and S₃, are discernible at the mesoscopic scale. S₁ fabric is gneissic in character and is only preserved in strain shadow regions. Elsewhere, S₁ is transposed along the later S₂ fabric, which is axial planar to fold on S₁. Prevalence of high-temperature deformation conditions during D₂ and D₃ deformation stages is manifested by the presence of S₂-parallel stromatic leucosomes and diatexite leucosome along dilatant S₃ fabric, developed parallel to the axial planes of fold on S₂. Significant simple shear component during D₃ deformation is evidenced from the asymmetric nature of S₂ folds, asymmetric porphyroclast tail and other shear sense markers. Eigen vector analysis reveals a change of maximum eigen vector, i.e., pole to the mean foliation, from NW-SE (D₁: 311⁰/62⁰ NE) to N-S (D₃: 187⁰/80⁰ W). The maximum eigen vector of D₂ (125⁰/53⁰ SW), though similar in trend with D1, shows a reversal of dip direction. Fabric shape analysis reveals a progressive change from girdle to a strong clustered distribution of solid-state fabric from D₁ to D₃ deformation regime, suitably accounting for intense ductile shearing and transposition of earlier fabric during the D₃ deformation stage.

Minor conjugate faults are ubiquitous at different locations along the AKSZ. Dihedral angle of ~60 or less for these faults suggests a shear or hybrid fracture origin, diagnostic of a compressive stress regime. Also, the observed slip of striations on slickensides suggests a consistent oblique reverse kinematics. Fault kinematic and paleo-stress analysis further reveals two distinct stress regimes with NW-SE and NE-SW directed maximum compressive stress (s₁). Stress ratios for these faults imply a compressional to transpressional tectonic regime. Superposition of the slip tendency of NW–SE directed stress tensor over NE–SW directed stress tensor and vice-versa suggests that the NW-SE stress tensor precedes the NE-SW stress tensor during a progressive brittle deformation regime. Summarily, the cooling and exhumation and the switch over from ductile to brittle deformation regimes of the granulites took place under a compressive stress field developed during terrane accretion along the AKSZ. The brittle faults seemingly result from the relaxation of the orogenic far-field stress.

How to cite: Manilal Girija, A. and Bhadra, S.: Ductile to brittle tectonic evolution of the Achankovil Shear Zone, Southern Granulite Terrane – Constraints from statistical analysis of fabric and paleostress inversion, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20066, https://doi.org/10.5194/egusphere-egu26-20066, 2026.

We investigate the heterogeneity of the Indian subcontinent using seismic recordings from the Mw 7.7 Myanmar earthquake that occurred on 28 March 2025. This event was recorded by broadband stations across India. These variations in waveforms at different stations highlight the influence of radiation pattern, crustal structure, wave-propagation paths, and local site conditions. Sedimentary basins, characterized by relatively soft sediments, are known to amplify seismic energy and modify ground motion characteristics, often resulting in enhanced shaking. Understanding these effects is essential for assessing seismic hazard.

We use time-series data from approximately 88 seismic broadband stations provided by the National Centre for Seismology (NCS), India. We apply frequency spectrum analysis, horizontal-to-vertical spectral ratio (HVSR) analysis, and surface wave dispersion analysis. The frequency spectrum helps identify frequency bands where seismic energy is amplified while HVSR analysis is used to estimate the site’s fundamental resonance frequency and the corresponding amplification factor. Surface wave dispersion analysis provides shear-wave velocity information, which is crucial for characterizing near-surface geological conditions.

Together, these analyses help us to understand the influence of local geological conditions at the receiver sites and contributes to a better analysis of regional seismic wave propagation and site-specific ground motion characteristics across the Indian subcontinent.

How to cite: Gupta, S., Silwal, V., and Bora, S. S.: Dynamic Triggering and Effects of Crust Heterogeneities on Propagating Waves due to the 2025 Mw 7.7 Myanmar Earthquake, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21372, https://doi.org/10.5194/egusphere-egu26-21372, 2026.

The northeastern margin of the Iberian Peninsula, extending from the Vallès-Penedès Graben to the Valencia Depression, constitutes a passive margin related to the opening of the Valencia Trough during the Neogene. It comprises a series of extensive basins bounded by several mountain ranges, where numerous NNE–SSW-oriented normal faults are present, commonly associated with mountain fronts. Previous studies show that some of these faults, such as the El Camp fault, remain active, although at very low slip rates, as low as 0.02 m/kyr for Late Pleistocene.
Recent analyses of high-resolution Digital Elevation Models (DEMs) have revealed several additional morphological escarpments crosscutting different Quaternary alluvial fan systems across the region. These scarps, found from the Sant Jordi Plain to the La Salzadella Basin, share the same orientation as the Neogene faults, suggesting a common tectonic origin. They are discontinuous, arranged in a right-stepped pattern, and locally display vertical offsets of up to 8 m. Furthermore, several geomorphic features indicate recent tectonic activity, including aligned fissures within the fans and entrenched channels developed on the upthrown block that fade out after crossing the escarpments. Each family of escarpments exceeds 10 km in length, with important implications for the seismic hazard of the region.
Geophysical surveys (ERT, GPR, SRT, HVSR, and MT), validated with borehole data, confirm the presence of faults beneath each analysed escarpment system. At the Vinaixarop escarpment, for instance, ERT profiles revealed a steeply dipping discontinuity plane with a vertical offset of approximately 40 m affecting upper Pliocene sediments.
The faulted alluvial fans are interpreted as Lower to Middle Pleistocene in age and are mainly composed of carbonate gravels. Paleoseismological trenches excavated at four sites (L’Ampolla, Vinaixarop, and two at Sant Rafael del Riu) on different scarps revealed consistent evidence of late Quaternary faulting, such as ruptured strata. Additionally, ground-based hyperspectral cameras (400–1700 nm) were deployed on trench walls as an ancillary tool to map faulted
stratigraphic layers and to detect subtle coseismic deformation. As sedimentation on the fans ceased by the Middle Pleistocene, subsequent activity has been primarily recorded through the deformation of pedogenic features (calcretes), commonly found in the study area. To constrain the timing of this activity, U–Th dating was performed on fault-related carbonates and deformed calcretes. Preliminary results indicate that tectonic activity along these faults persisted at least until the Late Pleistocene. However, a deformed colluvial wedge and the presence of open fissures observed on trench walls suggest Holocene activity.
Additionally, three cosmonuclide depth-profiles were sampled to date displaced geomorphic surfaces, and hence estimating the long-term slip rates of the faults. Ongoing analyses aim to demonstrate Holocene seismic activity and to further characterize paleoearthquakes and their seismic parameters.
In summary, studies of active tectonics in regions of very low strain are inherently challenging, especially when recent sedimentation is scarce and pedogenic processes are intense. Nevertheless, this work highlights that integrating multiple complementary techniques is the most effective approach to address such settings.

How to cite: Ollé-López, M., García-Mayordomo, J., Gallego-Montoya, J., Molins-Vigatà, J., Bellmunt, F., Gabàs, A., Andrés, J., Macau, A., Hasözbek, A., Martí, A., Figueiredo, P., Rodés, Á., García, D., Ortuño, M., and Masana, E.: Constraining Late Pleistocene to Holocene seismic fault activity in NE Iberia: The value of integrating complementary techniques in a low-strain region, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21385, https://doi.org/10.5194/egusphere-egu26-21385, 2026.

The Tbilisi Area is located within the eastern segment of the Achara-Trialeti fold-and-thrust belt, which is itself a part of the Kura foreland basin system. The region is characterized by an active convergent tectonic regime resulting from the ongoing collision between the Eurasian and Arabian plates. The convergence has induced extensive deformation features, including large-scale folding, faulting, and uplift, which have influenced the stratigraphy and lithological compositions of the Eocene sedimentary rocks of the study area.

A detailed, interdisciplinary study of these Eocene sedimentary rocks was implemented, integrating petrographic, geochemical, geomechanical, and petrophysical analyses to comprehensively understand physico-mechanical properties, and their interrelation. For this purpose, samples taken from the field were analysed at the Institute of Applied Geosciences, KIT. Petrographic examinations reveal a heterogeneous lithological assemblage comprising predominantly fine-grained clay mineral matrix-rich arkosic wackes, lithic wackes, arkosic arenites, and lithic arenites often containing foraminifera and glauconite, implying marine deposition. Geochemical analyses indicate the most prominent elements of the rocks are Si, Al, and Ca. Their content ranges 23.2-29.6%, 7.1-10.1%, 1.7-11.6%, respectively.

The rocks exhibit notably low permeability, generally in the range of 10^-4 to 3*10^-1 millidarcies (mD), with permeability strongly dependent on porosity metrics (ranging from 1.1% to 10.3%). This low permeability is primarily controlled by clay matrix content and to a lesser extent on cementation processes. The heterogeneity and complexity of these formations are further confirmed by the wide range of uniaxial compressive strength (UCS), from 36.9 MPa to 208.8 MPa, reflecting variations in lithology, degree of cementation, and diagenetic modifications across different sections.

This work presents an initial effort to showcase the diverse rock properties from the Tbilisi Area, as the Eocene sedimentary rocks show distinct lithological heterogeneity and complex mineralogical and petrophysical characteristics, strongly influenced by their depositional and tectonic history with implications for engineering utilization of the lithologies.

Acknowledgement: This work was supported by the Joint Rustaveli-DAAD fellowship programme, 2025. We thankfully acknowledge assistance in the lab by Martin von Dollen (KIT).

How to cite: Giorgadze, A., Busch, B., Chen, Y., Cheng, C., Alania, V., Gorgidze, L., and Enukidze, O.: Petrographic, physico-mechanical, and geochemical characteristics of Eocene sedimentary rocks from the Tbilisi Area (Georgia), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-159, https://doi.org/10.5194/egusphere-egu26-159, 2026.

Abstract

To identify and characterize reservoir and non-reservoir intervals across multiple formations, this paper examines comprehensive petrophysical evaluation and scattered cross-plot analysis using well-log data from Well 32-2-1 in the Smeaheia area of the Norwegian North Sea. The main objective of this study is to delineate lithology variations, detect fluid-bearing zones, and evaluate the key geological and petrophysical factors that control reservoir quality within a heterogeneous sandstone-shale succession.

A suite of well logs including gamma ray, resistivity, density, neutron porosity, and sonic measurements was thoroughly interpreted to identify lithological transitions and potential hydrocarbon-bearing intervals. Cross-plotting techniques were employed to analyze relationships between critical log parameters such as RHOB-NEU, GR-density, and neutron-density separation etc. These crossplots facilitated the differentiation of claystone, shaly-sand, clean sandstone, cemented sandstone, and coal units, while also revealing variations in porosity, mineral composition, and clay content across the well.

The integrated petrophysical analysis highlighted significant vertical heterogeneity, ranging from clean quartz-rich sandstones to shaly sands, claystone-dominated sections, calcite-cemented sands, and thin coal streaks. High gamma-ray, moderate density, and elevated neutron porosity responses delineate clay-rich zones where bound water inflates apparent porosity. Shaly-sand units are identified through intermediate clustering in RHOB-NEU and GR-density crossplots, reflecting mixed mineralogy and moderate effective porosity. Clean sandstone intervals, recognized by low GR and low NEU signatures, exhibit variable reservoir quality controlled by burial compaction and cementation intensity. Zones showing lower-than-expected density and reduced sonic velocity indicate undercompaction and suggest localized overpressure conditions.

Overall, the results of this cross-plot-driven petrophysical evaluation provide new insights into the distribution of porosity, clay volume, compaction state, and lithofacies variability throughout the stratigraphic interval. The interpretation enhances the understanding of reservoir and non-reservoir facies and improves the identification of intervals with potential hydrocarbon significance. The findings of this study contribute to more reliable reservoir characterization and support improved exploration and development strategies within the complex sandstone systems of the Norwegian North Sea.

Keywords:  Rock Physics, Petrophysics, Reservoir zone, Cross-Plot, Smeaheia Region.

How to cite: Rani, P. and Yadav, A.: Cross-Plot-Driven Petrophysical and Rock Physics Characterization of Reservoir and Non-Reservoir Zones in the Smeaheia Area, Norwegian North Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-688, https://doi.org/10.5194/egusphere-egu26-688, 2026.

Rocks exhibit pronounced nonlinear viscoelastic behavior such as modulus softening and loading–unloading hysteresis, even under micro-strain dynamic loading. However, classical nonlinear elastic constitutive models based on higher-order expansions of Hooke’s law show limitations in capturing these nonlinear viscoelastic features. To address this, we extend the classical nonlinear elastic framework by incorporating strain-rate terms and formulate a nonlinear viscoelastic constitutive relation in which nonlinear elastic and viscoelastic parameters jointly describe modulus softening, dynamic response, and hysteresis loops. We use a copropagating acousto-elastic testing system to acquire time series of elastic modulus variation ΔM/E and the corresponding hysteresis loops for Crab Orchard sandstone at five dynamic strain levels. We then invert these data to estimate model parameters and test the constitutive relation. The model reproduces the main nonlinear viscoelastic features observed at all strain levels and, compared with two classical nonlinear elastic models without viscoelastic terms, better captures the phase information in the ΔM/E time series and the geometry of the hysteresis loops. The proposed nonlinear viscoelastic constitutive relation provides a practical way to constrain micro-strain nonlinear viscoelastic parameters of rocks in the laboratory and offers a basis for linking laboratory measurements with field relative velocity changes monitoring to study stiffness evolution during processes such as hydrological loading and fault slow slip.

How to cite: Bai, H., Feng, X., Fehler, M., Brown, S., and Bokelmann, G.: Nonlinear viscoelastic constitutive relation for rocks under micro-strain dynamic loading, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1712, https://doi.org/10.5194/egusphere-egu26-1712, 2026.

Characterizing and predicting reservoir heterogeneity is essential for efficient hydrocarbon production. In this study, multifractal theory is employed, using both numerical and experimental data, to quantitatively describe the pore structure heterogeneity of a Lower Cretaceous limestone reservoir from the United Arab Emirates. Fractal dimensions derived from three-dimensional digital images show a strong correlation with high-pressure mercury injection (HPMI) measurements (R² = 0.69). Furthermore, both experimental and numerical fractal dimensions correlate well with HPMI-derived porosity. Multifractal parameters—including the non-uniformity degree of the pore structure (Δα), the asymmetry in the vertical axis (Δf(α)), the concentration of the pore size distribution (α₀), and the asymmetry in the horizontal axis (Rd)—calculated from digital and experimental data, are consistently correlated and demonstrate a robust ability to quantify sample heterogeneity. The ranges of these digital and experimental multifractal parameters provide an effective basis for distinguishing between homogeneous and heterogeneous samples.

How to cite: Jouini, M. and Bouchaala, F.: Multifractal Parameters as Indicators of Pore Structure Heterogeneity in a Lower Cretaceous Limestone Reservoir (UAE), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2023, https://doi.org/10.5194/egusphere-egu26-2023, 2026.

Natural stones used in most buildings undergo degradation due to their exposure to outdoor environments, particularly to sometimes extreme variations in temperature and humidity conditions. In historical buildings, consolidants are often used to restore the strength of the damaged building materials and prevent further deterioration. The most commonly used consolidant in this context is ethyl silicate but it is incompatible with salt-contaminated stone. A salt-compatible alternative, lithium silicate, has been developed but remains rarely used. This is partly due to the limited number of available scientific studies and the difficulty of detecting lithium using conventional analytical methods. The aim of this study was to compare the effectiveness of these consolidants on two porous limestones subjected to thermal and mechanical stresses. Saint-Maximin and Leitha limestones were selected for this study because both rocks were widely used as building stones, in France and Austria, respectively. Cylindrical samples of 40 mm in length and 20 mm in diameter were prepared from large blocks. The average porosities of our Saint-Maximin and Leitha limestones were 38% and 41%, respectively. The permeability of both limestones was greater than 1 Darcy. Three groups of samples were tested: intact samples, samples thermally treated up to 400°C and samples deformed uniaxially to the peak stress. Consolidants were introduced in these samples through imbibition experiments that lasted a minimum of 48 h. Within the limestones, the consolidants underwent hydrolysis and condensation reactions to first form a gel on the pore surface. This gel progressively polymerized to form a thin solid layer on the pore surface. Treated samples were typically left to cure in a dry environment for at least a month. Minor variations of the porosity and permeability were observed in all the consolidated samples. The Uniaxal Compressive Stress (UCS) of the intact samples predictably increased by a factor of two for both rocks and both consolidants. After thermal treatment up to 400°C, the UCS of samples of Saint-Maximin and Leitha typically decreased by about 25% due to the thermal expansion of the grains and thermal microcracking. We found that consolidation with ethyl silicate erased this weakening effect. For lithium silicate, the samples also recovered part of their strength, but the effect was less pronounced. When damage was introduced into the rocks through uniaxial compression, ethyl silicate produced a more significant strengthening effect than lithium silicate. In the context of cultural heritage conservation, it is essential that consolidated stones present petrophysical properties similar to the original material in order to prevent further mechanical alterations. Both products exhibit a consolidating effect, but stones consolidated with lithium silicate display properties closer to those of the original rock.

How to cite: Baud, P., Schloegel, P., Surma, F., Reuschle, T., and Heap, M.: The impact of consolidants on the properties of intact and damaged porous limestones, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3035, https://doi.org/10.5194/egusphere-egu26-3035, 2026.

Elastic and electrical properties of a fluid-bearing rock are strongly affected by cracks. Effective elastic compliance of a rock is determined by number density and compliance of cracks. When cracks form an interconnected network, effective electrical conductance is determined by number density and conductance of cracks. Cracks have rough surfaces and asperities on them come into contact under pressure. Elastic compliance and electrical conductance of a crack decrease with its closure to govern changes in elastic and electrical properties with pressure. Though surface topography of cracks is a key to understanding physical properties under pressure, it is still difficult to investigate. We thus conduct numerical experiments of crack closure to understand changes in elastic and electrical properties with pressure. For simplicity, a 2D 50×50 square lattice is used to model a fluid-filled crack. The initial aperture of cells has a Gaussian distribution and is randomly given. For a closure of aperture, the required stress is calculated through Hertzian contact theory and electrical conductance of the lattice is calculated with the finite difference method. The lattice is electrically conductive when more than 50% of cells are open. When more than 70% of cells are open, the conductance is insensitive to the distribution of apertures. The elastic compliance is also calculated as a function of aperture closure. Both elastic and electrical properties are thus obtained as a function of confining pressure and compared with measured elastic wave velocity and electrical conductivity reported in Watanabe et al. (2019, 2024).

How to cite: Watanabe, T.: Numerical experiments of crack closure and its influence on elastic and electrical properties, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3760, https://doi.org/10.5194/egusphere-egu26-3760, 2026.

We compare and contrast physical properties (density, P-wave velocity) of volcaniclastic sediments with other uncemented marine sediments. We study cores collected by International Ocean Discovery Program (IODP) Expedition 398, which recovered more than 2200 m of volcaniclastic deposits from 12 sites and 28 holes from Santorini Caldera, Greece, and the surrounding rift basins in the South Aegean Volcanic Arc. The grain density (mass of solids divided by their volume, including any isolated vesicles) of volcaniclastic deposits is typically lower than that of volcanic glass and crystals and is sometimes less than 2 g/cm³, indicating the preservation of isolated gas-filled vesicles in erupted and then deposited materials. To complement bulk measurements, we also measured the total and isolated porosity in individual lapilli-sized volcanic clasts from four different volcanic deposits. We use xray computed tomography to image isolated pore space. Collectively, these measurements confirm that volcaniclastic sediments can preserve vesicle textures and isolated porosity for hundreds of thousands of years and at depths >500 m below sea-level and > 100 m below the seafloor

Volcaniclastic deposits typically have higher P-wave velocities but lower bulk densities than oozes and other marine sediments. In volcaniclastic deposits, lapilli have higher P-wave velocities and lower bulk density than ash, the opposite trend of most sediment in which higher density is correlated with higher seismic velocity. We use granular physics models to show that the higher volcaniclastic P-wave velocity originates from two effects: 1) lower pore volume outside clasts that increases elastic moduli and P-wave velocity and 2) isolated gas vesicles in volcanic clasts that lower bulk density with proportionally less effect on elastic modulii. In volcaniclastic sediments there is relatively little change in physical properties to depths of several hundred meters below the seafloor, which we attribute to rough grain surfaces and lower intergranular (external) porosities that resist compaction and the decrease of intergranular pore space relative to background marine sediment.  These trends lead to distinctive signatures of volcaniclastic sediments in reflection seismic images.

How to cite: Manga, M., Wright, V., Cadena, T., Susman, I., Escogido, C., Ward, S., Kelly, L., Fauria, K., McIntosh, I., Preine, J., Tominaga, M., Nomikou, P., Druitt, T., Kutterolf, S., Ronge, T., Hübscher, C., Karstens, J., and Yamamoto, Y. and the IODP Expedition 398 Scientists: Contrasting seismic velocity and compaction of marine calcareous oozes and volcaniclastic deposits, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4113, https://doi.org/10.5194/egusphere-egu26-4113, 2026.

In situ stress and wave-induced fluid flow (WIFF) jointly influence the velocities of waves propagating through the formations. However, for partially saturated porous media commonly encountered in the subsurface, the stress- and frequency-dependent characteristics of wave velocity dispersion and attenuation are not yet fully understood. To address this, we propose a new poro-acoustoelasticity model to characterize wave velocities in patchy-saturated porous media under in-situ stress. This model simultaneously accounts for macroscopic global flow arising from wave-induced relative motion between the pore fluid and the solid frame, mesoscopic WIFF associated with patchy fluid saturation, and microscopic squirt flow induced by fluid pressure gradients between pores and cracks. Furthermore, by considering the influence of effective stress on the pore structure, the nonlinear deformation of cracks is incorporated into our rock physics model, thereby extending its stress applicability. The modelling results indicate that two compressional waves (fast P- and slow P-waves) and a shear wave (S-wave) coexist. As the effective stress increases, the velocities of the fast P- and S-wave increase, accompanied by reductions in dispersion and attenuation, which can be attributed to crack closure. In addition, with increasing frequency, the fast P-wave velocity exhibits three successive attenuation peaks, corresponding to the effects of WIFF at meso-, micro-, and macro-scales. In contrast, the slow P-wave velocity appears only at higher frequencies, and its variation is more significantly influenced by water saturation than by effective stress. The validity of the proposed model is demonstrated through comparison with previously published experimental data. Furthermore, our model is used to establish a rock-physics approach to estimate the wave velocities with the well-logging data. The predicted results agree well with the logging measured data, further confirming the feasibility of our approach. Our study and results provide a useful tool for hydrocarbon exploration, CO₂ storage monitoring, and hydrogeology.

How to cite: Li, J., Zong, Z., Chen, F., and Zheng, X.: Rock physics model for patchy saturated porous media under in-situ stress, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4727, https://doi.org/10.5194/egusphere-egu26-4727, 2026.

    The gravel deposit, widely distributed in Taichung City, central Taiwan, is a composite material in which gravels and cobbles are embedded within a fine-grained soil matrix. Due to the presence of large particles ranging from 100 mm to 300 mm, the characterization of shear strength commonly relies on large-scale direct shear tests or triaxial tests. In practice, however, these measurements are costly and difficult to perform in sufficient numbers to represent the high variability of mechanical properties at the site. To address this limitation, this study proposes an integrated framework that incorporates the hardness value of the matrix, obtained from the Yamanaka soil hardness tester, and the geometric features of gravels into Discrete Element Method (DEM) simulations to estimate the shear strength of the gravel deposit.

    Specifically, three-dimensional DEM simulations were conducted using EDEM, employing the Hertz-Mindlin contact model coupled with the Bonded Particle Model (Bonding V2) to replicate a full-scale direct shear test. The gravel deposit was discretized into matrix (< 4.75 mm) and gravel (> 4.75 mm) fractions based on the field investigation results. To represent the geometric heterogeneity, image analysis was employed to characterize the gravel fraction and extract morphological parameters for the simulation. Under this classification, distinct approaches were adopted for the micro-parameters. Contact parameters were specified for each fraction, while bonding parameters for all interaction types (Matrix-Matrix, Matrix-Gravel, and Gravel-Gravel) were governed by the matrix properties. To determine the appropriate bonding and contact parameters specific to the matrix fraction, the Yamanaka soil hardness tester was utilized to bridge the gap between field conditions and numerical simulations. By employing Response Surface Methodology (RSM), a quantitative relationship between micro-parameters and penetration depths was established to identify the parameters from the field data. Subsequently, the calibrated micro-parameters corresponding to the target in-situ penetration depth were assigned to the composite model for full-scale direct shear test simulations to evaluate the shear strength. Preliminary verification confirms the feasibility of the penetration test simulation in EDEM. Furthermore, complementary uniaxial compression tests demonstrate that the calibrated bonding parameters correspond to realistic physical properties, thereby ensuring the reliability of the shear strength estimation in the full-scale DEM simulations.

How to cite: Liu, P.-C. and Chang, K.-T.: Estimating the Mechanical Properties of Gravel Deposits by Integrating Field Investigation and DEM Simulation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4766, https://doi.org/10.5194/egusphere-egu26-4766, 2026.

The Paleogene sedimentary series of the Paris Basin, characterized by mixed carbonate and siliciclastic deposits in continental and marine environments, exhibit significant lithological, facies, and diagenetic heterogeneities. However, the lack of data and conceptual frameworks linking petrophysical properties to geological knowledge of these sedimentary series remains a major limitation for predicting physical properties across different scales. Understanding and predicting reservoir heterogeneities is essential for addressing major societal challenges, such as subsurface planning and geothermal energy development in the Île-de-France region. To tackle this issue, a multi-scale study is conducted, integrating sedimentary geology and petrophysical analyses of the Paleogene series. The study focuses on key areas of the Paris Basin, defined by the alignment of the Grand Paris Express (GPE) project, a 200 km network of new metropolitan lines around Paris. These areas are ideal to lead to a very high spatial resolution characterization of the sedimentary system, thanks to the availability of an exceptionally dense dataset of core drillings.

The analysis of cores from 15 boreholes and gamma-ray logs from 34 additional boreholes allowed the identification of twenty-two facies grouped into seven facies associations, corresponding to seven depositional environments ranging from open marine environments to palustrine settings. Based on these observations, twelve transgressive–regressive cycles spanning the Danian to the Rupelian were identified and correlated along two multi-kilometer transects (≈20 km).

Petrophysical measurements performed on 633 samples reveal a strong heterogeneity within the studied Paleogene successions. For instance, palustrine limestone facies affected by intense recrystallization and silicification display relatively uniform P-wave velocities (4.1–6 km s⁻¹) and porosities (1.5%–12%). In contrast, marine limestones exhibit a wide range of P-wave velocities (0.9–5.6 km s⁻¹), partly related to facies variability and primarily controlled by porosity (5–46%), with decreasing velocities at increasing porosity. This parameter is mainly governed by the abundance and nature of diagenetic cements, with mosaic calcite cements (drusy, granular, blocky) leading to a stronger porosity reduction than isopachous cementation, while depositional facies exert a secondary control (e.g. Miliolid grainstone vs Bioclastic floatstone). Additional controls contributing to the petrophysical heterogeneity of marine limestone facies include pore spatial distribution and connectivity, pore size distribution, and pore type. Pore spatial organization exerts a first-order control on acoustic velocities and porosity: uniformly distributed and well-connected pore networks are associated with low P-wave velocities and high porosities, whereas patchy pore distributions lead to higher velocities and reduced porosity. Pore size also influences petrophysical properties, with macroporosity (>62 µm) generally associated with relatively high P-wave velocities and low porosities, while meso-microporosity (<62 µm) does not show a clear relationship with either parameter. Pore type also plays a significant role, as interparticle porosity is associated with low velocities and high porosities, in contrast to intraparticle, moldic, and vuggy porosities, which are characterized by higher velocities and lower porosities.

Although locally homogeneous from a petrophysical perspective, these sedimentary series display strong heterogeneities governed by multiple geological controls, emphasizing the key role of petrophysical characterization in reservoir prediction.

How to cite: Marconnet, C., Bailly, C., Briais, J., Regnet, J.-B., Andrieu, S., Lasseur, E., and Brigaud, B.: Controlling factors of petrophysical and sedimentary heterogeneities in Paleogene rocks beneath Paris, France, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7308, https://doi.org/10.5194/egusphere-egu26-7308, 2026.

Seismic velocity at the near surface drops during ground motions due to remote earthquakes and can recover afterwards over decades. In the laboratory, seismic velocity of rock samples decreases after dynamic deformations (e.g., shaking) and gradually recovers towards the original level. These observations at different scales are referred to as slow dynamics in granular materials (e.g., rocks and concrete), but the underlying mechanisms remain debated.

We explore the physics behind seismic velocity transients during and after dynamic deformations using Stór Mjölnir — a triaxial pressure loading apparatus featuring two piezoelectric transducers mounted in the top and bottom pistons and an X-ray transparent aluminium pressure vessel that houses a cylindrical core sample of Clashach sandstone (10 mm in diameter and 25 mm in length).

We present the mechanical, acoustic, and X-ray microtomography results of two triaxial loading experiments, conducted at room temperature with a confining pressure of 20 MPa and a pore fluid pressure of 5 MPa. Both experiments involve first increasing the ram pressure at a constant strain rate of 1x10^-5 s^-1 until the onset of sample yielding, indicated by a deviation from the linear stress–strain curve. In the first experiment, we further hold the ram pressure constant and then abruptly reduce the pressure by 30 MPa before rapidly returning the pressure to the previous hold level; this perturbation is repeated for 32 cycles until catastrophic failure of the rock sample. In the second experiment, we apply the same cyclic loading protocol after sample yielding, except for the abrupt pressure drop of 150 MPa; the sample survives only two loading cycles before catastrophic failure. These cyclic loading protocols are designed to induce transient seismic velocity responses, which are monitored by active acoustic surveys acquired every 8 s and in-situ 3D X-ray tomography synthesised every 6 min at the beamline I12-JEEP, Diamond Light Source (Oxfordshire, UK).  

We observe nearly linear relationships between the small stress perturbations (30 MPa) and corresponding seismic velocity changes, indicating minimal slow dynamics in the rock sample. In contrast, large stress perturbations (150 MPa) cause nonlinear velocity changes, although the recovery time scale is limited by the small size of the experimental sample. The time-resolved 3D X-ray volumes from both experiments show no resolvable transient structural changes in the rock samples, despite ongoing microfracture accumulation and pore enlargement driven by background creep until catastrophic failure. These results demonstrate that active seismic waves can detect nonlinear velocity transients in triaxial loading experiments, which likely originate from microstructures (e.g., grain contacts) below the X-ray imaging resolution (voxel edge length ~ 7.9 µm). These experiments also motivate further study on seismic velocity transients using our next-generation experimental apparatus that accommodates larger samples (18 mm in diameter and 45 mm in length) and six acoustic transducers. Ultimately, we aim to assess seismic velocity transients as a proxy for rocks’ susceptibility to small stress perturbations, which could provide a method to map the proximity to catastrophic failure and hence help mitigate induced seismicity associated with hydraulic fracturing.

How to cite: Li, X., Chandler, M., Cartwright-Taylor, A., Freitas, D., Mangriotis, M.-D., Woldemichael, B., Liptak, A., Atwood, R., Chapman, M., Fusseis, F., Butler, I., Curtis, A., and Main, I.: Exploring seismic velocity transients using in-situ acoustic monitoring and synchrotron-based X-ray tomography in triaxial dynamic loading experiments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8295, https://doi.org/10.5194/egusphere-egu26-8295, 2026.

Geothermal exploration drilling plays a crucial role in advancing the energy transition. To make the prospecting process more economical and time-efficient, the EU-funded GeoHEAT project has set the goal of developing methods that allow for obtaining the maximum amount of information about the subsurface with as little expense as possible. The project includes the development of a ground-penetrating radar (GPR) that can be used during borehole logging at elevated ambient temperatures. For the accurate interpretation of the GPR data, as well as for estimating the porosity and water content of the rocks surrounding the borehole, properties such as permittivity and electrical conductivity of these rocks are required. Here, we want to present the current progress of our research, which aims to determine the aforementioned characteristic properties using digital rock physics (DRP). Moving away from standardized cylindrical samples and using irregularly shaped by-products of the drilling process, known as drill cuttings, we can provide a more comprehensive understanding of the subsurface, thereby improving the characterization of potential geothermal reservoirs.

Core sampling during exploration drilling is costly and time-consuming, as it interrupts the operation. In addition, cores are often taken from only a few sections in order to keep the added costs low. However, these samples are necessary for laboratory testing, as sufficiently large and smooth contact surfaces must be available to ensure that the respective measurement devices deliver accurate results. No such requirements exist in DRP, as simulations are performed at the pore scale and therefore very small samples without flat surfaces, such as irregular drill cuttings, can be used.

The DRP workflow consists of three main steps. First, high-resolution computed tomography scans are taken of a small sample. These are then processed into a digital twin using segmentation, where the individual phases, such as minerals or pores, are distinguished from one another so that specific properties can later be assigned to them in this location-dependent volume. In combination with our finite volume method code, which solves a stationary potential equation, this digital model can be used to simulate the desired effective properties.

In previous studies, an early implementation of our code demonstrated reliable results for frequencies greater than 1 MHz. By implementing preconditioners, we now simulate lower frequencies with highly accurate results where before the increase in polarization led to code instabilities. Additionally, we fully validated the code on comparative data, such as analytical solutions and laboratory measurements of a high-porosity sandstone and a low-porosity granite. In the future, we will investigate how changes during the creation and transport of drill cuttings influence the accuracy of the results, thereby contributing further to a more efficient approach to geothermal exploration.

How to cite: Kerkmann, N., Siegert, M., Finger, C., Shakas, A., and Saenger, E. H.: Frequency-Dependent Effective Electrical Properties of Suboptimal Rock Samples through Digital Rock Physics, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8989, https://doi.org/10.5194/egusphere-egu26-8989, 2026.

Petrophysical models linking geophysical observables to subsurface hydraulic states are fundamental to the interpretation of hydrogeophysical data. For electrical methods, power-law formulations such as Archie’s law are commonly used to relate different components of complex conductivity to water saturation, with the associated saturation exponents in these power laws describing the saturation-dependent behavior of complex conductivity. However, due to the lack of the ability to directly visualize pore-scale fluid distributions for traditional laboratory or field investigations, the physical origin and variability of these saturation exponents remain poorly understood, which hinders reliable interpretation of geoelectrical data in dynamic or heterogeneous subsurface environments.

In this contribution, we present results from our recent studies that quantitatively investigate how pore-scale fluid and interface distribution govern the saturation dependence of complex conductivity. First, a dedicated milli-fluidic micromodel was developed to enable simultaneous spectral IP measurements and direct visualization of pore-scale fluid configurations during drainage and imbibition. By combining laboratory observations with finite-element and pore-network simulations, we demonstrate quantitatively that both the in-phase and quadrature saturation exponents are controlled by the rate of change of pore-water connectivity with saturation. In parallel, by extending Archie’s laws to interfacial polarization using fractal theories, we establish that surface and quadrature conductivity in fractal porous media follow power-law relationships with specific surface area, with the corresponding exponents linked to the pore-volume fractal dimension. Building on these results, we further explore a commonly observed yet poorly explained anomaly in IP measurements: the decrease of quadrature conductivity (or normalized chargeability) with increasing saturation during drying. Using desiccation experiments combined with pore-network modeling, we show that this anomalous behavior arises from a coupled mechanism involving the sequential drying of pores of different characteristic sizes and the persistence of thin water films on solid surfaces.

Together, these studies advance the petrophysical understanding of IP signatures by linking macroscopic electrical parameters to microscale fluid topology and interfacial processes. Our findings underscore the importance of incorporating pore-scale fluid connectivity and interfacial effects into petrophysical models, thereby improving the quantitative interpretation of geoelectrical data in hydrogeological, biogeochemical, and reservoir monitoring applications.

How to cite: Qiang, S., Shi, X., and Revil, A.: Elucidating Pore-Scale Mechanisms Governing the Saturation-Dependence of Complex Conductivity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9096, https://doi.org/10.5194/egusphere-egu26-9096, 2026.

How to cite: Dannak, H., Pehme, P., Munn, J. D., and Parker, B. L.: Quantification of Low-Frequency Stoneley Wave Energy Attenuation in a Carbonate Fractured Bedrock Aquifer: An Evaluation of Its Relationship with Fracture Hydraulic Properties  , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12242, https://doi.org/10.5194/egusphere-egu26-12242, 2026.

The Dogger geothermal reservoir of the Paris Basin is one of the most actively exploited carbonate aquifers in France and serves as a key target for sustainable district-heating systems. It consists mainly of Middle Jurassic carbonates deposited on a ramp, where sedimentary facies and diagenetic overprinting produce strong spatial heterogeneity. Understanding the heterogeneous petrophysical distributions is essential for predicting fluid circulation and designing subsequent geothermal operation plan. However, petrophysical interpretations derived from geophysical methods remain scale-dependent: laboratory acoustic measurements, well logs, and seismic data have different resolutions, making it challenging to reconcile acoustic signatures and to map heterogeneity consistently across scales.

To address these challenges, we conducted ultrasonic transmission experiments on core fragments taken from the SEIF-01 geothermal well in Melun area to determine Vp and Vs across key facies types. We further measured pressure-dependent Vp-Vs variations on cylindrical plugs, to better understand how the sedimentary microstructures and crack closure control the seismic velocity. Using seismic rock velocity models, we interpret the influence of pore shape (measured by pore aspect ratio) and fluid on the seismic velocity, to provide a quantitative link between micro-scale pore geometry and macroscopic elastic properties.

Finally, we will integrate the laboratory results with in-situ sonic logs and 2D seismic reflection data to bridge acoustic observations across scales. This multi-scale integration provides new insights into the internal heterogeneity of the Dogger reservoir and improves the interpretation of geophysical datasets for geothermal development. Our results highlight the potential of combining ultrasonic experiments, well logs, and seismic data, to better constrain reservoir properties and support more reliable geothermal resource assessment in heterogeneous carbonate systems.

How to cite: Wang, H.-M., Bailly, C., Brigaud, B., Bordenave, A., Issautier, B., Pwavodi, J., Samuel, C., Fortin, J., Le Romancer, C., Ould braham, R., Hamm, V., Maurel, C., Bonte, D., Sosio, G., and Stopin, A.: Integrating the multi-scale elastic velocities for interpreting and predicting the Dogger carbonate geothermal reservoir, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13896, https://doi.org/10.5194/egusphere-egu26-13896, 2026.

     The thermal conductivity and thermal diffusivity of minerals and rocks under high temperature and pressure can provide data supporting for historical simulation of geological processes, and they are also necessary parameters for establishing the thermal structure of deep layer under a stable state.

     Amphibolite is a crucial component of the continental crust and subduction zone. The physical properties of amphibolite are of great significance for understanding the physical structure of the continental crust and subduction zones. In this study, using a thermal properties testing platform on a hinged cubic press, the thermophysical properties of two types of amphibolite were studied at 0.5-1.5 GPa and 300-973 K by the transient plane heat source method. The experimental results show thermal conductivity and thermal diffusivity decrease with the increasing of temperature and increase with the increasing of pressure. The influence of temperature on these parameters is much greater than that of pressure. Under fixed pressure, the thermal conductivity can vary up to 15.1% with temperature changes, while the thermal diffusivity can vary up to 40.0%. Under fixed temperature, the influence of pressure on the thermal conductivity can reach up to 23.4% and the influence on the thermal diffusion coefficient can reach up to 15.7%. The experimental data were successfully fitted using empirical formulas. The thermal conductivity and thermal diffusivity of garnet amphibolite in this study is notably higher than those of amphibolite without garnet and those reported in previous studies, so it is believed that the presence of garnet significantly enhances the thermal conductivity of amphibolite.

     Combining high-temperature and high-pressure data from previous studies as well as this study and the parameters of the thermal lithosphere model, the thickness of the thermal lithosphere in different regions of the North China Craton under different surface heat flows was calculated. The results reveal that within a surface heat flow range of 50-80 mWm^-2, the thermal lithosphere thickness of the Bohai Basin varies between 49.8 and 145.7 km, the thickness of the central orogenic varies belt between 53.3 and 201.5 km, and the thickness of the Ordos Basin varies between 54.4 and 231.2 km.

How to cite: Yi, L. and Ma, W.: Thermal properties of garnet-bearing amphibolite at high temperature and pressure and its impact on the thermal structure of the lithosphere, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15505, https://doi.org/10.5194/egusphere-egu26-15505, 2026.

CO₂ geological storage is key to combat global warming. During the site screening process, impact of seismic activity on the storage site is required [1]. As the Japan islands located in the convergent zone of four tectonic plates and are known as one of the most earthquake-prone countries in the world, evaluating and predicting the impact of great earthquakes on reservoirs and cap rocks and disseminating this information to society is especially important issues in terms of gaining social acceptance at the project planning stage.

　The authors are developing an earthquake response analysis method for evaluating the stability of CO₂ geological storage sites in advance in the event of a great earthquake, but the input physical parameters of the cap and storage rocks under in-situ stress condition are not enough obtained. 

As the most of Japanese candidate storage sites are at young sedimentary formations, we prepare specimens (height 100mm, diameter 50mm) from Early Pleistocene sandstone and mudstone block sampled from outcrops. Triaxial compression tests were conducted under confining pressures corresponding to the depths of CO₂ storage sites. The loading rate was performed at strain rates of 180%/min, 100%/min, and 10%/min. For instance, when a mudstone specimen was loaded at an effective confining pressure of 14 MPa, a back pressure of 9 MPa, and a strain rate of 180%/min, the maximum strength (approximately 6 MPa) appeared near an axial strain of 1%, after which a gradual softening trend was observed. Distinct strain-softening characteristics were not observed in this experiment. The pore pressure reached its peak slightly earlier than the maximum strength. Although it decreased thereafter, a slight upward trend was observed despite the decrease in axial deviator stress.

   In this presentation, we will report the strength and pore pressure characteristics of rocks based on rock types and loading rates, and propose strength parameters effective for dynamic analysis.

[1] International Organization for Standardization,2026, ISO standard 27914; Carbon Dioxide Capture, Transportation and Geological Storage – Geological Storage.

How to cite: Horikawa, S., Takemura, T., and Kusunose, K.: High Pressure Triaxial Compression Test in Soft Sedimentary Rocks　—Relationship Between Loading Rate and Strength-Deformation properties—, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15718, https://doi.org/10.5194/egusphere-egu26-15718, 2026.

Characterizing rock pore structure is crucial for geothermal energy extraction, hydrocarbon assessment, and carbon capture and storage. The pore structure is conventionally estimated from core samples and well-log data. However, 2D images derived from drill cuttings offer an abundant and cost-effective alternative to complement lab and log analysis. Digital Rock Physics, combined with Artificial Intelligence, has the potential to provide robust tools to generate representative digital rock models directly from these 2D images. Specifically, Growing Neural Cellular Automata (GNCA) offer a distinct advantage; they follow simple learned local update rules and are efficient for emulating complex systems and natural phenomena, such as the regeneration of biological patterns and self-organizing textures. Moreover, they demonstrate training stability and low computational resource demands. Therefore, we propose training GNCA on 2D images to stochastically reconstruct the pore structure of selected volcanic rock samples to demonstrate the feasibility of the method. 

GNCA treat reconstruction as an evolving morphogenetic process, growing the pore structure iteratively from a seed state. Their lightweight architecture enables efficient training on consumer-grade hardware by utilizing gradient accumulation to handle input resolutions useful for pore-scale analysis (≥320² px). A key contribution of our work is the physically-informed hybrid loss function, L_total, designed to bridge the gap between perceptual texture and physical topology:

L_total = W_vgg L_vgg + λ (W_tpcf L_tpcf + W_vt L_vt + W_α L_α + W_por L_por),

where: L_vgg captures local perceptual texture, while the physical constraints include L_tpcf for spatial statistics via the two-point correlation function, L_vt to regulate specific surface area via Total Variation, L_α to constrain global pore aspect ratio using the Global Inertia Tensor, and L_por for porosity compliance. The weights W_i balance individual loss contributions, while λ modulates the trade-off between perceptual quality and physical fidelity. 

This model was trained using micro-CT slices from distinct volcanic samples from the Los Humeros Geothermal Field, Mexico. For validation, we compare the stochastic reconstructions against randomly selected reference slices. We also evaluate the standard two-point correlation function S₂(r) and the two-point cluster function C₂(r) to assess the pore spatial distribution and topological connectivity, respectively. In addition, the morphological fidelity is assessed via non-cumulative Pore Size Distribution and Aspect Ratio Distribution histograms, ensuring that the model captures the shape diversity of volcanic vesiculation. Furthermore, we implement a spectral analysis using the indicator function's Fourier transform, χ_V(k), which demonstrates that GNCA reproduce power spectral density across spatial frequencies, from macro-structures to fine details. Finally, the trained model successfully generates complete stochastic slices that are statistically equivalent to the original images at a 95% confidence level. This demonstrates that GNCA are efficient for reconstructing the studied volcanic samples.

In conclusion, the proposed GNCA framework, constrained by a physically-informed hybrid loss function, constitutes a viable alternative for the stochastic reconstruction of complex pore topologies in 2D images, yielding high-fidelity results on the analyzed volcanic rock samples.

How to cite: Lechuga Lagos, F. M. and Vega Ruiz, S.: Stochastic reconstruction of 2D volcanic rock pore structure using Growing Neural Cellular Automata, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15788, https://doi.org/10.5194/egusphere-egu26-15788, 2026.

Earthquakes are produced by slip events along faults driven by the accumulation and release of elastic energy in the Earth’s crust. They are cyclic, and a broad spectrum of slip behaviors is observed along seismogenic faults. The irregular and chaotic nature of earthquakes makes it difficult to establish a predictive law. Furthermore, the constitutive behavior of active fault zones remains a subject of debate, particularly regarding the relative roles of static and dynamic energy controls during seismic events. Elastic energy accumulation in the crust leads to rupture nucleation, while rupture propagation and arrest are likely governed by the physical properties of the fault zone, which vary between events and evolve during slip. While numerous parameters have been proposed to influence these processes, we emphasize that fault geometry and fault–fault interactions represent fundamental controls on rupture behavior and the evolution of the seismic cycle. To better understand how the size and distribution of asperities along faults control the earthquake cycle, we conducted laboratory experiments on analog material samples with root-mean-square (RMS) roughness values ranging from 0.5 to 30 micrometers. We used the Energy-Controlled Rotary Shear (ECoR) apparatus to replicate the earthquake cycle in the laboratory. The ECOR allows for spontaneous nucleation of laboratory earthquakes at velocities, accelerations, displacements, and magnitudes comparable to those observed in natural earthquakes. In these experiments, we used a loading spring with an effective elastic constant and varied the sample-averaged normal stress. Across experiments, we observe a range of slip behaviors, from stick–slip to steady creep, over the lifetime of the laboratory fault. We hypothesize that the size and distribution of asperities along the fault control the style of fault slip. Furthermore, over the course of the seismic cycle and in the presence of frictional weakening, we propose that the power density, another aspect that we will explore in the future, and the critical nucleation size control the magnitude of earthquakes.

How to cite: Lambert, I. and Tisato, N.: Insights into Fault Roughness Throughout the Seismic Cycle of Laboratory Earthquakes , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15944, https://doi.org/10.5194/egusphere-egu26-15944, 2026.

Across the central and western expanses of China, where soft rock formations dominate the geological landscape, arch bridges have long reigned supreme as the preferred structural choice. Their foundations, ingeniously engineered with a stepped-block configuration, boast exceptional overall rigidity—striking a masterful balance between structural integrity and economic efficiency by curbing infrastructure costs without compromising on deformation resistance. Drawing upon the foundational engineering of the Lantian Yangtze River Five Bridges, this study embarks on a profound investigation into the load-bearing behavior of stepped block foundations embedded within soft rock strata. It introduces a refined rigid-flexible judgment criterion and unveils an advanced, standard-anchored Optimization Specification C Method for stress-displacement analysis—a paradigm shift in computational precision. A novel analytical formula for the local shear failure angle is derived, shedding light on the underlying mechanics of shear collapse, while the verification framework for foundation deformation characteristics is meticulously enhanced. The culmination of this research is a comprehensive, high-fidelity bearing performance analysis system, exquisitely tailored for practical engineering application. Key findings reveal that both the rigid-flexible classification and the Optimization Specification C Method are eminently suited for assessing the bearing capacity of stepped arch foundations in soft rock environments, with design protocols firmly advocating for rigid foundation behavior under displacement-controlled criteria. The newly developed computational model transcends traditional limitations by delivering multidimensional output—capturing not merely singular values but the intricate spatial distribution of stress and displacement across the foundation zone. Remarkably, the Optimization Specification C Method achieves a 34% reduction in relative error compared to conventional standards, underscoring its superior accuracy, reliability, and real-world applicability. Critically, under conditions of global horizontal sliding of the arch structure, localized shear failure may initiate within the frontal rock mass adjacent to the stepped foundation. Furthermore, four distinct failure modes have been identified, each intrinsically linked to specific geometric configurations of the stepped block foundation—implying that optimal design must be guided by precise evaluation of the failure angle. By integrating bearing capacity assessments with stringent displacement control benchmarks, a holistic evaluation of foundation performance is achieved. While the current arch bridge foundation design successfully satisfies all load-bearing requirements, its deformation response reveals considerable untapped potential for refinement. Engineering case analyses further confirm that conventional single-dimensional performance checks, though inherently conservative and generally safe, fall short of capturing the full complexity of foundation behavior.

How to cite: Yang, X., Deng, W., and Yang, H.: A Masterful Symphony of Strength and Stability: An Exquisite Analysis System for the Load-Bearing Behavior of Stepped Block Arch Bridge Foundations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18283, https://doi.org/10.5194/egusphere-egu26-18283, 2026.

A swarm of m-scale dikes, made of diabase and porphyritic microgranite, related to a ca. 420 Ma bimodal tholeiitic magmatism crops out in the Rocroi Inlier, a Lower Paleozoic inlier exposed in the Ardenne Allochton (western Rhenohercynian Zone, European Variscan Belt). These dikes are affected by a more or less penetrative cleavage and their magmatic mineralogy has been replaced to various degrees by secondary minerals such as albite, chlorite, sericite, epidote and calcite, as a consequence of Pennsylvanian Variscan deformation and associated low-grade metamorphism. The thickest dikes (thickness from 3-4 to 10-15 m) display a deformation gradient from the border zones which has a penetrative cleavage to the core which is little or even apparently not deformed.

We have conducted measurements of the anisotropy of magnetic susceptibility on samples collected along transversal sections across five thick dikes from the Rocroi Inlier, two made of diabase and three of microgranite. The bulk magnetic susceptibility (K_m) has typical paramagnetic values, 5.8-14.0 x 10^-4 SI in the diabase and 0.4-6.4 x 10^-4 SI in the microgranite, except in the core of one diabase dike where magnetite occurs, ca. 580-890 x 10^-4 SI. The corrected anisotropy degree (P’) tends to have high values, up to ca. 1.8, in the deformed border zones and decreases towards the core. This parameter is therefore a proxy of the petrofabric strength. The shape parameter (T) reveals a predominantly oblate magnetic fabric, which suggests a prevailing coaxial deformation, in agreement with a previous finite strain study.

Magnetic foliation in the deformed border zones is parallel to dike margin, as well as to cleavage both in diabase or microgranite and in metapelites at the contact with the intrusion. It rotates a few degrees when moving away from the dike walls. Magnetic lineation is orientated down-dip on the foliation plane and is therefore also slightly deflected across the dikes. The mean magnetic foliation dip, hence the mean lineation plunge is ca. 30-60° to the S-SE. This orientation is roughly similar to that of the main (Variscan) cleavage in the metapelitic host rocks which bears a down-dip stretching lineation. However, magnetic fabric in the dikes and petrofabric in the country rocks outside the contact zone with the intrusions are slightly oblique (up to ca. 30°). Such a discrepancy possibly results from fabric refraction due to competence contrast with the host rocks. Incomplete transposition of the magmatic fabric by the Variscan deformation, as observed in particular in some microgranite samples, could also play a role here, by influencing the orientation of the magnetic fabric.

How to cite: Bolle, O., Bopda Tala, N., and Custine, E.: Heterogeneous Variscan deformation in late Silurian–early Devonian diabase and microgranite dikes of the Caledonian Rocroi Inlier (Ardenne Allochthon, France) quantified using anisotropy of magnetic susceptibility (AMS), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18866, https://doi.org/10.5194/egusphere-egu26-18866, 2026.

Digital Rock Physics (DRP) methods are increasingly used to link high‑resolution 3D imaging with the numerical determination of rock properties. In this work, we present new results from the SimBoL project, which applies this approach to drill cuttings from a borehole in Traunreut, southern Germany. The goal is to evaluate the potential of non‑core material for reliable petrophysical characterization relevant to geothermal applications.​

High‑resolution X‑ray computed tomography (CT) scans were segmented to obtain representative digital samples of carbonate cuttings, from which mineral composition, thermal conductivity and permeability were derived. A subset of these properties was computed using an updated numerical solver that incorporates periodic boundary conditions, enabling the treatment of irregular cutting geometries without relying on subvolumes or sample reshaping, and thereby allowing the use of larger rock volumes and a more realistic representation of heat and fluid transport processes than previous approaches restricted to cubic domains.​

The ongoing simulations yield quantitative estimates of rock properties that are compared with available borehole data and complemented by observations from thin sections. The analysis illustrates how digital twins of rock cuttings can deliver additional information on the internal architecture of reservoir rocks, reducing dependence on costly core material and strengthening the conceptual basis for geothermal reservoir characterization.

How to cite: Serje Gutierrez, A., Siegert, M., Gurris, M., and Saenger, E. H.: From Rock Cuttings to Physical Properties: Integrating Digital Rock Physics and Borehole Data from Traunreut, Southern Germany., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20706, https://doi.org/10.5194/egusphere-egu26-20706, 2026.

Thin-bedded Miocene formations of the Carpathian Foredeep represent a major challenge for reliable petrophysical characterization due to strong lithological heterogeneity, high clay minerals content, and the vertical resolution of standard well log interpretations. Accurate assessment of porosity and permeability in such reservoirs is essential for hydrocarbon exploitation, geothermal applications, and the evaluation of Carbon Capture and Storage (CCS) potential.

This study presents an integrated pore-structure modeling workflow applied within a multi-well correlation framework, allowing the transfer and validation of petrophysical models across laterally variable, thin-layered deposits. The methodology combines multiscale laboratory measurements (NMR, nitrogen adsorption, MICP, X-ray CT, and FIB-SEM) with machine-learning–assisted interpretation of well log data to generate high-resolution continuous profiles of porosity and permeability. Models calibrated on core-scale laboratory data are propagated between correlated wells, enabling consistent characterization of reservoir properties beyond a single well.

To increase the geological credibility of the multi-well interpretation, seismic data are incorporated as an independent constraint. Seismic attributes support stratigraphic correlation, identification of thin-bed architecture, and the lateral continuity of petrophysical units. This integration facilitates the upscaling of pore-scale information from laboratory and well log data into a seismic framework, reducing uncertainty related to heterogeneity and thin layering.

The results indicate that the combined use of artificial neural networks, advanced statistical methods, and seismic support significantly improves both vertical and lateral resolution of petrophysical properties in thin-bedded reservoirs. The proposed workflow enables reliable application of pore-network–based models within a multi-well context and provides a scalable approach for reservoir characterization in complex clastic systems. The methodology is particularly relevant for unconventional reservoirs and mature fields considered for CCS or geothermal repurposing, where accurate representation of thin-layered architectures is critical for realistic resource assessment.

How to cite: Waszkiewicz, S., Krakowska-Madejska, P., Kwietnak, A., and Starzec, K.: Advanced Petrophysical Characterization of Thin-Bedded Reservoirs Through Integrated Laboratory, Well Logging, and Seismic Data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21976, https://doi.org/10.5194/egusphere-egu26-21976, 2026.

Understanding how water influences slow fracture growth in rocks remains a major gap in our ability to predict time-dependent failure. In particular, it is still unclear how moisture-related weakening push a subcritically stressed rock from stable deformation into sudden collapse.

In this study, we investigate how hygroscopic weakening—caused by liquid water entering a notch—affects the creep behavior of a clastic rock loaded below its short-term strength. Using a sandstone beam (400 mm×90 mm×90 mm) in an inverted three-point bending setup, we first load the sample to about 67% of its failure strength more than 5 days, then introduce a controlled water drip directly into the notch.

We track the fracture response using digital image correlation, ultrasonic transmission, acoustic emission, and crack-opening measurements. The results show two distinct stages after water arrives:

• a rapid increase in crack opening and loss of stiffness, consistent with moisture-driven softening; and
• a slower but sustained rise in microcracking activity, leading to accelerated creep and, in some cases, catastrophic failure.

In contrast, identical dry beams remain stable over several days, confirming that water—not load alone—initiates the transition to instability.

These findings demonstrate that even small amounts of liquid water can sharply alter the long-term mechanical stability of brittle stressed rocks. This work highlights a potential pathway through which more frequent or intense wetting events could increase the likelihood of sudden rock failure in natural and engineered settings.

How to cite: Wu, R., Kang, H., Gao, F., Peng, X., Dong, S., Zhao, C., Li, B., Leith, K., Lei, Q., and Selvadurai, P.: Hygroscopic weakening accelerates the transition to catastrophic failure during brittle creep in clastic rocks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-317, https://doi.org/10.5194/egusphere-egu26-317, 2026.

How to cite: Dore-Ossipyan, C., Quacquarelli, A., Sulem, J., Bornert, M., Dimanov, A., and King, A.: Multi-scale control of initial porosity distribution on deformation processes in a heterogeneous porous carbonate rock, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-338, https://doi.org/10.5194/egusphere-egu26-338, 2026.

Granular materials compact, increase in density, and degas as they accumulate. This changes the material properties, storage capacities, and fracture mechanics.  We developed a mechanical model for compacting granular old snow.  Based on minimal assumptions and data, we address a general phenomenon in compacting granular medium: propagating ruptures or “firnquakes”.  

Compacting snow becomes firn then ice. As the snowpack consolidates, it transitions from a non-homogeneous granular material to a more elastic continuum material. We propose that the granular legacy produces spatial variations in density, stiffness, and pre-stress. This creates an internal structure of supports in unconsolidated snow at depth. Firn can quake when these supports collapse. By combining granular with brittle fracture mechanics and making use of statistical percolation theory, we can explain the conditioning, triggering, and progression of firnquakes in a bulk homogeneous material, with near constant boundary conditions.

Our model provides means to assess ruptures in granular materials, which unlike firnquakes, can have hazardous consequences, like landslides, avalanches, powder tailing failure. It also provides mechanistic explanations and statistical approaches to assess storage structure and capacity, which, in the case of Antarctic’s firn, has been linked to icesheet disintegration.

How to cite: Voigtländer, A. and Gee, B.: Why firn (old snow) quakes - a continuum mechanics theory with granular legacy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1649, https://doi.org/10.5194/egusphere-egu26-1649, 2026.

How to cite: Chandler, M., Li, X., Cartwright-Taylor, A., Butler, I., Freitas, D., Woldemichael, B., Liptak, A., Atwood, R., Main, I., Mangriotis, M.-D., Curtis, A., Fusseis, F., and Chapman, M.: Grain-scale 4D visualisation of strain partitioning during brittle creep in sandstone, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3514, https://doi.org/10.5194/egusphere-egu26-3514, 2026.

As a giant hydropower hub on the Jinsha River in China, the underground powerhouse and diversion tunnel of the Wudongde Hydropower Station (WHS) pass through phyllite strata. During tunnel excavation, the unloading stress path has a significant impact on the changes in the mechanical properties of the surrounding rock, while the deformation and failure of the rock mass accumulate continuously, making it urgent to conduct an in-depth study on the time-dependent characteristics of mechanical parameters. In view of this, phyllite samples from the engineering site were selected to carry out a comparative study on the instantaneous and long-term mechanical properties under loading-unloading conditions. To simulate the instantaneous effect of unloading during tunnel excavation, triaxial loading-unloading tests on phyllite were performed using the MTS815.03 test system, with synchronous acoustic emission (AE) monitoring. The test results revealed the evolution law of rock strength parameters under different unloading paths and the differences in the micromechanical mechanisms of unloading-induced failure. To simulate the evolution of mechanical behavior during the long-term operation of the tunnel, triaxial creep tests under loading-unloading stress paths were conducted using a TLW-2000 rock triaxial creep testing system. A creep constitutive model was established to describe the evolution relationship between the deformation and failure of the surrounding rock and time, and the degradation characteristics of its long-term strength parameters were clarified. The study indicates that for phyllite tunnels under long-term operation, the long-term degradation effect of mechanical parameters should be taken into account, and it is more reasonable to adopt the obtained long-term triaxial rheological strength parameters to assess the long-term operational stability of the tunnel. The research results provide a technical basis for the stability analysis and excavation support design of the tunnel.

How to cite: Zhu, J.: Study on Instantaneous and long-term Mechanical Properties of Phyllite under Loading and Unloading Conditions at Wudongde Hydropower Station, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4582, https://doi.org/10.5194/egusphere-egu26-4582, 2026.

Volcanic rock can be subjected to high and fluctuating pressures and stresses associated with volcanic activity and geothermal production. When subject to a high constant stress in the brittle regime (shallow depths), strain and porosity increase as a function of time, eventually leading to macroscopic failure—a process called brittle creep. In the ductile regime (deep depths), rare experiments have shown that strain accumulates and porosity decreases at a constant stress. During this process—called compaction creep—the rates of strain accumulation and porosity reduction decrease as a function of time. Here, we performed triaxial constant strain rate experiments and triaxial compaction creep experiments on three samples of tuff sampled from boreholes drilled into Krafla volcano (Iceland). The tuffs differ in terms of their source depth (~395, ~505, and ~690 m), macroscopic texture (grain size and distribution), and mineral content (different quantities of clay minerals/chlorite). The connected porosities of the tuffs, however, are very similar (0.29–0.35). We first performed X-ray computed tomography on each tuff in order to provide a quantitative description of their microstructure (grain size and distribution, and pore size, distribution, shape, and orientation). Triaxial constant strain rate experiments were then performed at different effective pressures to map out the yield cap for each tuff. Finally, triaxial compaction creep experiments were performed at effective pressures corresponding to the same position on the yield cap for each tuff. The constant differential stress used in these experiments was selected as the same proportion between the onset of inelastic compaction and the inflection point in the stress-strain curve from the constant strain rate experiment performed at the same effective pressure. All three tuffs accumulated strain and lost porosity as a function of time under a constant stress, although the rates of strain accumulation and porosity reduction, and therefore the maximum strain and porosity loss achieved at the end of the experiment, were different. For example, the porosity loss at the end of the experiments (after 100 hours) for the three tuffs was 0.014, 0.015, and 0.023. Because the connected porosity of the three tuffs is the same, differences in their compaction creep behaviour can be explained by differences in their microstructure and mineral content. The time-dependent compaction of porous volcanic rocks, demonstrated here for tuffs, has implications for volcano stability and geothermal production.

How to cite: Heap, M., Bayramov, K., Baud, P., and Mortensen, A.: Time-dependent compaction creep in tuffs from Krafla volcano (Iceland), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5089, https://doi.org/10.5194/egusphere-egu26-5089, 2026.

During uniaxial compression testing, the sample fails macroscopically in the form of longitudinal splitting. This indicates that, although only axial load was applied, internal tensile forces caused the sample to fail. In the classic Mohr-Coulomb failure criterion, these tensile forces are not taken into account and the minimum principal stress is considered to be zero. In our modified Mohr-Coulomb failure criterion, we assume that during a uniaxial compression test, tensile stresses are generated in the rock, causing the specimen to fail. Under this assumption, it is possible to extend the stress state during a compression test into the tensile range. The hypothesis is that during a uniaxial compression test, failure is also determined by the tensile strength perpendicular to the load axis. Based on Mohr-Coulomb theory, it is now possible to determine the cohesion and internal friction coefficient from this stress state, knowing only the compressive and tensile strength of the rock.

This method has been tested for various rock types with known values for cohesion, internal friction coefficients, and tensile and compressive strength. Our method provides a good estimate of the intrinsic rock properties.

We present the theoretical basis for our modified Mohr-Coulomb failure criterion and its applicability to various rock types.

How to cite: Blöcher, G. and Cacace, M.: Estimation of cohesion and internal friction coefficient using a modified Mohr-Coulomb failure criterion., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5143, https://doi.org/10.5194/egusphere-egu26-5143, 2026.

In the upper crust, rock pore spaces may be occupied by fluids of diverse chemical compositions. Pore spaces can be naturally filled with water, carbon dioxide, oil or gas, or artificially saturated with reactive fluid for geo-engineering purposes, including geothermal energy, wastewater disposal, carbon dioxide or hydrogen storage. The presence of water and other fluids modifies the mechanical strength of porous rocks in both the brittle (i.e., localised) and ductile (i.e., distributed) regimes. According to micromechanical models, the strength of porous rock in the brittle regime is controlled by both frictional parameters and fracture toughness of the material, while inelastic compaction by cataclastic pore collapse is governed exclusively by fracture toughness. Experimental studies indicate that the presence of fluid affects the fracture toughness and static friction of limestones and sandstones. Accordingly, for a given rock type, fluid-induced weakening of the rock strength should be explained entirely by a decrease in fracture toughness and/or frictional parameters.

This interpretation is supported by measurements of the mode-I fracture toughness (K_Ic) and static friction (µ_s) of sandstones and limestones, under both dry and water-saturated conditions, which allow for the estimation of the uniaxial compressive strength and quantification of the degree of water-weakening. In this context, we investigate the influence of fluids and fluid composition on the mode-I fracture toughness and frictional strength of Adamswiller sandstone. This sandstone was selected because its mechanical behaviour is well-documented in the literature, and because both fluid presence and fluid composition have been shown to affect its response under uniaxial and triaxial compression. We tested a range of fluid-saturated conditions, including dry, deionised water, 6 mol NaCl solution, 0.1 mol HCl solution and 0.1 mol NaOH solution. For K_Ic, most of the weakening occurs between dry and fluid-saturated conditions, with additional reductions observed for acidic and basic solutions, with the greatest under basic conditions. For a saline solution, the extent of weakening relative to water-saturated conditions is unclear. In contrast, the measured static and peak friction coefficients are unaffected by either the fluid presence or the fluid composition. Incorporating the measured toughness and frictional strength into micromechanical models (wing crack model and pore collapse model) successfully reproduces fluid-weakening under uniaxial and triaxial conditions. The models capture the effective pressure dependence of fluid-weakening in both the brittle and ductile regimes, reproducing the observed strength variation associated with different fluid compositions. This experimental dataset provides new insight that constrains the micromechanical mechanisms governing porous rock deformation in natural and anthropogenic fluid-saturated environments, with direct implications for the safe exploitation of geo-reservoirs.

How to cite: Noël, C., Baud, P., Lazari, F., Shahin, G., and Violay, M.: The effect of fluid chemistry on sandstone’s fracture toughness and frictional strength: Implications for brittle and ductile strength, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5261, https://doi.org/10.5194/egusphere-egu26-5261, 2026.

Temperature fluctuations can influence the internal stress field of a rock mass, especially in its surficial layer. Inherent properties (mineral composition, porosity, and fracturing) and external forcing (air temperature, humidity, and solar radiation) control the heat flux and temperature within the rock. Long-term thermal forcing, particularly when combined with wetting-drying cycles, can exacerbate rock deterioration and weathering, leading to progressive changes in mechanical properties, as shown by laboratory experiments.

In this study, granodiorite samples from the Požáry field laboratory (Central Czechia) were subjected to thermal cycling in a controlled environment of a climate chamber, with repeated and increasing cycles reaching 80°C, a temperature that was most probably never reached after the rock´s formation.  During the cycling, repeated UPV measurements were made (P and S waves) to observe the changes in their velocity and the inferred dynamic elastic moduli.

The results showed slow but progressive decrease in the P and S wave velocities, suggesting rock damage after only a few cycles. This indicates possible increased rock wear in case of an expected future temperature rise.

How to cite: Blahůt, J., Lokajíček, T., Polezhaev, A., Racek, O., and Loche, M.: Changes in rock dynamic elastic moduli after thermal cycling in a controlled environment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5766, https://doi.org/10.5194/egusphere-egu26-5766, 2026.

 Large-magnitude earthquakes in the continental crust predominantly occur near the brittle–ductile transition zone, where the deformation behavior of rocks plays an important role in earthquake nucleation and energy release.Rocks deforming under high-temperature and high-pressure conditions within the brittle–ductile transition zone may exhibit mechanical responses controlled not only by temperature and stress level, but also by pre-existing microstructural features; in particular, perthitic feldspar, a widespread feldspar solid solution in the crust, commonly contains exsolution-related lamellar structures that may introduce orientation-dependent deformation behavior.Despite its common occurrence in mid-crustal rocks, the influence of pre-existing lamellar fabric orientation on the deformation behavior of perthitic feldspar, especially under brittle–ductile transition conditions, remains poorly constrained by experiments.Based on this background, we conducted high-temperature and high-pressure deformation experiments using a Griggs-type solid-medium apparatus to systematically investigate the deformation behavior of perthitic feldspar with different pre-existing lamellar fabric orientations.Samples were prepared with lamellar orientations at angles of 0°, 45°, and 90° relative to the maximum principal stress, and deformed at a confining pressure of 1 GPa, over a temperature range of 600–1050 °C, at strain rates ranging from 5 × 10⁻⁵ to 2 × 10⁻⁶ s⁻¹. Microstructures of the samples before and after deformation were characterized using scanning electron microscopy and electron backscatter diffraction, and the mechanical responses and microstructural features were compared among samples with different fabric orientations.The mechanical results show significant differences in peak strength among the three lamellar fabric orientations, with sample strength decreasing in the order of 45°, 0°, and 90° at the same temperature.All samples entered a plastic deformation regime above 800 °C (σ_d<Pc).Microstructural observations reveal that at low temperatures (<900 °C), pervasive brittle cracks crosscut both feldspar phases and are accompanied by localized ductile shear zones; at intermediate temperatures (900–950 °C), cracks are mainly confined within albite grains and are commonly oriented perpendicular to grain boundaries; at high temperatures (>950 °C), samples exhibit bulk plastic flow with a marked reduction in cracking.Notably, samples with a 45° lamellar orientation experienced pronounced bulk fragmentation at 1000 °C and 1050 °C.EBSD results show that K-feldspar does not develop significant changes in crystallographic preferred orientation during deformation, whereas albite exhibits progressively heterogeneous orientation patterns with increasing temperature, consistent with plastic deformation associated with subgrain rotation recrystallization.Together, the mechanical and microstructural results demonstrate that pre-existing lamellar fabric orientation exerts a significant influence on the deformation behavior of perthitic feldspar under brittle–ductile transition conditions, providing experimental constraints on strength anisotropy in feldspar-rich rocks.

How to cite: yaqi, C., jiaxiang, D., and yongsheng, Z.: Deformation behavior of perthitic feldspar under brittle–ductile transition conditions: effects of pre-existing lamellar fabric, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6075, https://doi.org/10.5194/egusphere-egu26-6075, 2026.

Seismic monitoring is an effective tool for studying rock mass stability, playing a crucial role in detecting the precursory assessment of damage and cracking processes preceding and accompanying macroscopic failures.

We first present the seismic monitoring results from the Lorgino Quarry in Crevoladossola (NW Italy) where a 6000 m³ rock-fall occurred on January 26, 2023 shortly after deploying a small-aperture array (about 100 meters) of three seismic stations, equipped with a tri-axial, velocimetric sensor and data-loggers sampling at 250 Hz. The rock fall took place about a month after the site-specific seismic array installation at the lithological contact between folded gneisses and a dolomitic limestones unit, mainly composed of dolomites and dolomitic saccharoid marbles. The rockfall seismic signature lasted 15s and the spectral analysis shows the occurrence of multiple sub-episodes of slip triggered by the initial rupture. 

As no obvious correlations between precursory activity and the rockfall occurrence were observed via traditional seismological approaches, we applied an unsupervised deep-learning method that combines a deep scattering network, for automatic feature extraction, with Gaussian mixture model clustering. This approach successfully identified low-amplitude signals occurring nearly one hour before the rockfall, nearly undetectable in raw seismic records and likely associated with a nucleation phase occurring well before the acceleration to failure.

In order to investigate the physical mechanisms driving the nucleation phase, we carried out rock deformation laboratory experiments, where marble cylindrical samples (100x40mm) from the quarry were triaxially loaded in compression to failure at constant effective pressure (20MPa) while an array of 16 Piezoelectric Transducers recorded the ongoing Acoustic Emissions (AE). The time and spatial distribution of AE reveal the nucleation and growth of patches led by limited occurrence of low energy AE events and the coalescence of microfractures into cm-scale macroscopic ruptures planes leading to AE clustering and stress drop and a peak in number of events and energy. Preliminary source mechanism analysis, carried out by developing an automated focal mechanism inversion workflow for AE based on P-wave first-motion, integrating polarity and amplitude measurements, suggests that the inverted focal mechanisms are stable and broadly consistent with the imposed stress conditions, highlighting the potential of the workflow to improve source mechanism quality by identifying and excluding unreliable solutions.

How to cite: Vinciguerra, S., Adinolfi, G. M., Zhou, W., and Comina, C.: Unravelling Precursory Rockfall seismic signatures via multiscale clustering analysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7457, https://doi.org/10.5194/egusphere-egu26-7457, 2026.

With the extension of mining and tunneling engineering into deep complex water-bearing strata, the interaction between groundwater and rock mass has become a critical factor governing mechanical excavation efficiency. The presence of water fundamentally alters the rock fragmentation characteristics, and understanding this hydro-mechanical coupling is prerequisite for optimizing conical pick performance. In this study, a comprehensive experimental framework combining macroscopic indentation tests and microscopic characterization was established to evaluate the breakability of twenty distinct lithologies under dry and saturated conditions. The variation of Peak Indentation Force (PIF) and cutting work was monitored, alongside micro-analysis using SEM and XRD to reveal the intrinsic controls of mineral composition and pore structure. The results demonstrate a lithology-dependent bifurcation: porous sedimentary rocks exhibit significant degradation in strength due to pore pressure wedging and chemical softening, whereas dense magmatic rocks remain largely insensitive to saturation. Furthermore, to bridge the gap between experimental data and field application, an Extreme Gradient Boosting (XGBoost) model was used. Feature importance analysis reveals that under water-saturated conditions, the Brittleness Index surpasses hardness as the dominant predictor for rock breakability. This study quantifies the water-weakening mechanism and provides a data-driven approach for predicting cutter performance and improving excavation efficiency in water-bearing environments.

How to cite: Shi, X. and Wang, S.: Experimental investigation and machine learning prediction of water-weakening effects on rock breakability by conical pick, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8640, https://doi.org/10.5194/egusphere-egu26-8640, 2026.

Frost heave and thaw settlement are among the most widespread and destructive geotechnical hazards in cold regions, posing serious threats to the safety and long-term performance of infrastructure. The initiation and evolution of these hazards are highly dependent on the mechanical properties of frozen soils, such as compressive strength, cohesion, internal friction angle, and deformation modulus. These properties are jointly controlled by temperature, ice content, water content, and freeze–thaw cycling, resulting in strong nonlinearity, temporal variability, and spatial heterogeneity. As a result, conventional laboratory testing and empirical approaches often suffer from high cost, low efficiency, and limited applicability in parameter determination and prediction.　In recent years, machine learning techniques have been increasingly applied to predict soil mechanical parameters due to their ability to handle multi-source data and capture complex nonlinear relationships. However, the strong temperature sensitivity of frozen soil behavior makes it difficult to achieve high prediction accuracy by solely establishing mappings between temperature–moisture–structural characteristics and mechanical responses. This challenge highlights the necessity of data-driven modeling frameworks that explicitly consider stress states and thermomechanical coupling effects.　In this study, a machine learning–based framework was developed to predict the strength characteristics of frozen clay. A total of 116 sets of directional shear test data were used to train and validate four machine learning algorithms. The intermediate principal stress coefficient, principal stress axis orientation angle, mean principal stress, and temperature were selected as input variables, while frozen clay strength was taken as the output. Model performance was systematically evaluated using cross-validation and further verified through comparison with supplementary experimental data.　Based on the optimal model, the distribution of frozen clay strength within a multi-dimensional input parameter space was analyzed. In addition, model interpretability techniques were employed to conduct sensitivity analysis, enabling quantitative evaluation of the relative importance of different input parameters. The results demonstrate that machine learning approaches can accurately reproduce the stress–strain behavior and failure strength of frozen clay, while effectively capturing the complex nonlinear relationships between strength and controlling factors.　Overall, this study shows that machine learning provides a robust and efficient alternative for predicting frozen soil mechanical parameters. The proposed framework enhances prediction.

How to cite: Wang, D.: Prediction of Frozen Clay Strength Under Different Temperature Conditions Using Machine Learning Approaches, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11103, https://doi.org/10.5194/egusphere-egu26-11103, 2026.

Excavating large-scale tunnels in tectonically active regions, such as the Himalayan seismic zone, challenges the stability of underground structures due to high in-situ and induced stresses. The Diamer-Basha Dam (DBD) project involves complex tunneling through heterogeneous bands of granite and diorites, necessitating an engineered support system to mitigate progressive rock mass failure. In Pakistan, the tunnel support often relies on empirical classifications like the Rock Mass Rating (RMR) and Q-system. These systems provide a useful initial estimate; however, their direct application without site-specific calibration frequently results in conservative or over-designed support systems. This study investigates an optimized support framework by integrating empirical characterization with numerical Finite Element Method (FEM) analysis. Using geological data acquired from the site, including face maps and borehole logs, we classified rock mass and simulated its response to excavation using RS2 software. The research specifically evaluates the mechanical efficacy of Fiber Reinforced Shotcrete against optimized combinations of plain shotcrete and active rock bolts. Numerical simulations indicate that the in-situ rock mass possesses sufficient self-supporting capacity in specific zones to allow for a reduction in shotcrete thickness when supplemented with bolting. The models demonstrate that optimized designs maintain the required structural stability while reducing material consumption. These findings suggest that a hybrid empirical-numerical framework offers a cost-effective engineering solution for large excavations. By validating support performance through numerical modelling, this study provides a repeatable framework for optimizing tunnel support in complex geological environments.

How to cite: Aman, U., Ali, Z., Hussain, S. N., Nazeer, S., and Ayub, M.: Optimization of Tunnel Support Systems in High-Stress Geological Zones: A Case Study of Diamer-Basha Dam, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11234, https://doi.org/10.5194/egusphere-egu26-11234, 2026.

Yosemite National Park (California, USA) is characterized by high granitic rock walls affected by diffuse rock slope instabilities. These pose high rockfall hazards to roads and threaten the lives of millions of people that every year access the park to visit the natural beauties of Yosemite Valley, walk trails and climb iconic rock walls like El Capitan. Here, rockfalls are chiefly triggered by the progressive failure of portions of exfoliation sheets (“flakes”) bound to the cliff by rock bridges. In this context, identifying potentially unstable flakes is crucial for risk mitigation, yet the field characterization of such flakes remains difficult, highlighting the need for remote sensing mapping methods.

At the southeastern face of El Capitan, in situ time-lapse infrared thermographic (IRT) surveys, conducted in October 2024, revealed that exfoliation sheets cool faster than the surrounding rock mass heated by the same daily solar forcing. To lay foundations for a remote detection methodology, we carried out a combined laboratory and numerical study of the IRT signature of daily heating and cooling of exfoliation sheets and the underlying physical processes.

We conducted 37 laboratory experiments in a controlled setup, where the cooling of 20 cm by 20 cm granite slabs with variable thickness (1-6 cm) and opening of a simulated exfoliation joint (2-54 mm), oven-heated at 85°C, is monitored by contact thermocouples and a high resolution thermal camera. For each tested combination of slab thickness and joint aperture, we recorded detailed temperature time series and modelled cooling curves using the lumped capacitance solution of Newton’s law of cooling.

Experimental results show that, until a threshold value of the thickness/aperture ratio is reached, IRT can detect a dependence between the cooling rate of the external slab face and the aperture of the simulated exfoliation joint, with two contrasting trends. For very small aperture, cooling speed decreases with aperture. Beyond a certain aperture value, varying with slab thickness, the slab face cools faster as joint aperture increases.

To investigate the physical processes underlying this behaviour, we reproduced our experiments by 2D and 3D finite-element numerical simulations with the software Temp/W-Geostudio^TM, considering different conditions (i.e. initial temperature of the cliff rock behind the flake, conduction, and air convection parameters). Model results suggest that convective heat transport in the open simulated joint strongly controls the thermal energy dissipation within the cooling flake. For very small joint apertures or limited convective circulation, the insulating effect of air results in slower flake cooling. However, for increasing joint aperture and thus greater air convection, the results indicate more effective heat dissipation and associated faster cooling. Our study provides a quantitative framework towards the development of remote mapping of unstable rock features upon proper methodology upscaling to in situ conditions.

How to cite: Giorgi Spreafico del Corno, F., Agliardi, F., Castellanza, R., Stock, G. M., Bruschetta, R., and Collins, B. D.: Investigating the thermal behavior of exfoliation sheets in granitic cliffs (Yosemite, USA) through laboratory experiments and numerical modeling , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11286, https://doi.org/10.5194/egusphere-egu26-11286, 2026.

Fault zones in clay-rich formations play a critical role in controlling deformation, fluid flow, and stability in both natural and engineered subsurface systems, including earthquake rupture, underground storage, and radioactive waste disposal. However, the coupled hydro-mechanical behavior of natural fault rocks remains poorly constrained due to the difficulty of obtaining representative samples and performing fully coupled laboratory experiments. Here, we present results from petrophysical and fully hydro-mechanically coupled triaxial compression tests on preserved natural fault material from scaly clay sections of the Main Fault intersecting the Opalinus Clay formation at the Mont Terri Underground Research Laboratory, Switzerland.

The hydraulic properties of scaly clay Opalinus Clay were measured using flow-through experiments on back-saturated specimens. Permeability coefficients determined sub-parallel to the orientation of bedding and tectonic shears are up to three orders of magnitude larger than those of the intact rock. The shear experiments were conducted under undrained conditions at different effective confining stresses, allowing direct observation of stress–strain behavior, pore pressure evolution, and effective stress paths up to large axial strains. In contrast to intact Opalinus Clay, the faulted scaly clay exhibits continuous strain hardening without a distinct peak stress or post-peak weakening. Deformation is distributed and accommodated by the reactivation of multiple pre-existing tectonic micro-shear. The shear strength analysis within a Mohr–Coulomb framework reveals that the scaly clay fabric has effectively zero cohesion and a shear strength that is lower than even the residual strength of intact Opalinus Clay. Microstructural observations confirm that deformation proceeds through distributed sliding along an anastomosing network of polished micro-shears surrounding undeformed microlithons.

These results demonstrate that inherited fault-zone fabric exerts a first-order control on both mechanical strength and hydro-mechanical coupling in clay-rich faults. Incorporating fabric-and stress-dependent behavior as well as critical-state deformation into constitutive models is therefore essential for realistic predictions of fault reactivation, pore pressure evolution, and long-term stability of low-permeability clay formations.

How to cite: Winhausen, L., Ziegler, M., and Amann, F.: The hydro-mechanical coupling, reduction of effective strength, and critical state shearing of faults: Evidence from laboratory testing on natural fault rock, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13046, https://doi.org/10.5194/egusphere-egu26-13046, 2026.

The behaviour of slowly moving rock slope instabilities in alpine regions is governed by the interaction between inherited structural discontinuities and externally imposed environmental forcing, yet the mechanisms linking these controls across scales remain poorly constrained due to the lack of continuous subsurface observations. Rock slope toppling represents a typical form of such structure-controlled deformation. Here, we present a comprehensive investigation of a structurally complex toppling rock slope in pre-Variscan metamorphic units in the Bedretto Valley (Swiss Alps). This landslide is intersected by the unlined Bedretto Tunnel over a length of approximately 450 m. The presence of the tunnel and the associated facilities of the Bedretto Underground Laboratory provides a unique opportunity to resolve spatially variable deformation.

To understand the controls on the motion of this landslide, we installed a multi-parameter monitoring network, which integrates surface and subsurface measurements. It combines meteorological stations covering the toppling crown and toe, sectional groundwater pressure monitoring, and a 2-km-long distributed fiber optic sensing (DFOS) cable anchored along the Bedretto Tunnel. This configuration provides a unique internal view of deformation within the rockmass, enabling continuous, decimeter-scale observations of microstrain and temperature beneath up to 1500 m of overburden.

The measurements reveal two distinct deformation responses of the landslide. First, reversible, centimeter-scale strain oscillations correlate with surface temperature fluctuations but exhibit anomalously high amplitudes and penetration depths, which cannot be explained solely by conductive heat transfer. This points to a non-local thermoelastic response, whereby far-field thermal stresses are generated and anisotropically transmitted through the fracture network within the rockmass. Superimposed on this cyclic thermoelastic background, the data reveal discrete, irreversible strain steps near critical fracture zones. These steps temporally coincide with major seasonal hydrologic events including sustained snowmelt and intense rainfall when a two-layer bucket model predicts corresponding peaks in groundwater storage and pressure transients. This correlation provides direct evidence for hydro-mechanically driven, progressive damage within the fracture network of the slope.

Multivariate decomposition of the deformation time series isolates not only the dominant thermo-hydraulically driven cyclic signal, but also residual components characterized by spatially variable, and locally opposing, monotonic strain trends. These opposing trends are partially explained by the mechanical and geometrical heterogeneity of the fracture network and reveal the accumulation of progressive inelastic deformation. Beyond direct thermal or hydraulic forcing, such components suggest a creep-like weakening mechanism of the rockmass under quasi-static gravitational stress.

These findings reveal the dynamics of a coupled thermo-hydro-mechanical system, in which seasonal forcing drives both reversible deformation and irreversible damage. The study thus highlights the critical role of discontinuities in controlling slope behavior, showing how transient hydrology and thermal cycling progressively degrade rockmass strength along pre-existing fractures and joints, ultimately weakening large rock slope failures.

How to cite: Yuan, M., Hirschberg, J., Oestreicher, N., de Palézieux, L., and Aaron, J.: Resolving Thermo-Hydro-Mechanical Coupling and Progressive Damage in a Metastable Toppling Rock Slope using Integrated Fiber-Optic Monitoring, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13221, https://doi.org/10.5194/egusphere-egu26-13221, 2026.

Understanding damage accumulation under cyclic loading is critical for assessing the stability of deep excavations and heterogeneous crustal rocks subjected to repeated stress perturbations. Subvolcanic and volcanic lithologies typical of porphyry copper systems, and analogous to shallow volcanic crust, exhibit strong mineralogical and structural heterogeneity due to intrusive processes, veining, and hydrothermal alteration, challenging models derived from homogeneous rocks. We present results from uniaxial and triaxial cyclic loading experiments on five lithologies from the El Teniente porphyry copper deposit (tonalite, diorite, porphyritic dacite, veined andesite, and hydrothermal breccia), conducted under confining pressures up to 25 MPa and coupled with continuous acoustic emission (AE) monitoring. Cycles of increasing stress amplitude were used to quantify stiffness degradation and acoustic memory through the Felicity Ratio (FR). Elastic moduli were derived from unloading branches, allowing direct comparison with elastic reversibility frameworks proposed for crystalline rocks. Homogeneous to moderately heterogeneous lithologies exhibit gradual stiffness loss and limited departure from elastic reversibility, whereas strongly heterogeneous rocks display pronounced stiffness fluctuations, early deviation from elastic behaviour, and broad FR dispersion, indicating intermittent strain localization and partial loss of elastic memory. Increasing confinement reduces mechanical and acoustic scatter, highlighting the stabilizing role of lateral stress. Ongoing work integrates photogrammetry-based quantification of grain-size distributions, vein density, vein thickness variability, and alteration intensity. These micro- and mesoscale descriptors are used to explore correlations with mechanical degradation rates and acoustic reactivation patterns observed during cyclic loading. This combined mechanical–microstructural approach aims to clarify how lithological heterogeneity governs the style, rate, and intermittency of cyclic damage in subvolcanic crust, with implications for deep mining stability and stress cycling in volcanic systems.

How to cite: Clunes, M., Valdés, F., Roquer, T., Cortez, J., Garrido, M., Browning, J., González, R., and Orellana, L. F.: Multiscale controls on cyclic damage and elastic memory in heterogeneous rocks from a porphyry copper system, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15252, https://doi.org/10.5194/egusphere-egu26-15252, 2026.

Hydrothermal alteration exerts a first-order control on the mechanical behaviour of rocks in deep mining environments by modifying mineralogical composition, grain bonding, and internal structures. In porphyry copper systems, quartz–sericite, potassic and chloritic alteration produces strong contrasts between mechanically competent and weak mineral phases, influencing damage accumulation and failure under cyclic stress conditions. However, experimental constraints on how alteration intensity governs mechanical degradation and acoustic response during cyclic loading remain limited. We investigate the mechanical and acoustic behaviour of rocks exhibiting variable degrees of alteration from a porphyry copper deposit. A total of 57 specimens, classified according to their alteration intensity, were subjected to single-cycle and multi-cycle compression tests under unconfined and confined conditions (σ₃ = 15 MPa). Acoustic emission (AE) monitoring was performed continuously to track microcrack activity and damage evolution during loading. Analysis of the unloading modulus throughout cycles reveals a progressive stiffness degradation that correlates with internal damage accumulation. In general, ‘perfect elasticity’, where loading and unloading gradients converge, is only observed at low stress levels, typically between 20% and 40% of the peak strength. These results contrast with previous studies on more homogeneous rocks, where a broader elastic range were reported. Our findings indicate that beyond 40% threshold, the divergence between loading and unloading moduli increases sharply as a function of cycle accumulation. Samples enriched in softer mineral phases (sericite-rich) exhibit distinct acoustic signatures that reflect a more distributive damage mechanism, whereas quartz- and K-feldspar–dominated rocks, characterized by higher mineral hardness, show a greater damage and microcrack accumulation. This is quantitatively supported by the Felicity Effect analysis: under unconfined conditions, rock dominated by harder mineral phases exhibit lower Felicity Ratio (FR) values, indicating significant pre-peak damage. However, the introduction of a 15 MPa confining pressure leads to a homogenization of the FR across all alteration intensities, as the external stress suppresses micro-cracking regardless of the initial mineralogical heterogeneity. Ongoing analysis explores relationships between alteration degree, mineralogical composition, cyclic damage thresholds, and post-test fracture patterns. By integrating mechanical measurements and acoustic emission data, this work aims to clarify how hydrothermal alteration governs damage accumulation and failure processes in heterogeneous rocks subjected to cyclic stressing, with implications for deep mining stability and induced seismicity.

How to cite: Valdés, F., Clunes, M., Roquer, T., Cortez, J., Garrido, M., Browning, J., and Orellana, L. F.: Alteration-related damage thresholds in cyclically loaded rocks from deep mining environments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15305, https://doi.org/10.5194/egusphere-egu26-15305, 2026.

Rock fracturing driven by temperature fluctuations, rainfall, and freeze–thaw cycles governs both long-term landscape evolution and the onset of rock-slope instabilities. However, the thermo-mechanical stress field that develops within the first decimeters of exposed rock—and its role in sub-critical crack growth—remains poorly constrained, largely because it cannot be directly observed at relevant spatial and temporal scales in-situ.

We present a new dense ultrasonic monitoring experiment designed to image near-surface stress, rigidity, and damage in an unstable limestone cliff. The system consists of more than 50 permanently installed ultrasonic transducers deployed over a 4 m² area on a 50-m-high limestone pillar located in the foothills of Larzac, southern France. Half of the sensors operate as emitters and half as receivers, allowing repeated, highly redundant measurements of travel times and waveforms across hundreds of ray paths. Using acousto-elasticity, temporal changes in ultrasonic velocity provide a quantitative proxy for stress and crack evolution, while waveform decorrelation enable tracking of micro-damage and scattering.

The high spatial density of the array enables 2-D and potentially 3-D tomographic imaging of stiffness and damage within the rock surface layer, resolving gradients that are invisible to sparse instrumentation or bulk resonance methods. First results reveal pronounced diurnal velocity variations that correlate with surface temperature and solar radiation, indicating strong thermo-elastic control on near-surface stress and fracture opening.

This new monitoring approach opens the door to direct, time-lapse imaging of climate-driven damage in rock slopes, providing a critical link between environmental forcing, sub-critical cracking, and the progressive weakening that precedes rockfall and cliff collapse.

This work was funded by the European Research Council (ERC) under grant No. 101142154 - Crack The Rock project.

How to cite: Rolland, A., Rousseau, R., Bontemps, N., Starke, J., Moreau, L., Baillet, L., and Larose, E.: Dense ultrasonic imaging of thermo-mechanical stress changes in a limestone cliff, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17417, https://doi.org/10.5194/egusphere-egu26-17417, 2026.

Salt caverns are formed by solution mining and may be used for energy storage purposes after the production phase. During the operational lifetime, and in particular during abandonment, the spatial and temporal distribution of stress changes and deformation around caverns leads to convergence of salt around the caverns. In turn, this may lead to surface subsidence and potentially affect cavern integrity. Deformation around salt caverns is governed by different creep mechanisms, encompassing transient and steady-state creep stages. Steady-state creep is governed by a combination of dislocation and diffusion creep mechanisms. Deformation due to dislocation creep is dominant at relatively high stresses, whereas grain size-sensitive diffusion creep, particularly pressure solution, contribute significantly at low stresses. When modelling rock salt behaviour, these mechanisms need to be properly accounted for. Recently, studies have suggested that a threshold differential stress may exist below which pressure solution does not take place, which needs to be accounted for. In addition, dynamic recrystallisation may take place through grain boundary migration, driven by differences in energy stored in neighbouring grains due to dislocation creep strain. The process of grain boundary migration reduces the (work hardening) energy in the system as old grains are consumed by new ones, causing weakening. Furthermore, incorporation of transient creep is generally based on the description given in the Munson and Dawson model.

In this study, we aim to simulate different cavern operation phases, such as leaching, production, and abandonment, to analyse the effect of transient creep, pressure solution creep and its threshold stress on the stress and deformation evolution around the cavern. The coupled effects of these complex creep characteristics on cavern behaviour have not yet been studied in detail. Such, more extensive coupling, are needed to better align laboratory- and field-based observations of salt mechanical behaviour, and apply it to large-scale numerical models. The commercial mechanical simulator FLAC (Fast Lagrangian analysis of continua) has been used to develop a 2D model for a single cavern system, which can be used to examine cavern convergence, subsidence and cavern integrity. An empirical model is used to define the threshold strain limit for dynamic recrystallisation by grain boundary migration, analogous to the Munson-Dawson strain limits for transient creep.

The results show a significant effect of pressure solution creep on stress and deformation behaviour around the cavern. In the production phase, the transient creep does not show any significant effect on cavern behaviour; however, it could be important under varying loading conditions. The extent and magnitude of convergence and subsidence are dependent on the rate of pressure solution creep and its threshold stress. A preliminary analysis of the onset of the dynamic recrystallisation around the cavern suggests that DRX may be active in the lower regions of the cavern.

How to cite: Jain, G., Wassing, B., Hangx, S., Ter Heege, J., and de Bresser, H.: Impact of creep mechanisms on stress and deformation behaviour around salt caverns, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17519, https://doi.org/10.5194/egusphere-egu26-17519, 2026.

Progressive damage and failure in rock masses is governed by multi-scale processes ranging from micro-crack growth to meter-scale fracture opening. We present an active acoustic monitoring approach that captures these evolving processes through time-lapse waveform fingerprinting, providing a quantitative measure of the temporal evolution of rock mass stiffness and scattering properties.

We deployed acoustic sensors on three highly fractured rock cliffs (two limestone sites in southern France and one gneiss site in western Switzerland) and conducted repeated active acoustic measurements every few minutes over periods of several weeks to more than one year. Each source-receiver path yields a unique acoustic response whose complexity increases with fracture density and scattering. By tracking phase shifts and waveform distortions, we 'draw' time-lapse waveform fingerprints that are highly sensitive to small changes in crack density, fracture aperture, and contact stiffness.

The waveform fingerprints reveal strong repeatability under similar meteorological conditions, with coincident patterns observed on days sharing comparable temperature and moisture regimes. Distinct fingerprints emerge under different rock cracking and damage states reflecting reversible thermo-hydro-mechanical effects. Some rocks are indeed more reactive to external forcings than others. At longer timescales, partial but incomplete recovery of the fingerprints is observed. In the one-year data set, major fingerprint features reappear under similar climatic conditions, but with persistent residual changes, indicating the accumulation of irreversible damage within the rock mass.

Future work could apply diffuse acoustic wave spectroscopy and acoustic correlation-based imaging to spatially locate damage and quantify fracture growth, enabling the transition from qualitative fingerprints to quantitative maps of rock degradation.

How to cite: Starke, J., Rousseau, R., Rolland, A., Baillet, L., and Larose, E.: Acoustic Fingerprints - Tracing Irreversible Damage in Natural Cliffs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17611, https://doi.org/10.5194/egusphere-egu26-17611, 2026.

El Teniente, located in central Chile, is the largest underground copper mine in the world, with ore extraction reaching depths of up to one thousand meters below the surface. The primary ore body is hosted in volcanic rocks of basic composition, extensively intersected by a stockwork of closely spaced veins. At present, the combination of the regional stress regime, localized stress concentrations around excavations, and the heterogeneous nature of the veined rock mass poses significant challenges to operational safety, particularly when considering an extension of the mine’s productive life. Discontinuities, ranging from large-scale faults to small-scale veins, are closely linked to failure processes in mine excavations. In highly homogeneous, intact rock, spalling failure in tunnels is typically initiated at about 50% of the rock’s uniaxial compressive strength (UCS). This relationship is not consistently observed in veined rocks, where standard UCS tests often fail to account for the mechanical influence of discontinuities during deformation. In this study, an experimental setup that integrates four strain gauges, acoustic emission (AE) monitoring, and digital image correlation (DIC) during uniaxial compressive strength (UCS) testing was developed. The method was applied to 20 mm-diameter cylindrical rock specimens from El Teniente to investigate the role of sulfide-rich veins in rock deformation. Samples were selected to minimize the occurrence of multiple veins, containing instead a single primary vein (< 4 mm thick) oriented between 0° and 90° relative to the axial loading direction. Particular emphasis was placed on strain partitioning, with DIC employed to obtain full-field strain measurements, enabling the quantification of strain differences between the rock matrix and the veins. Experimental results indicated that veins accommodated greater strains than the surrounding rock matrix during both the elastic and plastic regimes. All the samples shown a rotation of the local stress tensor on the vein when reaching the onset of dilatancy. Veins oriented between 0 to 40° yielded before the bulk material, with yield onset occurring at 50-60% of the UCS. These findings suggest that precursory shear strain within favorably oriented veins, evidenced by the onset of AE activity, dilatancy and DIC, may play a critical role in initiating rockmass failure at excavation boundaries.

How to cite: Robbiano, F., Violay, M., Orellana, L. F., Guggisberg, A., and Heinkel, E.: The role of veinlets in the unconfined behavior of El Teniente Mine rock samples: Implications for mining-induced rockmass failure., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19556, https://doi.org/10.5194/egusphere-egu26-19556, 2026.

Understanding how fluid injection perturbs stressed faults and triggers induced seismicity has become an urgent challenge in geophysics and hazard mitigation. Observations from subsurface fluid injection associated with geoenergy exploitation show that variations in injection pressure and rate and injected volume can strongly modulate seismicity rates and magnitudes. Yet, comparable injection operations may result in stable creep, slow slip, or dynamic rupture, highlighting persistent gaps in our understanding of the physical processes governing fluid-driven fault reactivation.

Here, we investigate fluid-induced fault reactivation through decimetric-scale laboratory experiments on granite samples containing a 45° precut fault. Experiments are conducted in a biaxial apparatus under critically stressed conditions at 3 MPa normal stress, with independent control of normal and shear stresses. Fluids are injected directly into the fault surface while fault slip is measured using fibre-optic sensors (mini-SIMFIP) installed across the fault. Seismic activity is monitored through passive acoustic emission recordings, and repeated active ultrasonic surveys are performed throughout the experiments to track wave velocity changes and map fluid diffusion along the fault.

By systematically varying the injection rate, we observe a clear transition from aseismic creep to slow slip and dynamic rupture. In all cases, fault slip nucleates at the injection point and subsequently propagates within the pressurized region of the fault. High injection rates generate localized overpressure near the injection point, triggering abrupt and seismic fault reactivation. During high-rate injection, we observe a pronounced drop in P-wave velocity, indicating strong mechanical perturbation of the fault zone, followed by a progressive velocity increase as fluids diffuse along the fault. In contrast, low injection rates lead to stable, aseismic slip confined to the pressurized zone, while intermediate rates produce a progressive reactivation sequence in which slip initiates aseismically, evolves into slow slip, and eventually transitions to dynamic rupture as the pressurized region expands.

Our results show that injection rate governs fault slip behavior by controlling where slip nucleates and whether it remains confined to, or propagates beyond, the pressurized zone and accelerates dynamically.

How to cite: Pignalberi, F., Osten, J., Selvadurai, P., Spagnuolo, E., Jalali, M., and Amann, F.: Illuminating Fluid-Induced Fault Reactivation: Laboratory Insights into Injection-Rate Control on Slip Evolution and Seismic Nucleation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2082, https://doi.org/10.5194/egusphere-egu26-2082, 2026.

In Kochi, JAMSTEC, Japan, we have developed rotary shear apparatuses to understand fault slip behavior of geologic materials at coseismic slip rates. However, coseismic fault behavior under hydrothermal conditions has remained challenging to reproduce in the laboratory. To address this issue, we developed a novel apparatus (HDR: hydrothermal rotary shear apparatus) in 2017, which is capable of high-velocity slip (~2 m/s) at temperatures of up to 600 ℃ and pore fluid pressures of up to 120 MPa. Using this apparatus, we have successfully carried out hydrothermal high-velocity friction experiments on various types of materials, including bare gabbro surfaces and gouges derived from quartz, gabbro, granite, olivine, calcite, and clay minerals under a wide range of pressure-temperature-velocity conditions. The experimental data obtained under hydrothermal conditions are sometimes markedly different from those typically observed in room-temperature experiments due to dynamic changes in fluid properties and chemical reactions in supercritical water. Further understanding of coupled interactions between fault slip, frictional heat, fluid properties, chemical reactions, etc. under hydrothermal conditions will be essential for constraining fault slip behavior in seismogenic-zone settings. In this presentation, we introduce some of our latest results obtained using HDR.

How to cite: Okuda, H., Tanikawa, W., Hamada, Y., Okazaki, K., Bedford, J., Hidaka, M., and Hirose, T.: Development of hydrothermal high-velocity rotary shear apparatus in Kochi, Japan: towards understanding fault slip behavior in the seismogenic zone environment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2718, https://doi.org/10.5194/egusphere-egu26-2718, 2026.

Understanding dynamic weakening is of paramount importance because it involves thermally triggered physical-chemical processes that reduce the fault frictional resistance and facilitate earthquake propagation. Investigating the evolution of temperature (T), pore fluid pressure (P_f), and chemical reactions during slip therefore allows exploration of dynamic weakening mechanisms. However, direct in situ measurements of T and P_f within slip zones remain technologically challenging in laboratory high-velocity rotary shear experiments. Consequently, numerical simulations constrained by mechanical data are essential for inferring these critical parameters. We utilize a series of rotary-shear mechanical data on a combination of kaolinite and quartz. The experiments are conducted under undrained/drained conditions with thermocouples for temperature measurements. On the basis of these data, we develop a Thermo-Hydro-Mechanical-Chemical (THMC) modeling framework using COMSOL Multiphysics to estimate physical conditions within the Principal Slip Zone (PSZ) and to infer dynamic weakening mechanisms responsible for the observed frictional behavior. Under undrained conditions, the friction coefficient (µ) reaches a peak friction (µ_p) at ~0.28 and undergoes abrupt weakening, followed by a steady-state low-friction (µ_s) at ~0.1. This behavior corresponds to a measured T stabilizing at 320–360°C and a simulated P_f rapidly increasing and is maintained at ~2.4 MPa. It suggests that frictional heating induces pore water pressurization. Under drained conditions, µ reaches a µ_p ~0.3 at 0.7s and undergoes abrupt weakening during 1.2-1.7s, maintains µs ~0.17 between 2-4s and followed by a re-strengthening behavior at 4s. µ was accompanied by the changes of T and P_f. T increased to ~280°C at 1.2s, followed by a decrease to ~200°C. Meanwhile, the simulated P_f achieved the highest value (~1.9 MPa) at 1.2s and gradually decreased and reached a relatively lowest value (~0.3 MPa) at 4s due to the pore fluid drainage. Whether P_f is present is corresponding to the weakening and re-strengthening times. In addition, microstructural and mineralogical observations show thermal decomposition of kaolinite. Because thermal decomposition is a strong endothermic reaction, the temperature decreases during the later stages of the process due to the thermal decomposition of kaolinite. We suggest that thermal pressurization operates as the dynamic weakening mechanism during the initial slip stage, consistent with theoretical predictions and experimental documentation of thermal pressurization during rapid shear, as described by Rice (2006) and Ferri et al. (2010). Later, thermal pressurization ceases under drained conditions, resulting in complex frictional behavior. In general, our study provides essential insights into the dynamic weakening mechanism during experimental seismic slip. In addition, we suggest that drainage conditions may influence frictional behavior by affecting the generation and maintenance of pore fluid pressure during seismic slip.

How to cite: Huang, Y.-Q., Kuo, L.-W., Hung, C.-C., and Nguyen, T. T.: Thermo-hydro-mechanical-chemical modeling of synthetic wet gouges sheared at experimental seismic slip under fluid drainage conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3257, https://doi.org/10.5194/egusphere-egu26-3257, 2026.

While dynamic fluctuations in effective normal stress affecting fault zones are ubiquitous, arising from both complex natural phenomena like remote dynamic triggering of earthquakes and anthropogenic activities like industrial subsurface fluid injection, the precise influence of these perturbations and specifically their frequencies, on the macroscopic fault strength remains insufficiently characterized.The frequency of these pore-pressure variations is likely a key factor setting the timescale for the drained-to-undrained transition, thereby driving markedly different mechanical responses.

In this work, we present results from a coupled hydromechanical-discrete element model simulating a pre-stressed, fully saturated granular fault gouge subject to cyclic pore-pressure variations across three orders of magnitude in frequency. We observe that fault failure consistently occurs before the system reaches the traditional Mohr-Coulomb failure criterion. This early failure indicates that additional dynamic mechanisms, often neglected in effective stress analyses, play a dominant role in triggering instability. We investigate the driving forces responsible for this pre-Mohr-Coulomb failure and find they evolve distinctly with frequency. We evaluate three primary candidates driving this behavior: 1) seepage forces arising from the pore-pressure gradients, 2) contact weakening induced by granular agitation (vibration), and 3) inertial effects driven by acceleration from cyclic pore-pressurization. Our analysis isolates the contribution of each mechanism across the frequency spectrum, offering a new physical basis for understanding why dynamic pore-pressure perturbations can trigger slip earlier than what static friction laws predict.

How to cite: Sarma, P., Parez, S., Toussaint, R., and Aharonov, E.: Mechanisms of failure in a fluid-saturated fault gouge subject to cyclic pore-pressure oscillations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4224, https://doi.org/10.5194/egusphere-egu26-4224, 2026.

Cyclic fluid injection can promote repeated fault reactivation and transient permeability changes, a behavior often discussed within the fault-valve concept where faults alternate between acting as hydraulic barriers and conduits. Such cycles are relevant to both natural hydrothermal systems and industrial activities that modify pore pressure in the subsurface.

The complex evolution of fault permeability and strength during and after fault reactivation calls for a more complete description of the underlying physical processes. Current state-of-the-art fault reactivation models generally represent these weakening and strengthening mechanisms at the macroscopic scale using phenomenological laws, such as the widely used rate-and-state framework. Although these formulations have proven successful in reproducing some observed fault behaviors, they rely on empirically determined parameters and still leave part of the relevant physics insufficiently described.

Here we investigate these processes using a discrete element method (DEM) approach coupled with a pore-scale finite volume (PFV) scheme. Similarly to the framework proposed by Nguyen et al. (2021), the DEM models the granular gouge, while PFV simulates pore-pressure evolution and fluid flow through the evolving pore geometry, with full two-way coupling between solid deformation and fluid pressure, thus relating the macroscopic response of the system to the micromechanical phenomena at work.

Using this coupled approach, we simulate both monotonic and cyclic injection protocols designed to represent fault-valve cycles. We quantify how permeability evolves before, during, and after reactivation, and we explore the influence of key controlling factors: (i) initial permeability, (ii) initial stress state prior to injection, and (iii) confining stress. We also estimate seismic moments associated with individual reactivation events where the recovered moments remain bounded by the injected-volume constraint M_0,max =GΔV  (McGarr, 2014). Overall, by adding grain-scale observations to trends reported in laboratory and in situ studies, this work helps interpret permeability transients and their implications for triggered seismicity, in order to provide more realistic models of fluid-induced fault reactivation.

References
Nguyen, H. N. G., Scholtès, L., Guglielmi, Y., Donzé, F. V., Ouraga, Z., & Souley, M. (2021). Micromechanics of sheared granular layers activated by fluid pressurization. Geophysical Research Letters. https://doi.org/10.1029/2021GL093222
McGarr, A. (2014). Maximum magnitude earthquakes induced by fluid injection. Journal of Geophysical Research: Solid Earth, 119, 1008–1019. https://doi.org/10.1002/2013JB010597

How to cite: Larkem, O., Scholtès, L., and Golfier, F.: A Multi-Scale Approach to Fault-Valve Systems and Their Evolution, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6001, https://doi.org/10.5194/egusphere-egu26-6001, 2026.

How large earthquakes rupture across mechanically distinct fault systems remains a fundamental unresolved problem in seismology. New Zealand straddles the Australia–Pacific plate boundary, where relative plate motion is partitioned between deep Hikurangi subduction and widespread dextral strike-slip faulting in the upper plate, such as the Marlborough Fault System. This tectonic configuration favours complex, multi-fault earthquakes, as illustrated by the 2016 Mw 7.8 Kaikōura earthquake and the 2013 Mw 6.5 and Mw 6.6 Cook Strait sequence in central New Zealand. Investigating the source physics of such complex ruptures and their relationship to plate-boundary architecture will provide essential constraints on fault interaction processes and seismic hazard in large-scale convergent margins.

On the evening of 23 January 1855, a major earthquake struck central New Zealand with intense ground shaking across much of the North and South Island. Empirical ground intensity was estimated up to ten, and local tsunami waves reached nearly 10 m in the Cook Strait (Grapes and Downes, 1997; Clark et al., 2019). This event is remarkable for having generated significant uplift - maximum uplift of 6.4 m measured in the southern Wellington coast (McSaveney et al., 2006) - and extreme dextral slip, measured as ~18.7 m at Pigeon Bush along the Wairarapa fault (Rodgers and Little, 2006).  Seismological evidence suggests an offshore hypocentre at ~25 km depth and a magnitude exceeding 8.0; however, in the absence of instrumental observations, it remains unclear whether significant slip also occurred on the Hikurangi subduction interface  (Beavan & Darby, 2005). Resolving the role of the deeper Hikurangi megathrust, particularly its potential synchronous activation with upper-plate faults, is therefore crucial for understanding the rupture mechanics of this event and for improving seismic hazard assessments in densely populated plate-boundary regions.

In this study, we investigate the source complexity and associated ground motions of multi-fault earthquakes using physics-based dynamic rupture simulations of the 2016 Kaikōura and 1855 Wairarapa earthquakes in central New Zealand. We construct dynamic source models accounting for updated geological and seismological constraints, including regional tectonic stress fields (Townend et al., 2012), national fault networks (Seebeck et al., 2024), nonlinear rheology, and three-dimensional subsurface structures. These key geophysical constraints are essential in reproducing the instrumental observations in the case of the 2016 Kaikōura earthquake (e.g. Ulrich et al. 2019).  Rupture magnitude and ground shaking of historical earthquakes are validated against geological measurements, landslide inventories, and tsunami run-up. Beyond observation-driven scenarios, we systematically explore the sensitivity of rupture dynamics and ground motions to variations in tectonic conditions in historical earthquakes. These simulations will provide physical constraints on rupture kinematics and fault interactions, offering insights into improving near-source ground-motion models and regional seismic hazard assessments.

How to cite: Li, D., Howell, A., Coffey, G., Clark, K., Litchfield, N., Langridge, R., Caballero Leyva, E., Benites, R., Williams, C., Bora, S., and Gerstenberger, M.: Filling a missing piece of the 1855 Wairarapa earthquake: Rupture characteristics and implications for regional seismic hazard, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6574, https://doi.org/10.5194/egusphere-egu26-6574, 2026.

The Gutenberg-Richter law for the distribution of earthquake magnitude and the Omori law for the decay of aftershocks are two universal laws in seismicity. Although numerical models have been developed to reproduce these laws, they sometimes produce many more foreshocks and less aftershocks than observed. In this study, we simulate earthquake sequences on a 2D random fault network. The fault lengths follow a power-law distribution. Our simulations reproduce the Omori law, without producing many foreshocks. The event size distribution follows Gutenberg-Richter's law with the b-value expected from the fault length distribution, even though many earthquakes are multi-fault ruptures or partial ruptures. Ruptures sometimes propagate into other faults, though there are more partial ruptures than multi-fault ruptures. The frequency of partial and cascading ruptures increases with higher fault density or stronger velocity-weakening friction. Overall, this work illuminates how fault interaction controls the spatiotemporal pattern of seismicity.

How to cite: Ozawa, S.: Partial ruptures, cascading multi-fault ruptures, and aftershocks in 2D random fault network, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8601, https://doi.org/10.5194/egusphere-egu26-8601, 2026.

Understanding where and how rupture terminates on a fault is crucial because it controls earthquake magnitude and associated damage. But the in situ stress state, which is one of the key parameters governing rupture dynamics, is not directly measurable on natural faults. Laboratory experiments therefore provide an essential approach for investigating rupture termination (e.g., Bayart et al., 2016, 2018; Ke et al., 2018). Previous studies showed that termination can often be interpreted using an energy balance at the rupture tip within linear elastic fracture mechanics (LEFM), while also suggesting that incorporating additional processes such as long-tailed weakening or rate-dependent friction may improve the description (e.g., Paglialunga et al., 2022; Brener and Bouchbinder, 2021). In order to deepen our understanding of rupture dynamics, including how they terminate, it should be efficient to conduct a systematic investigation that controls the stress state and resulting rupture behavior in the laboratory. From this point of view, we have started a large-scale rock friction experiment. In our experiments, two metagabbro specimens are stacked vertically within the experimental frame. The contacting nominal area is 6.0 m long by 0.5 m wide. Six hydraulic jacks apply normal load to the upper block, and a single hydraulic jack applies shear load to the lower block, which is supported on low-friction rollers. Strain gauge arrays along the fault measure local shear stress every 130 mm at a sampling rate of 1 MHz. In experiment GB01-051, we first imposed 5 mm of shear displacement under a macroscopic normal stress of 2.8 MPa, generating repeated stick-slip events that nucleated at either the leading or trailing edge. We then gradually reduced the normal load on one of the normal jacks on the trailing-edge side while maintaining the shear load. This procedure produced clear nucleation near the unloaded jack followed by a full rupture across the entire fault. After restoring the loads to near-critical conditions, we repeated the procedure at the leading-edge side to generate fault ruptures. In a subsequent trailing-edge attempt, however, rupture terminated approximately halfway along the fault, despite a similar macroscopic stress level. Local stress measurements indicate that previous ruptures reduced the shear stress on the leading-edge side, lowering the available energy release rate for propagation and promoting termination. These results demonstrate that rupture initiation and termination can be manipulated through the evolving stress heterogeneity. We also estimated a lower bound on fracture energy from the measured stress drop using LEFM. Accounting for uncertainty in the termination location, the inferred value ranges from 0.032 to 0.29 J/m², consistent with prior experiments on the same rock type (Xu et al., 2019). Ongoing work will further quantify how controlled stress heterogeneity governs rupture termination.

How to cite: Yamashita, F., Fukuyama, E., Okubo, K., and Matsumoto, Y.: Rupture termination controlled by tuned stress heterogeneity on a 6-m-long laboratory rock fault, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8994, https://doi.org/10.5194/egusphere-egu26-8994, 2026.

The critical nucleation length (L_c) is a fundamental parameter that characterizes the earthquake nucleation process and is theoretically proportional to the fracture energy and the inverse square of shear stress drop (Andrews, 1976, JGR). However, quantitative verification of this relationship has been limited due to difficulties in controlling the nucleation location and in achieving spatially dense measurements capable of resolving the nucleation extent and associated stress drop on the fault.

In this study, we conducted large-scale rock-friction experiments using the biaxial friction apparatus to investigate the scaling characteristics of the critical nucleation length L_c. The experimental configuration consists of vertically stacked Indian metagabbro blocks, comprising an upper block (L6.0 m × W0.5 m × H0.75 m ) and a lower block (L7.5 m × W0.5 m × H0.75 m ), forming a simulated fault with a nominal contact area of 6.0 m × 0.5 m. Strain gauges were installed at a distance of 15 mm from the fault surface with a spacing of 130 mm, and continuous measurements were recorded at a sampling rate of 1 MHz. Normal loading was applied using six independently controlled jacks, enabling controlled rupture nucleation confined within the fault and high-resolution measurements of local shear-stress time history.

To establish a robust measurement criterion for L_c based on rupture velocity evolution, we performed 2D dynamic rupture simulations using spectral boundary integral equation software UGUCA (Kammer et al., 2021) with a linear slip-weakening law. We compared the theoretical L_c predicted from the frictional parameters and prescribed initial stress with L_c inferred from rupture-velocity-based criteria. By varying the critical slip distance D_c, we simulated nucleation processes with different L_c values. The results show that the preslip extent at which the rupture velocity reaches approximately 0.06V_s (where V_s is shear-wave velocity) is in good agreement with the theoretically predicted L_c, supporting the use of this criterion for quantifying and discussing the scaling of L_c.

Applying this criterion to the laboratory experiments, the estimated L_c values range from 0.4 m to 4.0 m, spanning nearly one order of magnitude. The average local shear stress drop within the estimated nucleation region was evaluated as the difference between the initial and residual shear stresses measured before and after the main shock. We observed that L_c clearly scales with the inverse of the shear stress drop, rather than the inverse square, which persists under different normal stress conditions. This scaling is consistent with the observation that the initial shear stress is close to the peak strength in the nucleation region, under the assumption that D_c remains nearly constant among events (approximately 1 μm in this study). These findings provide insight into the quantitative dependence of L_c on the shear stress drop and place important constraints on our understanding of earthquake nucleation processes.

How to cite: Matsumoto, Y., Okubo, K., Yamashita, F., and Fukuyama, E.: Earthquake Source Processes inferred from a 6-meter-long laboratory fault (1) Quantitative Evaluation of Critical Nucleation Length under Variable Stress Drops, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9293, https://doi.org/10.5194/egusphere-egu26-9293, 2026.

Fault zones play a fundamental role in controlling subsurface fluid circulation and mineralization. Their capacity to alternately behave as barriers or conduits—commonly conceptualized within the fault-valve model—results from complex interactions among multiphysical processes (thermal, hydraulic, mechanical, and chemical) acting across a wide range of spatial and temporal scales within the fault structure.

Focusing on the gouge scale, we investigate the hydromechanical behavior of faults using a three-dimensional pore-scale modeling framework that couples a Discrete Element Method (DEM) with a pore-scale finite volume (PFV) scheme. Building on the approach of Nguyen et al. (2021), the DEM is used to model the gouge material as a granular assembly, while the PFV method is used to model fluid flow within the evolving pore space.

Using this coupled approach, we simulate a sheared, fluid-saturated granular gouge subjected to hydromechanical loading through a controlled cyclic fluid injection protocol applied at constant shear stress. The DEM-PFV model captures emergent behaviors consistent with laboratory and in situ observations, and reveals pronounced cycle-to-cycle variability in both the onset of reactivation (Sarma et al., 2025) and the magnitude of slip under repeated pressurization and depressurization. In particular, some cycles produce large slip episodes whereas others exhibit comparatively small slip under similar loading conditions. To connect this macroscopic variability to micromechanics, we track the evolution of the force-chain population throughout the cycles using the characterization method proposed by Peters et al. (2005). The results provide grain-scale insights into how internal load-bearing structures reorganize across cycles and how these force-chain dynamics relate to the occurrence of large-slip events in fault-valving sequences.

References
Nguyen, H. N. G., Scholtès, L., Guglielmi, Y., Donzé, F. V., Ouraga, Z., & Souley, M. (2021). Micromechanics of sheared granular layers activated by fluid pressurization. Geophysical Research Letters. https://doi.org/10.1029/2021GL093222
Sarma, P., Aharonov, E., Toussaint, R., & Parez, S. (2025). Fault gouge failure induced by fluid injection: Hysteresis, delay and shear-strengthening. Journal of Geophysical Research: Solid Earth. https://doi.org/10.1029/2024JB030768
Peters, J., Muthuswamy, M., Wibowo, J., & Tordesillas, A. (2005). Characterization of force chains in granular material. Physical Review E. https://doi.org/10.1103/PhysRevE.72.041307

How to cite: Scholtès, L., Larkem, O., and Golfier, F.: Micromechanical Investigation of Fault-Valving Cycles Using the Discrete Element Method, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9616, https://doi.org/10.5194/egusphere-egu26-9616, 2026.

In fluid-rich faults, thermal pressurization (TP) is theoretically predicted to induce rapid fault weakening and facilitate large earthquakes, whereas dilatant strengthening (DS) can counteract this process through pore-space expansion. This study experimentally investigates the relative efficiency and condition of TP and DS using triaxial stick-slip tests with on-fault pore pressure (Pp) measurements. Dynamic stick-slip events were generated on four saturated, saw-cut, thermally cracked Westerly granite samples under varying effective confining (30–60 MPa) and pore pressures (25–45 MPa). Results reveal a systematic rupture transition from co-seismic Pp rise (TP-type) at low shear stress to Pp drop (DS-type) at higher shear stress, accompanied by a coupled fast–slow spectrum: fast TP → slow TP → slow DS → fast DS. Fast events reach slip velocities up to three orders of magnitude higher (0.5–10 mm s^-¹) than slow ones (0.001–0.5 mm s^-¹). The TP model under undrained, adiabatic conditions reproduces the measured Pp evolution, indicating a progressive shear-zone widening (0.05–0.5 mm, as overestimated values) that reduces TP efficiency and promotes slow events. For DS sequences, an increasing dilatancy coefficient is inferred, consistent with enhanced Pp drops. Breakdown energy shows no clear difference between TP and DS events, suggesting similar rupture energetics despite opposite pore-pressure evolution. Overall, this study provides the first direct experimental evidence of TP–DS transitions, demonstrating that TP governs early-stage weakening but diminishes as the shear zone widens, allowing DS to dominate. These results imply that in mature fault zones, after several seismic cycles, fault weakening may be mainly governed by co-seismic dilatancy, although strong, fast ruptures can still occur when the dilatant strengthening is not sufficient to stop the on-going rupture.

How to cite: Fan, C., Lin, G., Aubry, J., Deldicque, D., Giorgetti, C., S. Bhat, H., and Schubnel, A.: From thermal pressurization (TP) to dilatant strengthening (DS) during stick-slip ruptures on saturated saw-cut thermally cracked westerly granite, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9839, https://doi.org/10.5194/egusphere-egu26-9839, 2026.

The largest earthquakes on planet Earth occurred along the frictional interface between subducting and overriding plates (i.e., the megathrust) at convergent margins. Some of these destructive events have occurred over the last 20 years, such as the 2004 Mw 9.0 Sumatra‐Andaman, 2010 Mw 8.8 Maule, and 2011 Mw 9.0 Tōhoku‐Oki earthquakes. These large events are among the most devastating expressions of Earth's dynamics and, along with tsunamis, they represent a major hazard to society. Therefore, it is crucial to understand to which extent it is possible to predict the final size of a large rupture from the early stage of its propagation. We studied analog earthquakes in an apparatus - Foamquake (Mastella et al., 2022) - in which we used foam rubber to reproduce the upper plate and a 1 cm thick layer of granular materials to reproduce the subduction channel. The set-up is made of an elastic foam wedge with a dimension of 145 × 90 × 20 cm3 (i.e., the overriding plate analog) that overlies a planar, 10° dipping, rigid plate. Along the plate, a basal conveyor belt is driven with constant velocity (0.01 cm/s), reproducing a steady, trench-orthogonal subduction. To constrain the dynamics of analog earthquakes, we used a network of 11 Micro-Electro-Mechanical Systems (MEMS) accelerometers, distributed on the model surface, and measured the evolution of the trench orthogonal component of acceleration at 1 kHz. Additionally, we also used a top-view high-resolution camera (100 Hz), that allows us to derive surface displacements via Particle Image Velocimetry (PIV), that enables characterization of the final static rupture properties, while MEMS monitoring resolves the temporal evolution of spatiotemporal slip. We report 21 models with different frictional configurations of the analog megathrust, including asperities and barriers of varying dimensions, to produce thousands of events with different magnitudes. MEMS monitoring allows for characterization of the Source Time Function (STF) of each event. Preliminary analysis of the STFs indicates a weak correlation  (i.e., R²<0.2) between the moment accumulated over different time windows during the early stages of rupture propagation and the final size of individual events. These results contribute, from an experimental perspective, to the ongoing debate on the stochastic versus deterministic nature of earthquake rupture growth.

How to cite: Pardo, S., Corbi, F., Guastamacchia, S., Mastella, G., Tinti, E., and Funiciello, F.: Can Earthquake magnitude be predicted at rupture onset? insights from scaled seismotectonic models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12207, https://doi.org/10.5194/egusphere-egu26-12207, 2026.

The seismic potential of a fault is controlled by its ability to regain strength between earthquakes (fault healing). Variations in healing rate among different rock types can cause local locking and elastic strain energy accumulation, potentially leading to earthquake nucleation. The 2016 Mw 6.5 Norcia mainshock nucleated in the Triassic Evaporites, the seismogenic layer of central Italy, composed of dolostones and anhydrites. Despite its relevance, the mechanical behavior of anhydrite remains relatively less constrained compared to other common crustal lithologies. Only a limited number of studies have systematically investigated the frictional properties of anhydrite, suggesting that its mechanical behavior is strongly sensitive to boundary conditions.

We conducted room humidity (RH) and water-saturated (WS) Slide-Hold-Slide (SHS) friction experiments on anhydrite gouge to assess and isolate the role of water on its healing properties. To inform mechanical data with the microphysical evolution of the fault, we used piezoelectric (PZT) sensors in transmission mode, which record ultrasonic S-wave (UW) propagation through the sample. Finally, mechanical and ultrasonic measurements were complemented by microstructural analyses of the post-mortem sample.

Our results show that fault healing follows a log-linear dependence on hold time under both RH and WS conditions, but with markedly different magnitudes. Water-saturated experiments exhibit a healing rate nearly three times larger (β ~ 0.024) than RH experiments (β ~ 0.009). Ultrasonic measurements reveal a systematic log-linear increase in S-wave velocity during hold periods. This growth is significantly more pronounced in WS samples, where S-wave velocity increases by more than ~2.8% per decade of hold time, compared to ~1% in RH conditions. Microstructural observations indicate that RH samples deform through distributed cataclastic processes accommodated by multiple R-shear bands, whereas WS samples exhibit extreme strain localization along B-shear zones characterized by intense grain-size reduction and the development of a compressive foliation, consistent with semi-brittle deformation.

These results demonstrate that water fundamentally alters the healing efficiency and deformation style of anhydrite faults. Moreover, they show that ultrasonic wave measurements provide a powerful, independent tool to track fault restrengthening during simulated interseismic periods. The observed increase in S-wave velocity can be directly linked to an increase in the shear modulus of the gouge, which appears to be greater in the presence of water, probably due to fluid-assisted healing processes. Together, the high healing rates and the mechanical stiffening of the microstructure inferred from S-wave velocity measurements suggest that anhydrite gouge may be capable of efficiently accumulating elastic strain energy during interseismic periods. Our findings suggest that fluid-assisted healing in anhydrite-bearing fault zones may play a critical role in controlling fault stability and seismic behavior in natural settings.

How to cite: Mauro, M., Guglielmi, G., Trippetta, F., and Scuderi, M.: Fluid-assisted frictional healing revealed by ultrasonic waves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12872, https://doi.org/10.5194/egusphere-egu26-12872, 2026.

In laboratory studies, the tendency for earthquake nucleation on major plate-boundary fault zones is typically evaluated by measuring the change in shear strength upon controlled step changes in driving velocity.  In such laboratory shear experiments simulating fault sliding, it is typical to neglect the cohesion and assume that all the measured shear strength is due to frictional resistance.  Therefore, the results of such experiments are evaluated in terms of a friction coefficient calculated as the ratio of the shear strength to effective normal stress, where the “velocity-dependent friction” is quantified by the parameter a-b.  However, previous work has shown that the sliding cohesion is not necessarily negligible, especially for water-saturated samples rich in clay-minerals.  This comes from recent experiments have shown that, using a single-direct type shear geometry, the cohesion can be directly measured as a peak shear strength with zero applied normal load (zero effective normal stress).  The technique can also be used for samples that have accumulated slip, thus providing a measure of the cohesion that exists during sliding or “sliding cohesion”.

Here, I use measurements of sliding cohesion to test the assumption that velocity-dependent fault strength is completely controlled by friction, and determine if cohesion plays a significant role.  For these tests I use water-saturated powdered illite-rich Rochester shale, a material that consistently exhibits velocity-strengthening behavior.  The velocity-dependent strength is first obtained with a series of standard 3-fold step increases in driving velocity in the range 0.1-100 μm/s under 10 MPa effective normal stress.  The sliding cohesion is then measured in a series of experiments in which the samples were sheared for 5 mm under 10 MPa effective stress, the normal stress subsequently removed, and then sheared at each of the velocities used in the velocity-step test.  From these tests, the velocity-dependent cohesion is calculated by substituting the cohesion for shear strength and calculating an equivalent “a-b” value that can be subtracted from the standard a-b value measured from the velocity step tests. 

Preliminary results show that velocity-dependent cohesion is of the same order as a-b values, and accounts for up to about a third of the measured a-b values.  The percentage of strength change related to cohesion rather than friction decreases as a function of increasing driving velocity.  Although cohesion is a significant proportion of the velocity-dependent strength changes, removing the velocity-dependence of cohesion is insufficient to cause negative a-b values.  However, this result can also be affected by the choice parameters used in the modeling technique that extracts the a-b values and must therefore be evaluated carefully.  The magnitude of velocity-dependent cohesion suggests that it may represent a signification proportion of velocity-dependent sliding strength.

How to cite: Ikari, M.: Considering velocity-dependent cohesion in fault sliding stability, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14228, https://doi.org/10.5194/egusphere-egu26-14228, 2026.

Fracture energy is a key scaling parameter governing the transition from quasi-static nucleation to dynamic rupture propagation, and it also controls the dynamic stress field around the rupture front. In principle, when fracture energy is uniform along a fault, a single intrinsic fracture energy should describe both quasi-static nucleation and the stress field during rupture propagation. However, this unifying role has not been fully validated experimentally, primarily because conventional laboratory faults are too small to capture the entire rupture process from nucleation to propagation toward limiting speed. Here, we compare the fracture energy evaluated from the shear stress changes associated with the dynamic rupture propagation (Γ) with that inferred from the independently observed critical nucleation length (G^Lc) on a 6-meter-long laboratory fault.

We conduct faulting experiments on a 6-meter-long laboratory fault and evaluate fracture energy by fitting a steady-state rupture model with a linear cohesive zone to local shear stress changes recorded 15 mm away from the fault. In the biaxial rock-friction apparatus, vertically stacked rock specimens form a simulated fault with dimensions of 6 m × 0.5 m. Six independently controlled jacks applying normal loading enable rupture nucleation at prescribed timing and location by unloading a selected jack while maintaining the shear stress near the peak frictional strength. Rupture velocity is constrained by cross-correlating shear stress histories between neighboring strain gauges spaced at 130 mm. Using the locally estimated rupture velocity, fracture energy Γ and cohesive zone size are determined by minimizing the residual between observed and modeled shear stress time histories.

Fracture energy inferred from critical nucleation length, G^Lc, is computed following the formulation of Palmer and Rice (1973) and Andrews (1976), using the critical nucleation length examined by Matsumoto et al. (2026, EGU) togather with the measured stress drops.

We analyze three nucleation-controlled events conducted under a macroscopic normal stress of 3 MPa. From local shear stress time histories associated with rupture velocities lower than 0.95 of the Rayleigh wave speed, we obtained an average fracture energy Γ of 0.04 ± 0.01 J/m². This value is consistent with G^Lc of 0.05 J/m², inferred from events with an average stress drop of 0.05 MPa. These results contribute to the quantitative interpretation of laboratory observations and to improved understanding of earthquake source processes on natural faults.

How to cite: Okubo, K., Yamashita, F., Matsumoto, Y., and Fukuyama, E.: Earthquake Source Processes inferred from a 6-meter-long laboratory fault (2): Fracture Energy Estimation from Dynamic Stress Fields, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14469, https://doi.org/10.5194/egusphere-egu26-14469, 2026.

Fault slip stability is governed by the competition between the elastic energy stored in the surrounding medium and the rate of frictional weakening of the fault, which depends on its constitutive frictional properties. Laboratory shear experiments validate this framework under simplified conditions and homogeneous boundary conditions. In contrast, natural faults are heterogeneous across multiple scales, with variations in stress, frictional properties, fault-zone structure, and elastic properties as documented in laboratory, numerical, and field studies. In gouge-filled faults,  heterogeneities are dynamically coupled: stress concentrations promote shear localization, which modifies gouge fabric that controls the frictional properties, ultimately dictating fault stability and rupture dynamics. Resolving how spatially heterogeneous stress and evolving fault-zone structure interact to control rupture nucleation and propagation therefore requires experimental approaches that move beyond homogeneous assumptions.

Here we present results from large-scale biaxial shear experiments on a quartz gouge–filled fault (75 cm × 8 cm) using two forcing-block materials—Nylon 6 and PMMA—with different elastic stiffnesses. The fault was densely instrumented to investigate how stress heterogeneity and evolving shear fabric control slip behavior under different nominal normal stresses from 3 to 10 MPa and a constant loading rate of 10 µm/s. Far-field stress and displacement were monitored using load cells and LVDTs (1 kHz), while forcing blocks deformation was measured using Digital Image Correlation (DIC, 2–30 Hz). During laboratory earthquakes, local fault slip and volumetric deformation were recorded using eddy-current displacement sensors (1.25 MHz) and high-speed DIC (10 kHz). Emitted acoustic waves were recorded at 3.125 MHz using an array of 33 piezoelectric sensors calibrated through ball-drop tests, active-ultrasonic survey, laser vibrometry, and spectral-element waveform modeling.

The experiments produce a broad spectrum of slip behaviors, from stable creep, slow ruptures to fast, dynamic events. Transitions from slow to fast slip are promoted by increasing normal stress and decreasing elastic stiffness. Co-seismic slip, peak slip velocity, and high-frequency acoustic energy increase systematically with cumulative fault slip, increasing normal stress, and decreasing loading stiffness. Direct measurements of slip enable estimation of the critical nucleation length, which decreases with increasing cumulative slip and normal stress, in agreement with theoretical predictions. Finite-element modeling shows that the experimental geometry induces heterogeneous stress distributions promoting the development of heterogeneous shear fabrics and spatially variable frictional responses. When shear fabric is well developed, normal stress is low, and the nucleation lengths are correspondingly large, stress heterogeneities have little impact  on slip dynamics, which is dominated by system spanning events with regular, periodic seismic cycles. Conversely, at higher normal stress—conditions associated with smaller nucleation lengths—and/or poorly developed shear fabric, stress heterogeneity drives complex slip behavior, including partial and full ruptures and rupture cascades characterized by strongly spatially variable stress drops. These results highlight how the coupled evolution of stress, shear fabric, and frictional heterogeneities controls slip dynamics in gouge-filled faults.

How to cite: Mastella, G., Volpe, G., Van den Ende, M., De Solda, M., Corbi, F., Funciciello, F., Marone, C., and Marco, S.: Heterogeneous Stress–Driven Shear Localization Governing Fault Slip in Decimeter-Scale Quartz Gouge under Variable Surrounding Stiffness, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14558, https://doi.org/10.5194/egusphere-egu26-14558, 2026.

How to cite: Barras, F. and Brantut, N.: Bridging inner fault shear localisation and the propagation of earthquake rupture, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15062, https://doi.org/10.5194/egusphere-egu26-15062, 2026.

Earthquakes represent the most visible manifestation of tectonic stress accumulation and release within the Earth’s lithosphere. Despite their fundamental importance, the absolute stress levels that drive earthquake rupture remain poorly constrained. In particular, fault shear strength is strongly influenced by fluid pressure at depth, yet its magnitude and variability, especially within the locked zones of megathrusts and across different tectonic regimes, are still not well understood. In this study, I estimate the shear strength of the lithosphere by quantifying the total energy budget of large earthquake ruptures. The total released energy is partitioned into radiated energy and energy dissipated during fault slip, commonly referred to as breakdown (or fracture) energy (G). Radiated energy can be robustly estimated from seismic waveforms or earthquake source time functions. In contrast, reliable estimates of fracture energy are more challenging. To address this, I employ a finite-width slip-pulse model for steady-state dynamic rupture propagation (Rice et al., 2005), combined with heterogeneous kinematic rupture models of large earthquakes. The analysis is based on 208 finite-fault rupture models from the NEIC database, spanning Mw ≥ 7 earthquakes between 1990 and 2025. The adoption of a self-healing pulse rupture framework is motivated by the widely observed property that earthquake rise times are approximately an order of magnitude shorter than total rupture durations, on average about one-seventh of the rupture time in my database.

My results indicate that fracture energy (G) is strongly controlled by the heterogeneous distribution of rupture velocity, rise time, and slip during earthquake rupture. Fracture energy is underestimated by approximately a factor of five when heterogeneity in the rupture process of large earthquakes is neglected, and by nearly an order of magnitude when estimates are based on a classical crack rupture model. Assuming that megathrust earthquakes undergo an almost complete strength drop during rupture, as observed for the 2011 Tohoku earthquake, our estimates represent lower bounds on fault shear strength across global subduction zones and the oceanic lithosphere. The results reveal pronounced fault weakening during megathrust ruptures, with a global average shear strength of approximately 6 MPa. Tsunami earthquakes correspond to the weakest faults, with shear strengths on the order of ~2 MPa, implying that fluid pressures are extremely elevated across most subduction interfaces worldwide. Using these shear strength estimates, I infer a global average pore-fluid pressure ratio (λ = P_f / σ_lith) of approximately 0.9 for subduction megathrusts. In contrast, the oceanic lithosphere at mid-ocean ridges, transform faults, and fracture zones is nearly an order of magnitude stronger, indicating fluid pressures close to hydrostatic conditions. These pronounced contrasts demonstrate that fluid pressure may play a first-order role in controlling the strength of the Earth’s lithosphere.

How to cite: Pulido, N.: Estimation of fault fracture energy and shear strength drop in large earthquakes: Implications for fluid pressure and tectonic regime, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16466, https://doi.org/10.5194/egusphere-egu26-16466, 2026.

Understanding earthquakes mechanisms still represents a challenge. The complexity of both the fault zone and the fault behaviour requires to make some simplifications and to downscale the studied system.

 In our work, we aim at creating a down-scaled experimental fault model where the behaviour of the asperities and the shearing of the granular gouge are both considered. In order to do so, we borrow from the tribological approach the pin-on-disk experiment: the original experimental apparatus consists in a centimetric pin with a hemispherical extremity representing the fault asperity while a large flat rotating disk stands for the opposite surface of the experimental fault. Both parts are made in the same carbonate rock with controlled roughness. Co-seismic conditions (Velocity in the range [0.001 – 1]  m/s, Normal stress in the range [4 – 400] MPa) are applied during the different experimental tests. A number of high-sampling-rate sensors are used to constrain the observation of the asperity-track contact during the simulated seismic events. Moreover, complete post-mortem analyses of the contact surfaces allow to quantify the mechanisms and to reconstruct friction scenarios in accordance with the time-series acquired during tests.

In a previous work, we determined the conditions most representative for a mature lab-fault. In the present study, we focus on the changes in wear and friction behaviours in the lab-fault linked to the slip velocity variations and the presence of granular gouge. Wear is mostly dependent on the slip velocity and the granular gouge layer thickness obtained at the mature conditions appears as an optimal thickness to limit wear. Here, velocity weakening is observed, with dramatic consequences on the microstructure of the contact surfaces. SEM and optical images show evidences of the combination of high stresses and heating on the first layer of minerals of the contact zone.

How to cite: Clerc, A., Mollon, G., Ferrieux, A., Lafarge, L., and Saulot, A.: Microstructural and Frictional Consequences of Slip Velocity Variations: Insights from a single-asperity lab-fault experiment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16867, https://doi.org/10.5194/egusphere-egu26-16867, 2026.

Understanding the mechanisms controlling fault slip requires dedicated experimental setups that bridge the gap between natural conditions and observable scales. These experiments may require the use of analogue materials adapted to the scale of the laboratory, which present interesting characteristics facilitating the observation of physical processes. Polystyrene, due to its low strength and elastic properties, and structure, offers an effective analog material to investigate mechanical fault processes at both large (metric) and small scales (cm). Its low elastic properties slows down deformation processes and enables in-situ small scale observations using X-ray microCT.

In this study, we performed uniaxial compression experiments on small polystyrene blocks with a pre-cut slip interface. Polystyrene of various initial densities were tested. Compression rates ranged from 0.1 to 1 mm/min to induce different slip modes, from slow slip to dynamic stick-slip events. Real-time 2D X-ray radiography was coupled with mechanical monitoring to capture the onset and evolution of slip along the interface. Additionally, 3D scans were acquired at various stages during compression, with the objective of evaluating the spatial distribution of deformation around the fault plane over time.

The aim of this study is to combine mechanical data and imaging in order to characterize internal density changes associated with deformation. Preliminary observations seem to highlight density contrasts in the bulk material around the fault plane, offering insight into potential precursory signs of slip.

This approach demonstrates the potential of X-ray microCT for high-resolution monitoring of analog fault models, with perspectives for quantifying strain localization and post-slip damage patterns. These results may contribute to the understanding of frictional behavior and rupture dynamics in scaled experiments.

How to cite: Walter, B. and Bonnelye, A.: In-situ X-ray imaging of stick-slip behavior in small-scale polystyrene fault analogs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17231, https://doi.org/10.5194/egusphere-egu26-17231, 2026.

Understanding the faulting dynamics of natural earthquakes is fundamentally limited by the scarcity of near-source observations and the incompleteness of knowledge about in situ conditions at depth. The Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab) in Switzerland addresses these limitations through controlled hydraulic stimulation experiments that generate seismicity beneath more than 1 km of rock overburden, thereby bridging the scale gap between laboratory studies and observed natural earthquakes. A key advantage of the BedrettoLab is the ability to characterize in situ conditions prior to seismicity induction. This includes imaging the geometry of the target fault, estimating the local stress state and pore fluid pressure, and examining host and fault rock properties.

Seismicity induced by controlled hydraulic stimulation is recorded by a comprehensive suite of near-source instrumentation, including strong-motion seismometers, borehole geophones and accelerometers, high-frequency acoustic emission sensors, and fibre-optic cables enabling Distributed Acoustic Sensing (DAS) and Fibre Bragg Grating (FBG) measurements. Past experiments have successfully generated seismicity sequences with mainshock magnitudes between Mw −0.5 and 0.0. We construct dynamic rupture models for one such mainshock, constrained by the available near-source observations, to image slip distribution, rupture directivity, and rupture velocity at meter-scale resolution. We find that rupture directivity has a substantially stronger impact on spectral amplitudes than average stress drop. The inferred stress and friction drops are interpreted in terms of the maximum possible confining pressure, providing insights into dynamic weakening processes during earthquake rupture.

The next experiment aims to induce Mw 1.0 earthquakes along a selected fault zone. Using constraints from hydraulic fracture tests, fault geometry imaging, and injection protocols, we seek to forecast the potential rupture dynamics of the induced mainshock by generating a suite of dynamic rupture models representing plausible rupture scenarios, against which the observed mainshock dynamics can be evaluated. In particular, we assess how reliably pre-experiment slip tendency analyses translate into the actual rupture behavior under these controlled conditions. Ultimately, this research will advance our understanding of earthquake source physics and contribute to improved forecasting and mitigation of worst-case scenarios associated with hydraulic stimulation.

How to cite: Schliwa, N., Mosconi, F., Tinti, E., Lambiase, A., Tuinstra, K., Gabriel, A.-A., Rinaldi, A. P., Meier, M.-A., Cocco, M., and Giardini, D.: Imaging and forecasting rupture dynamics of induced microearthquakes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17445, https://doi.org/10.5194/egusphere-egu26-17445, 2026.

Fault slip emerges from coupled frictional and hydromechanical processes, yet forecasting stress evolution remains challenging when fluid pressurization, drainage, and dilatancy modulate effective normal stress. We present laboratory double-direct shear experiments on quartz-rich natural fault gouge conducted under dry, 100% humidity, and constant fluid-pressure boundary conditions. Throughout quasi-periodic and irregular seismic cycles, we continuously acquire active-source ultrasonic waveforms transmitted across the gouge layer and derive cycle-resolved acoustic observables: P- and S-wave velocity changes and amplitude-based transmissivity metrics (band-limited RMS).

For our range of conditions, the acoustic properties exhibit robust, two-stage precursory evolution. During the early interseismic phase, acoustic transmissivity increases with contact stress, consistent with progressive asperity contact growth and rising contact stiffness (“healing”). This is followed by a late interseismic-to-preseismic transition characterized by gradual bulk velocity reduction, interpreted as distributed inelastic creep and microcrack growth within the gouge. For the 100% humidity condition, velocity changes track slip velocity, peaking as the fault locks and decreasing prior to dynamic slip. Under constant external fluid pressure, partial drainage and localized undrained behavior further modulate both elastic velocity and transmissivity through shear-induced porosity changes and associated pore-pressure transients. Localized slip regions can show transient acoustic velocity increases consistent with dilatancy hardening, while the bulk response trends toward overall velocity decrease as failure approaches.

We develop a mechanistic poromechanical framework that links the ultrasonic observables to evolving contact stiffness, porosity, and effective stress, providing a physical basis for interpreting travel-time and amplitude changes under fluid pressurization. As an additional validation, a lightweight sequence model trained on the acoustic observables can reconstruct cycle-scale shear-stress evolution and event timing, demonstrating that the acoustic measurements encode the state of the fault. These results highlight the role of fault zone elastic properties for detection of precursory processes prior to earthquake failure and illuminate the processes that occur during the preparatory stages of earthquake nucleation for fluid-saturated fault systems.

How to cite: Affinito, R., Yu, P., Elsworth, D., and Marone, C.: P- and S-wave precursors to lab earthquakes under variable drainage and fluid pressurization, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17533, https://doi.org/10.5194/egusphere-egu26-17533, 2026.

Earthquakes originate from frictional instabilities that nucleate and propagate along faults cutting through a multilayered crust. Geological observations show that fault zones are complex structures often composed of heterogeneous mineral assemblages with contrasting frictional rheologies, whose interaction strongly influences slip behavior during fault reactivation. Two major earthquakes (Mmax > 6) that struck central Italy in the past 30 years–the 1997 Colfiorito and 2016 Norcia events–nucleated within Triassic Evaporites (TE) consisting of anhydrites and dolostones. Previous laboratory experiments on stick-slipping faults in TE gouges highlighted the key role of shear zone fabric in controlling breakdown processes and slip dynamics. However, the individual contribution of each lithology to frictional failure mode remains unclear.

Here we present preliminary results from laboratory friction experiments on powdered TE samples aimed at disentangling the role of anhydrite and dolostone in controlling fault slip behavior and dynamics in TE faults. We conducted double-direct shear experiments on three gouge compositions: 100% anhydrite, 50:50 anhydrite-dolostone, and 100% dolostone. The experimental procedure comprises two main stages: (i) a fabric development phase, in which the gouge is sheared under 50 MPa normal stress at a load-point velocity of 10 μm/s; and (ii) a fault reactivation phase at boundary conditions designed to enhance frictional instabilities that are: normal stress reduced to 30 MPa, a low-stiffness element inserted in series with the shear loading axis, and re-shearing at 1 μm/s. We also monitored the evolution of the fault physical properties from fabric development to stick-slip, via an ultrasonic system that continuously transmits and receives acoustic waves through the experimental fault.

During the fabric development stage, all three fault gouges display stable sliding with a friction coefficient μ of ~ 0.6. In the fault reactivation stage, anhydrite faults exhibit slow (v < 20 μm/s), repetitive stick-slip with small stress drops (Δ𝛕 < 0.1 MPa), whereas dolostone faults accommodate shear through stable sliding. Interestingly, the 50:50 anhydrite-dolostone mixture does not exhibit intermediate behavior between the two  end-members but instead develops larger (0.1 < Δ𝛕 < 0.4 MPa), yet generally slower (v < 10 μm/s), slip instabilities, indicating a nonlinear mechanical interaction between anhydrite and dolostone.

Post-experiment microstructural analyses reveal that single-component gouges deform via cataclastic flow and frictional sliding along boundary and Riedel shear bands. In contrast, the 50:50 mixture exhibits extremely localized boundary shear planes dominated by nanometric dolostone particles embedded within foliated anhydrite-dolostone S-C structures. These features suggest a significant contribution of ductile, distributed deformation to energy dissipation during slow frictional ruptures. Ongoing ultrasonic wave analyses aim to characterize the relationship between the evolution of the elastic properties of the fault gouge, its internal structure, and the resulting slip behavior.

Our results provide new insights into the complex interplay between different frictional rheologies within fault zones of the seismogenic layer of northern Apennines, and highlight the role of compositional heterogeneity in controlling fault slip dynamics and energy dissipation.

How to cite: Guglielmi, G., Mauro, M., Scuderi, M., Collettini, C., and Trippetta, F.: Complex interaction between brittle and ductile rheologies in slow laboratory earthquakes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18859, https://doi.org/10.5194/egusphere-egu26-18859, 2026.

The physics underlying earthquake precursory phenomena remains poorly constrained, particularly in crustal materials containing long preexisting fractures.  We conduct axial compression experiments on pre-heated Fontainebleau sandstone with acoustic emission (AE) monitoring under confining and pore pressures of 30 and 5 MPa, respectively.  Preliminary results show no systematic differences in precursory AE behavior between pre-heated and as-is samples.  All samples exhibit b values between 0 and 1, with a wide range of overall b-value evolution toward failure, including both increasing and decreasing trends.  Despite this variability, many samples show a local decrease in b value immediately before and during failure, preceded by a local increase near peak stress.  These local b-value variations likely reflect distinct microfracturing processes in porous granular rocks, contrasting with the more monotonic b-value decrease commonly reported for crystalline rocks.  Our results suggest that subtle differences in initial granular microstructure promote diverse precursory behavior under otherwise identical experimental conditions.  Such variability may contribute to the range of precursory behavior observed in tectonic earthquakes, where many large events are not preceded by a decrease in b value.  To investigate the role of aseismic deformation in fault nucleation, we are currently quantifying the evolution of microfracture distributions in deformed samples.  In parallel, we are deforming samples containing long preexisting fractures by first pre-deforming them in the semibrittle regime at higher pressure–temperature conditions, followed by deformation to failure at lower pressure–temperature conditions.  These experiments constrain the evolution of seismic and aseismic precursory signals toward large earthquakes in highly fractured crust.

How to cite: Kanaya, T.: Effect of long preexisting fractures on fault nucleation processes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22410, https://doi.org/10.5194/egusphere-egu26-22410, 2026.

The roughness of natural fault surfaces means that faults make contact at discrete, high-stress bumps and other geometrical highs, producing extreme spatial heterogeneity in normal stress on the sliding interface. This heterogeneity plays a crucial role in nucleating and arresting earthquake rupture. However, heterogeneity resulting from contact of rough surfaces has not been systematically tested in laboratory experiments, which have previously been restricted to nominally flat surfaces. We investigate how the spacing of macroscopic contact regions controls slip stability by shearing cement blocks designed and manufactured to have prescribed contact spacing, as well as replicas of a natural fault surface. Arrays of hemispherical bumps were manufactured with initial spacings of 22-235 mm. Experiments were conducted in direct shear at normal loads ranging from 1 to 10 kN, resulting in local contact stresses of ~20-120 MPa.

Across 27 experiments, sliding ranged from stable creep to unstable stick-slip behavior. Instability is controlled by the minimum spacing between adjacent contact regions (λc), which evolves during wear. Faults remain stable when λc is less than the critical nucleation length Lc predicted by fracture mechanics (tens to ~180 mm for our measured G, Dc, σ, and Δf). When λc exceeds Lc, stick-slip initiates regardless of overall friction coefficient or surface type (regular hemisphere arrays, random bumps, or natural fault replicas). Increased contact normal stress also promotes instability by reducing Lc. These findings are corroborated by a case study of a single experiment, in which λc increased abruptly upon the loss of a few individual contacts, resulting in the immediate transition from stable to unstable sliding that occurred precisely as λc crossed Lc, independent of changes in contact radius.

Our results demonstrate that the spacing of high-stress contact patches may significantly influence slip stability on faults. Because this spacing length scale can be directly observed on real fault surfaces, it provides a physically grounded predictor of where rupture can nucleate or arrest across scales from hand samples to fault segments.

How to cite: Baumgarte, J., Yakuden, L., and Kirkpatrick, J.: Contact Stress Distribution and Slip Stability on Experimental Faults, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-313, https://doi.org/10.5194/egusphere-egu26-313, 2026.

Nephrite jade is a monomineralic rock composed of fine-grained interwoven amphibole fibres of the tremolite-actinolite series. This rock has a great cultural importance for many countries due to its combination of beauty, toughness, and high fracture resistance. In New Zealand, nephrite jade (pounamu) is culturally significant to Māori, with a documented history of use in the manufacture of tools, weapons, jewellery, and talismans. Despite this historical importance, there is a lack of systematic quantitative data on the physical and mechanical properties that contribute to its workability and durability.

Traditionally nephrite jade’s suitability for carving has been relying on carvers’ expertise and empirical knowledge. Drawing on the collective experience of Māori artisans, a preliminary survey of pounamu jade carvers (n=20) ranked the quality of a collection of specimens, showing positive correlation between perception of material quality and measured anisotropy in compressional (P-) wave velocity. Current research aims to evaluate the intrinsic origin of this anisotropy and it’s link to mechanical properties through combined microstructural and mechanical analysis.

Electron backscatter diffraction (EBSD) maps of three samples characterised microstructural patterns associated with perceived quality. A sample subjectively rated by carvers as high-quality is noted for homogeneous microstructure with a relatively small grain size (Equivalent diameter on average <8 μm), a weaker crystallographic preferred orientation (CPO) (M-index <0.04), and a lower density of pre-existing microcracks. In contrast, samples identified as poor quality exhibit a stronger CPO (M-indexes 0.14 and 0.09, J-indexes 5.36 and 3.66), particularly within larger grains, greater grain size variability (Equivalent diameter from first μm to 120 μm), and a generally coarser grain size.

Fracture toughness measurements (K1C) were conducted on 3×4×25 mm samples using a universal testing machine equipped with a four-point bending setup, following ISO 6872. Results of these measurements correlate with microstructural observations above. A sample with smaller, more homogeneous grain sizes demonstrates higher fracture toughness. This relationship is consistent with previously described toughening effect related to crack deflection. In nephrite jade, the fracture path is interpreted to be deviated or deflected around the fibres, thereby increasing the effective fracture surface energy. Thus, the coarser microstructures observed in lower-quality samples can contribute to a reduction in this toughening effect, leading to lower fracture resistance. A higher density of pre-existing microcracks observed in coarser-grained samples also leads to lower fracture toughness

Thus, the empirical assessments of nephrite jade quality by carvers correlate with quantifiable microstructural parameters, where a fine-grained, homogeneous fabric with weak CPO promotes crack-deflection toughening and better fracture resistance.

How to cite: Seliutina, N., Palmer, M., Li, K. C., Prior, D. J., Cox, S. C., Mortimer, N., Ford, A., and Kuo, L.-W.: Microstructural controls on the quality and fracture toughness of New Zealand nephrite jade (pounamu), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2630, https://doi.org/10.5194/egusphere-egu26-2630, 2026.

As society works to reduce carbon emissions and phase out fossil fuels, new and sustainable energy sources are increasingly sought. Hydrogen is typically viewed as an energy carrier, but naturally occurring geogenic hydrogen has emerged as a potential primary energy source. Its appeal, namely zero carbon footprint and continuous generation through subsurface processes, is tempered by major uncertainties. Although the mechanisms that produce natural hydrogen are reasonably well understood, successful exploration cases remain rare and its migration pathways and interactions within the subsurface are poorly constrained.

This project investigates hydrogen migration from a likely ophiolitic source along an active, highly segmented strike‑slip fault system in the Neogene Lavanttal Basin (Austria). We combine short‑term and long‑term soil‑gas measurements with subsurface information from the recently completed Koralm railway tunnel and vintage 2D seismic data. The integrated dataset suggests a possible link between elevated near‑surface hydrogen concentrations and structural features such as subsurface faults and surface lineaments. If confirmed, these results would improve our understanding of hydrogen migration in faulted crust and support more reliable site selection for future natural hydrogen exploration and production.

How to cite: Bezwoda, N., Schöpfer, M., Grasemann, B., and Tari, G.: Fault‑Controlled Migration of Geogenic Hydrogen in the Lavanttal Basin (Austria), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5645, https://doi.org/10.5194/egusphere-egu26-5645, 2026.

Natural hydrogen is emerging as a promising sustainable energy source with a negligible carbon footprint. Among the most striking surface indicators of subsurface hydrogen production are “fairy circles”, distinctive, sub‑circular features marked by anomalous vegetation patterns and subtle topographic depressions, often with depth‑to‑diameter ratios as low as 1:100. While previous numerical studies have examined soil‑gas hydrogen anomalies associated with active fairy circles, the mechanism responsible for the observed surface subsidence has remained unclear.

Here, a geomechanical model grounded in soil‑mechanics principles is developed to explain the formation of these depressions. Using coupled simulations of two‑phase flow and volumetric deformation driven by changes in effective stress, the model reproduces surface expressions consistent with those observed in natural hydrogen‑emitting fairy circles. These results provide a physically plausible mechanism for the development of fairy‑circle topography and offer a framework for interpreting surface indicators of subsurface hydrogen generation.

How to cite: Schöpfer, M., Detournay, C., and Tari, G.: The formation of hydrogen-emitting “fairy circle”, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7492, https://doi.org/10.5194/egusphere-egu26-7492, 2026.

The mechanical response of polymineralic rocks is the result of the combination of the individual strengths of all the mineralogical phases that are included in it. But which percentage of a softer mineral is sufficient to soften the mechanical strength of the entire rock? And, on the other hand, which percentage of a stronger material is enough to increase the strength of a soft rock? And do these percentages depend on the microstructure?

Gypsum-anhydrite rocks have the requisites to answer these questions, due to the high mechanical contrast between anhydrite and gypsum mineralogical phases and to the high heterogeneity of microstructures that are usually present even in a single rock mass because of the low temperatures of the phase transition between gypsum and anhydrite.

For these reasons, the present study investigated the strength and creep response under uniaxial compression of Triassic sulphates from the Italian Western Alps. The samples considered may be clustered in three main groups, depending on the occurring microstructural organization of gypsum and anhydrite mineralogical phases: i) pure gypsum, ii) gypsum with relicts of anhydrite at the nuclei of the crystals and iii) anhydrite with gypsum bordering the rims among the crystals.

Results of mechanical tests showed that even a low percentage of anhydrite present at the nuclei of the gypsum crystals strongly controls the mechanical response, causing an increase in the uniaxial strength from 20-25 MPa to 50-70 MPa. On the other hand, the presence of small quantities of gypsum at the rim of anhydrite crystals implies a decrease of mechanical strength of up to 50% with respect to the values expected for pure anhydrite.

Unlike the results about strength, creep strain rate data in gypsum showed a high predictability, suggesting that time-dependent deformation is mainly controlled by mechanisms occurring at the rim of crystals (e.g., pressure solution).

How to cite: Caselle, C., Paschetto, A., Baud, P., Bonetto, S., and Mosca, P.: Mechanical control of soft mineralogical phases on the global strength of the rock: the case of anhydrite and gypsum, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9311, https://doi.org/10.5194/egusphere-egu26-9311, 2026.

Accurately acquiring elastic and anisotropic parameters is  critical for hydrocarbon prediction. Studies have shown that pore and fracture aspect ratios significantly influence the elastic and anisotropic properties of rocks. However, obtaining accurate pore aspect ratio data is extremely difficult and costly, and conventional logging data typically lack this information. Consequently, pore and fracture aspect ratios are generally assumed to be constant based on experience, which does not accurately reflect the actual geological conditions of the formation. To address this limitation, this study proposes a nonlinear petrophysical inversion method based on the Tetragonula Carbonaria Optimization Algorithm (TGCOA), an algorithm inspired by the nest-building and temperature-regulating behavior of tetragonula carbonaria, notable for its structural simplicity and fast convergence. First, a complex fractured-vuggy petrophysical model and inversion objective function are developed by integrating the Xu-White dual-pore model with the Eshelby-Cheng model. Then, constrained by measured acoustic logging data, the TGCOA global optimization algorithm is employed to perform nonlinear petrophysical inversion, solving for the pore and fracture aspect ratios. Finally, these estimated ratios are used as inputs for the petrophysical model to calculate the elastic and anisotropic parameters of rocks. This method comprehensively utilizes various well-logging data to obtain more accurate elastic and anisotropic parameters. Application of the proposed approach to field data in eastern China demonstrates its high computational efficiency and accuracy.

How to cite: Xiong, Z., Yin, X., Ma, Z., and Xiang, W.: Petrophysical inversion of pore and fracture aspect ratios in complex fractured-vuggy reservoirs using TGCOA algorithm, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12392, https://doi.org/10.5194/egusphere-egu26-12392, 2026.

Digital image correlation (DIC) is a powerful tool in lab-scale rock mechanics, yet its application to cylindrical samples is compromised by perspective-induced distortions. These optical effects lead to significant inaccuracies in measuring deformations, especially impacting the reliability of Poisson's ratio calculations. To address this, we developed a specialised preprocessing workflow to rectify raw images before correlation.

The proposed method uses a custom Python script that performs image denoising, camera calibration, and lens distortion correction and an unwrapping algorithm that projects the cylindrical surface onto a 2D plane, effectively "flattening" the sample geometry. This allows standard 2D DIC software, such as NCORR, to process the data without the geometric bias inherent in radial perspectives.

To validate the workflow, results were benchmarked against a 3D-DIC system and physical sensors. Preliminary data shows that our rectification process significantly improves displacement accuracy on lateral surfaces, providing a low-cost yet precise alternative to complex 3D setups. This enhancement is crucial for characterising displacement over the full sample surface where traditional strain gauges are limited. Future work will focus on refining pixel-level interpolation to further minimise noise in high-strain zones.

How to cite: Chalé, A., bu, F., jaboyedoff, M., and derron, M.: Framework for mersuring deformation of cylindrical sample in 2D DIC , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12588, https://doi.org/10.5194/egusphere-egu26-12588, 2026.

Swelling in anhydritic claystones remains a major tunnelling risk. Although this phenomenon has been widely studied, knowledge gaps persist regarding its swelling behaviour. One of them being the effect of porosity on the volumetric strains that develop during the anhydrite to gypsum transformation. To address this gap, a series of laboratory tests was carried out on artificial specimens made from highly compacted mixtures of anhydrite and kaolin powders. The initial porosity was varied between 0.22 and 0.35, and volumetric strain development was monitored during gypsum formation. The experiments show that transformation-induced strains decrease with increasing initial porosity. The observations further suggest two distinct mechanisms: in more porous specimens, gypsum precipitation occurs largely within the existing pore space, reducing porosity and limiting bulk expansion; in more highly compacted specimens, gypsum growth forces matrix expansion, leading to larger macroscopic swelling. These results are applicable to porous media where crystallisation may occur within pores. Overall, the experimental campaign provides observations and a dataset that can support the development and calibration of coupled chemo-hydro-mechanical models for anhydrite swelling, enabling more realistic predictions of strain development due to gypsum growth in tunnelling applications.

How to cite: Nousiou, A. and Pimentel, E.: Effect of porosity on anhydrite swelling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12906, https://doi.org/10.5194/egusphere-egu26-12906, 2026.

Flysch rock masses pose significant challenges in geotechnical design due to their pronounced heterogeneity, anisotropy, and susceptibility to disintegration, particularly in deep urban excavations. These characteristics often result in unpredictable slope behavior, requiring a careful combination of geotechnical modelling and practical experience.

This study presents a case of an excavation approximately 11 m high, constructed for a building in the Pujanke area of Split, Croatia, situated in typical Dalmatian flysch. The original design proposed a wide excavation with relatively gentle slopes, based on conservative geotechnical parameters previously validated in similar conditions through the authors’ prior studies. During construction, the designed geometry was not fully respected, and significantly steeper slopes were implemented, leading to localized slope failure. This situation provided a rare opportunity to observe classical disintegration mechanisms, layer interactions, and the influence of flysch heterogeneity on excavation stability, complementing insights from previous research.

Following the collapse, a remediation project was successfully implemented. However, due to subsequent modifications of the excavation geometry and construction conditions, additional design iterations were required on the same slope. These successive redesigns illustrate the core of rework in AEC design, where changes in fundamental assumptions such as geometry, boundary conditions, and construction phasing necessitate repeated reinterpretation of the same geotechnical problem.

A back-analysis of slope stability demonstrated a strong correspondence between previously proposed design parameters and the actual behaviour of the rock mass, confirming their appropriateness and highlighting the critical importance of strict adherence to design assumptions during execution. The study further discusses various technical solutions and their robustness against potential deviations from planned conditions, including minor slope modifications and reinforcement measures.

The results contribute to a better understanding of the behaviour of flysch rock masses in deep excavations and provide practical guidance for safer and more resilient geotechnical design in urban areas with heterogeneous soft rocks, enabling a more stable continuity of design assumptions and a reduction in rework in AEC design.

How to cite: Vlastelica, G. and Salvezani, D.: Geotechnical Design and Rework in Flysch Excavations: A Case Study from Split, Croatia, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13985, https://doi.org/10.5194/egusphere-egu26-13985, 2026.

The characterisation of tuffaceous soft rocks represents a substantial challenge for slope stability evaluation. The geomechanical behaviour of these materials depends heavily on the microheterogeneity of pyroclastic rocks and varies considerably based on the degree of alteration and water-rock interaction processes.

Tuffaceous lithologies outcrop in a wide variety of areas worldwide, often forming landslide-prone vertical cliffs, both in coastal and continental settings. These instability processes are controlled by progressive rock failure (PRF) mechanisms that govern the enucleation and propagation of fractures, the evolution of which can lead the slope to a state of instability. PRF process can be numerically analysed by adopting hybrid stress-strain numerical solutions able to capture the transition from continuum to discontinuum behaviour. However, these tools require micromechanical input parameters governing the fracture mechanics (e.g., fracture toughness/energy), which are often difficult to calibrate numerically.

This study focuses on the tuffaceous cliffs of Ventotene Island (Italy), highly susceptible to rock-falls and topples and exposed to sea-wave actions, combining laboratory testing and numerical modelling. In the site the mechanisms of progressive fracturing in this type of soft rock and its relationship with environmental forcings are deepened through the design and implementation of the “Ventotene Field Laboratory”. This natural field laboratory, part of EPOS Field-Scale Laboratories, allows the integration of field observation, in situ monitoring data and numerical investigations.

To characterise the mechanical behaviour of the Ventotene tuffs, uniaxial compressive strength (UCS), indirect tensile (Brazilian) and fracture toughness (FT) tests were performed at Aachen Rock Mechanics Laboratory on representative rock samples under both dry and saturated conditions. The laboratory results highlight a strong influence of water content on the mechanical properties of the tuff, with a marked reduction in strength and stiffness under wet conditions. In addition, the thermal properties of the material were also investigated to support thermo-mechanical analyses.

The laboratory test results were used to provide (micro)mechanical input parameters to a FDEM slope numerical model using the Irazu software (Geomechanica Inc.). Overall, the results show that numerical calibration is essential to obtain a tuned parametrisation of tuffaceous soft rocks and to bridge the gap between laboratory-scale measurements and field-scale responses.

The laboratory tests were numerically simulated, and the calibrated parameters have been transferred to a numerical domain representative of the Ventotene sea cliffs. This latter model served to perform accurate slope stability analysis of coastal cliffs, by combining the action of different (marine and environmental) controlling factors.

The calibrated micromechanical parameters also provide a robust basis for future modelling FDEM studies since calibrations of this nature have rarely been conducted on tuffaceous lithologies.

How to cite: Montagnese, M., Feliziani, F., Marmoni, G. M., Grechi, G., Lemaire, E., Hamdi, P., and Martino, S.: FDEM numerical calibration of mechanical properties of tuffaceous rocks for slope stability analysis , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14093, https://doi.org/10.5194/egusphere-egu26-14093, 2026.

Compacting granular systems composed of brittle materials not only deform but can also fracture. In these systems, stress transmits across grain contacts and forms force chain networks. A fracture can occur when the heterogeneously distributed stress becomes locally high and critical. Here, we use photoelasticity to visualize the evolution of stress distributions within a uniaxially loaded, homogeneous 2D array of discs made of soda-lime glass, a transparent, isotropic non-crystalline material. We run experiments in water-saturated and nominally dry conditions, through loading, holding and unloading periods. We optically and acoustically (via acoustic emissions) monitor the evolution of stress field, bulk deformation, and crack propagation. Photoelasticity data are analyzed by image recognition and processed to map stress distributions. 

Preliminary results show five characteristics that set the fracturing of granular matter apart from continuum solids. First, we can extract the stress transmissions, which, despite the macroscopic homogeneity, show force chains and an inhomogeneous stress field. Second, these stress concentrations lead to a local excess of strength and disc fractures. The birefringence patterns in individual discs are altered by fractures but still carry load. During unloading, the fractures can slip or frictionally lock and the stress acting on them don’t fully relax. Third, unloading and reloading cause cracking before reaching the previous target. Fourth, cracking continues during holding periods in a time-dependent manner; perhaps subcritical crack growth redistributes stresses and thus leads to cascades or spurts of acoustic emission events. Finally, homogeneously highly stressed subdomains of discs develop that confine grains and thus suppress localization and fracturing.

How to cite: Nakagawa, S., Voigtländer, A., Zhang, Y., and Gilbert, B.: Stress evolution within a granular system undergoing subcritical failure: insights from photo-elastic imaging of a 2D glass disc pack, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15924, https://doi.org/10.5194/egusphere-egu26-15924, 2026.

Reliable characterization of crystalline geothermal reservoirs requires linking rock microstructure to effective physical properties across scales, from pore to reservoir level. Digital rock physics (DRP) provides a promising framework by combining three-dimensional imaging with numerical simulation. However, established DRP workflows for sedimentary rocks are often insufficient when applied to crystalline lithologies. Low porosity, complex mineral intergrowths, fine inclusions, and alteration textures complicate phase identification and introduce biases in predicted elastic, permeability, and thermal properties, limiting established DRP workflows for low-porosity crystalline rocks. This study presents a geologically driven workflow for a granitoid rock sample from the Frontier Observatory for Research in Geothermal Energy (FORGE) site in Utah, USA. High-resolution X-ray computed tomography (XRCT) of cylindrical core plugs (10 mm diameter, 40 mm length) at 6.9 µm voxel resolution provides the basis for digital pore-scale analysis. Multiphase segmentation, i.e., assigning gray-scale intensities in XRCT volume to specific mineral phases, was performed by integrating grayscale-based thresholding techniques with geological constraints derived from thin-section petrography and scanning electron microscopy (SEM). This integrated workflow reduces misclassification caused by overlapping gray-scale intensities, partial-volume effects at phase boundaries, and unresolved microporosity. The resulting segmentation distinguishes pore space, quartz, feldspar, ferromagnesian minerals (amphibole, biotite), titanite, and accessory phases (zircon, opaque oxides, apatite). Initial digital twin analysis shows results that deviate from laboratory measurements for porosity and the determined P- and S-wave velocities. We suspect that assigning completely intact single-crystal properties to the segmented phases may be incorrect, as the microstructure provides clear information about mechanical stresses, e.g., undulatory extinction or mineral alignment. Additionally, the analyzed subvolume (400³ with an edge length of 2.76 mm) does not yet constitute a representative volume element (RVE) relative to the coarse feldspar grain size (1-3 mm). This results in the following challenges for a DRP workflow in relation to crystalline rocks compared to established sedimentary rocks: (1) XRCT scans of larger field of views to encounter the larger minerals within the granitoid sample, (2) assigning reduced elastic properties to the individual segmented mineral phases due to microcracks and fluid inclusions, (3) Preserving high-resolution imaging to resolve the small volumes of porosity (~1.2 %). We present a refined DRP workflow that addresses these challenges through multi-scale imaging strategies and microstructure-informed elastic property assignments, validated against laboratory measurements on FORGE crystalline samples.

How to cite: Keutchafo Kouamo, N.-A., Balcewicz, M., Beiers, L. M., Renner, J., and Saenger, E. H.: A digital rock physics workflow for crystalline reservoirs: Developing digital twins through a geologically driven workflow, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17521, https://doi.org/10.5194/egusphere-egu26-17521, 2026.

How to cite: Kiss, A., Spagnuolo, E., Cornelio, C., Aretusini, S., Longobardi, V., Cocco, M., Taddeucci, J., and Colombelli, S.: Observations of laboratory earthquake rupture: implications for earthquake early warning, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17668, https://doi.org/10.5194/egusphere-egu26-17668, 2026.

The effects of high temperature on the tensile strength and physical properties of rocks were investigated using furnace heating and simulated fire treatments. Four rock types—basalt, granite, limestone, and sandstone—were examined under dry, wet, and saturated conditions (0%, 50–60%, and 100% water content, respectively). Tensile strength was measured before and after heating using the Brazilian test, while changes in porosity, thermal conductivity, mass, and P- and S-wave velocities were also evaluated. Thermal measurements indicate that both heating methods reached maximum temperatures of approximately 600 °C and produced comparable effects on rock properties. Initial water content had a negligible influence on post-treatment tensile strength and physical properties. In contrast, rock lithology strongly controlled the degree of thermal damage. Basalt, characterized by high initial tensile strength, exhibited minor reductions in tensile strength and wave velocities, whereas sandstone showed greater degradation. Granite and limestone exhibited pronounced reductions in P- and S-wave velocities. Rocks with higher thermal conductivity, such as sandstone, experienced larger decreases in thermal conductivity after heating, while basalt showed the smallest change. Conversely, basalt exhibited the greatest increases in porosity and mass loss. Overall, rock lithology and initial mechanical strength are the primary factors governing rock degradation under high-temperature exposure.

How to cite: Nguyen, X.-X., Blahůt, J., Racek, O., Rastjoo, G., Polezhaev, A., and Loche, M.: Assessing Changes in Rock Properties and Tensile Strength due to High Temperature from Laboratory Simulation Studies, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18000, https://doi.org/10.5194/egusphere-egu26-18000, 2026.

Predicting the geometry of natural fracture networks in the subsurface is a challenging endeavor. In this study, we present a model in which burial-related stress curves are combined with the theory of sub-critical crack growth to investigate the timing of propagation of fractures that constitute the background network, i.e. not genetically related to faults and/or folds.

To predict principal stress orientations and magnitudes as a function of crustal depth, we relate the vertical stress to the density of the overburden, and the horizontal stresses are estimated by calculating the Poisson effect caused by this overburden. In addition, we assume a tectonic stress in the direction of the maximum horizontal stress. We correct the resulting values for the effects of pore fluid pressure.

Based on the orientation and magnitude of the principal stresses at various depths, we calculate the normal and shear stresses on planes ideally oriented for opening and shear fractures (perpendicular to σ₃ and at 30 degrees with σ₁, parallel to σ₂ respectively). Following linear elastic fracture mechanics, the normal and shear stresses are used to compute the stress intensity at fracture tips and estimate the related fracture propagation rate adopting sub-critical crack growth theory.

This simplified model gives insight into the relationship between depth and i) magnitudes of horizontal and vertical stresses, ii) permutations of principal stresses and associated changes of stress regimes and iii) the magnitude of fracture stresses driving fracturing.

We test our model in the Lower Cretaceous limestones of the Geneva Basin, a naturally fractured formation targeted for geothermal exploitation. The burial curve of this formation is marked by two distinct burial phases. The first is in the Late Cretaceous with maximum burial depths of +-500 m. After this, the carbonate rocks have been exhumed to the surface in the Paleogene, followed by deep burial (up to 4000 m) in the foreland of the emerging Alpes in the Miocene. Our model predicts that sub-critical fracture growth only occurred during the latest burial phase, in a reverse and strike-slip regime.  

The results are compared with analogue outcrops of the Lower Cretaceous carbonate rocks. Multiple generations of calcite veins from different mountain ranges surrounding the Geneva Basin (Jura, Vuache, Bornes Massif) are sampled for absolute dating with the U/Pb geochronology. The obtained ages confirm a change in stress regime from reverse to strike-slip in the Oligocene to Miocene times

How to cite: Hupkes, J., Bruna, P.-O., Bertotti, G., Nuriel, P., Guillong, M., and Caudroit, J.: When do background fractures form? , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18501, https://doi.org/10.5194/egusphere-egu26-18501, 2026.

In geological reservoirs, pore fluid chemistry significantly impacts rock strength through mineral dissolution, precipitation, and surface charge modifications. Understanding these interactions is crucial for geological applications such as CO₂ storage and geothermal energy applications, where fluid chemistry controls reservoir integrity, exploitability, and aging.

With increasing depth, porous rocks transition from localized to ductile deformation regimes, with the latter characterized by compactant behavior that drastically reduces permeability and affects reservoir exploitability. The role of fluid chemistry in controlling this transition remains poorly understood and varies with fluid composition, mineralogy, and stress conditions.

We performed triaxial deformation experiments on Tavel limestone (84% calcite, 7.5 % quartz and 6% phyllosilicates,14% porosity) at effective confining pressures of 20 MPa (localised regime) and 100 MPa (ductile regime) under constant strain rate (10⁻⁶ s⁻¹). Samples were tested dry and saturated with: deionized water, 0.01 M HCl solution, CO₂-water solution, CaCO₃-saturated solution, 0.1 M MgCl₂, 6 M NaCl and 0.1 M NaOH solutions. During deformation, we continuously monitored spectral electrical conductivity (0.1 Hz–1 MHz), permeability, and P-S wave velocities. Pore fluid chemistry variations were analyzed using ICP-OES, and post-mortem sample were characterized at SEM.

Results reveal water weakening in the localised regime, while in the ductile regime water or fluid chemistry only marginally affect rock strength. These findings contrast sharply with previous results on sandstone under identical conditions (Lazari et al., 2025), where chemical effects were negligible in the localized regime but caused 30-35% weakening during ductile deformation.

In the localized regime, the presence of Mg⁺² or CaCO₃ leads to a slight increase of peak stress, while the presence of HCl creates dissolution patterns on the sample, though without altering the mechanical properties of the rock in the observed timescale.

Increased pore connectivity is evidenced by increasing electrical conductivity with deformation, while calcite dissolution is testified by increased Ca⁺² concentration in the fluid after deformation.

Our results have critical implications for reservoir management: (1) carbonate integrity in shallow reservoirs is more sensitive to formation water chemistry than siliciclastic rocks; (2) CO₂ injection requires careful assessment and evaluation of long-term processes; and (3) rock-specific understanding of chemical-mechanical coupling is essential—behaviors cannot be extrapolated across lithologies. These findings underscore the importance of accounting for specific rock-fluid interactions in geological reservoir management.

How to cite: Lazari, F., Meyer, G., Pluymakers, A., and Violay, M.: Effects of pore fluid chemistry on localised and ductile deformation of porous rocks., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19502, https://doi.org/10.5194/egusphere-egu26-19502, 2026.

Underground storage facilities are receiving increasing interest for a variety of geo-engineering applications in the realm of renewable and sustainable energy: geothermal systems, carbon capture and storage, nuclear waste repository and hydrogen storage. Particularly for the hydrogen storage case, this gaseous fluid in injected and withdrew cyclically, causing variations of the effective stress field of the reservoir and caprock.

Based on the Terzaghi effective stress law, changing the effective stress can be achieved by either changing the principal stress or the pore pressure. The first option, called cyclic loading and unloading, has been used extensively to study the effects of cyclic conditions on the hydro-mechanical properties of different rock types. However, the actual phenomenon occurring in a reservoir rock is the cyclic variation of pore pressure, or cyclic pressurization. The cyclic flow of pressurized fluids may mobilize particles, which can clog fluid pathways, and trigger chemical reactions such as dissolution. This leads to an alteration of the microstructure of the rock matrix differently from that caused by the cyclic loading and unloading case.

Although cyclic pressurization experiments cannot be run on every rock at the laboratory scale due to poor hydraulic properties, we chose a highly porous and permeable rock, Bentheim sandstone, which guarantee us a pore pressure equilibrium throughout a rock sample during this type of experiment. Apart from its hydraulic properties, Bentheim sandstone is regarded as a conventional georeservoir rock even at great depth, due to its mineral composition, homogeneity, micro- and macrostructure. Therefore, it has been extensively tested for a variety of applications to understand its physical and mechanical properties under changing environmental conditions.

As part of the TEN.efzn project, we performed a series of laboratory experiments on both intact and fractured rock samples, carried out in a servo-controlled triaxial apparatus, capable of simulating in-situ pressure and temperature conditions at relevant depths. By combining mechanical and hydraulic data with acoustic emission and ultrasonic velocity data, we observe that cyclic pressurization leads to higher sample compaction compared to cyclic loading and that the presence of a fracture zone leads to higher changes of the hydro-mechanical properties.

Our results suggest that the values of specific properties obtained during cyclic loading experiments underestimate the real values of reservoir rocks under cyclic fluid injection and withdrawal.

How to cite: Fazio, M., Gottlieb, M., and Sauter, M.: Experimental study on the variation of hydro-mechanical properties of reservoir rocks under cyclic loading and cyclic pressurization  , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19936, https://doi.org/10.5194/egusphere-egu26-19936, 2026.

Mode I fracture toughness, K_Ic, is a key measure of rock strength. In volcanology, K_Icis particularly relevant for dike propagation, as it quantifies the critical stress intensity factor required for fracture propagation under tensile stress. K_Ic has been extensively studied for rock types and construction materials relevant to civil engineering, mining, and hydrocarbon-related applications. However, there is a paucity of data for volcanic rock. In this study, we present K_Icvalues for basalt samples of varying porosity and texture from four different lava flows on Mt. Etna (Italy) and investigate the influence of key microstructural parameters on K_Ic.

We conducted 12 mode I fracture toughness experiments under dry, ambient pressure conditions on Cracked Chevron-notched Brazilian Disc specimens and determined K_Ic using a standardised method. Additionally, we characterised rock physical properties including porosity, elastic wave velocities, and permeability, and analysed thin sections to determine mineralogical composition and rock texture. We compared the physical and microstructural properties of the four lavas and then assessed those properties regarding any correlations with K_Ic.

The fracture toughnessmeasurements were successful for 10 of the 12 specimens, yielding K_Icvalues of 0.6–1.3 MPa·m^1/2. Average connected porosity varied between 9­ and 17%. P-wave velocities varied from 3.1 to 3.8 km/s, while permeability varied from 6.7·10^-17 to 6.3·10^-12 m².

Our fracture toughness data are consistent with experimental data from the literature, fitting the general trend of high K_Ic typically corresponding to low porosity. However, within our small data set of rather heterogenous porosity characteristics and rock textures, we observe no strict inverse correlation of K_Ic,and porosity, since the porosity range of our samples is only moderate and other microstructural factors (e.g. pore size and shape) can dominate fracture behaviour in individual cases. We observe no systematic relationship between elastic wave velocities and K_Ic.

How to cite: Papanagnou, L., Furst, S., Noël, C., Heap, M. J., Violay, M., and Urlaub, M.: The influence of porosity and microstructure on the fracture toughness of basalts from Mt. Etna: laboratory measurements, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21233, https://doi.org/10.5194/egusphere-egu26-21233, 2026.

Chemical weathering induced by reactive fluid circulation critically affects the mechanical properties of sedimentary rocks involved in subsurface energy applications such as geothermal systems and underground energy storage. This work investigates chemo-mechanical degradation in bonded granular geomaterials through a multiscale approach combining discrete and continuum modeling.

At the grain scale, Discrete Element Method (DEM) simulations are used to study dissolution-driven debonding under oedometric conditions. The evolution of the lateral earth pressure coefficient k0 used as a proxy for stress state, is analyzed as a function of cementation degree, confining pressure, initial stress anisotropy, and loading history. Progressive dissolution leads to convergence toward an attractor stress state, with k0​ stabilizing between 0.3 and 0.4 independently of initial conditions. This behavior results from the collapse of cement-stabilized force chains and chemical softening of grains.

At the continuum scale, a phase-field fracture model coupled with damage-enhanced reactive diffusion is developed, informed by micromechanically derived degradation laws from DEM simulations. The model reveals that higher initial cementation delays brittle fracture initiation, while increased acidity may induce a chemical ductilization effect that counter-intuitively postpones fracture due to localized softening ahead of crack tips. The competing effects of chemical softening and degradation of fracture toughness are quantitatively characterized.

How to cite: Rattez, H., Sac-Morane, A., Wu, F., Hu, M., and Veveakis, M.: From grain-scale dissolution to reactive fracture: A multiscale geomechanical study of chemo-mechanical couplings in reservoir rocks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21311, https://doi.org/10.5194/egusphere-egu26-21311, 2026.

The Calita landslide (Northern Apennines, Italy) is a large, complex phenomenon extending over approximately 0.9 km² and characterized by a roto-translational rockslide, in a flysch-dominated area, that evolves downslope into an earth slide–earthflow covering a length of approximately 2.5 km. The earthflow sector involves clay-rich soils derived from a chaotic and strongly tectonized melange partially mixed with the progressively degradated rocky head scarp. Landslides of this type are highly sensitive to hydro-mechanical perturbations, including pore water pressure increases and undrained–drained mechanisms induced by static loading. Moreover, the litho-structural setting controls deep filtration pathways, potentially promoting localized pressurization beneath the sliding surface.

This contribution presents ongoing work aimed at developing a geotechnical model of the Calita landslide and identifying its predisposing and triggering factors. Geological surveys, field instrumentation, laboratory tests have been integrated to characterize the hydro-mechanical behaviour of the landslide body. Direct shear, oedometer, triaxial and water retention tests were performed, allowing derivation of strength parameters, permeability, and pore pressure response under saturated and partially saturated conditions. Mineralogical analyses revealed the presence of gypsum and pyrite along the landslide’s shear surfaces, indicating possible chemo-mechanical weakening mechanisms and enhanced fluid–rock interaction.

GNSS, inclinometer, and piezometric monitoring delineated the spatial variability of displacement and hydraulic pressures, with piezometers recording artesian conditions in some portions of the earth slide-earthflow. DEM of differences were produced on high-resolution Digital Terrain Models obtained during the monitoring years, from 1973 to 2024, allowing to appreciate the areas where displacements occurred and the related mobilized volumes. Finally, numerical analyses were carried out using both finite element (hydro-mechanical) and limit equilibrium approaches to evaluate slope stability under different hydraulic regimes. The results provide a consistent geotechnical framework for future scenario analyses and mitigation planning.

How to cite: Fraccica, A., Bonasera, M., D'Angiò, D., Di Paola, G., Maggi, M., Chiessi, V., Monti, G. M., Pellegrini, F., De Simone, N., and Spagni, R.: Experimental and numerical study of a landslide in a complex geostructural context: the case of Calita (Italy), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21876, https://doi.org/10.5194/egusphere-egu26-21876, 2026.

The total organic carbon content is one of the key controls on hydrocarbon potential in unconventional shale systems. In the Indian Barren Measures Shales, TOC estimation from wireline logs remains challenging due to the strong heterogeneity, variable mineralogy, and limited core-calibrated geochemical measurements of shales. Standard empirical methods, such as the ΔlogR technique, capture first-order trends but often fail to generalise across mineral compositions. Purely data-driven machine learning models improve flexibility but may produce physically inconsistent predictions when sample sizes are small or when petrophysical responses are nonlinear.

This work presents a Physics Informed Convolutional Neural Network designed to estimate TOC from gamma ray, bulk density, and resistivity logs while embedding rock physics behaviour directly into the learning process. The network uses one dimensional convolutional filters to learn depth dependent patterns associated with organic richness, and incorporates the ΔlogR principle as a soft constraint in the loss function. The training workflow includes depth windowing and a hybrid loss function that balances data fidelity with physics consistency, which stabilises learning under limited sample availability.

Using 104 depth-indexed TOC measurements, the model was trained with five-fold cross-validation and a range of physics weighting factors. The final configuration achieved a mean absolute error (MAE) of 0.3, a root mean square error (RMSE) of 0.4, and a Pearson correlation coefficient (r) of 0.9, representing an improvement over both a standard multilayer perceptron (MAE = 0.6, RMSE =1, r =0.6) and the classical ΔlogR approach (MAE = 0.9, RMSE =1.3, r =0.4). These results show that physics informed learning provides a reliable and physically consistent framework for petrophysical characterisation in heterogeneous unconventional reservoirs, offering a generalizable workflow that integrates geological knowledge with machine intelligence to support improved formation evaluation and reduce uncertainty in reservoir assessment.

How to cite: Shaik, N. A. and Vedanti, N.: Physics informed convolutional neural network for TOC estimation in heterogeneous Barren Measures Shales, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-232, https://doi.org/10.5194/egusphere-egu26-232, 2026.

The precise quantification of mineral composition is a basis for the accurate geomechanical evaluation, brittleness assessment, and completion design of shale reservoirs. Currently, elemental logging has become an indispensable technical method for acquiring key mineral composition information. The standard industry practice involves solving an optimization problem to invert elemental dry weight fractions, measured downhole, into mineral contents. The accuracy of this inversion is fundamentally governed by a predetermined transformation matrix, which ideally requires rigorous calibration against a statistically robust suite of laboratory X-ray diffraction (XRD) analyses from rock samples. This prerequisite poses a significant constraint in poorly cored intervals, leading to substantial uncertainties in the derived mineralogy.

To reduce the core-dependency, the key innovation lies in formulating the element-to-mineral transformation as a joint inversion problem. The proposed algorithm operates by treating the transformation matrix not as a fixed input but as an optimizable variable within the inversion framework. Starting from a geologically reasonable initial model based on regional knowledge, the method employs an iterative optimization loop. In each cycle, it simultaneously adjusts the mineral volumes and refines the transformation matrix to minimize a dual-objective function. The misfit between the log-measured and model-predicted elemental yields, and a regularization term that constrains the matrix adjustments to physically plausible ranges. The iteration converges when the global error is minimized, yielding a formation-specific optimal transformation matrix alongside the final mineralogy.

The efficacy of the method was rigorously tested using data from offshore shale oil wells in China. Comparative analysis demonstrates that the mineralogical profiles produced by the iterative method achieve an excellent correlation with those derived from the XRD-calibrated approach in intervals where core data is available. More importantly, in zones lacking core control, the iterative method provides stable and geochemically consistent results. A detailed comparative analysis indicates that this method significantly enhances the prediction accuracy for critical brittle minerals such as quartz and plagioclase. The reduction in error for these key components directly translates to higher confidence in computed geomechanical properties.

In conclusion, this study presents a robust workflow that significantly enhances the reliability of mineralogical evaluation from elemental logs in core-constrained environments. The iterative inversion method reduces the critical need for extensive, expensive core-based calibration, offering a powerful and practical tool for the accurate and efficient characterization of offshore shale oil reservoirs. This advancement holds substantial value for optimizing drilling, completion, and stimulation strategies, thereby supporting the economical development of complex unconventional resources.

How to cite: Wang, M., Yin, L., and Yan, W.: An Iterative Inversion Method for Mineral Composition Evaluation in Offshore Shale Reservoirs Based on Elemental Logging, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1443, https://doi.org/10.5194/egusphere-egu26-1443, 2026.

Shale reservoirs, characterized by intricate fluid occurrence,elevated clay and organic matter (OM) contents, and diverse pore structures, present complexities that obscure the primary controlling factors of lacustrine shale conductivity and render existing saturation models insufficient in accuracy. Taking the medium-high maturity lacustrine shale of the Qing1 Member in the Changling Sag, Southern Songliao Basin as an example,  we use 2D nuclear magnetic resonance and pressure-maintained and sealed coring techniques to obtain the original fluid information of shale reservoirs. By combining geochemical, mineralogical, and geophysical properties, we investigate the impact of mineral components, physical properties, and fluid occurrence on the electrical conductivity properties of shale reservoirs. The clay and carbonate minerals play a primary role in  influencing the conductivity of shale, whereas effective porosity plays a secondary role. Conductivity variations within single lithologies are affected by total organic carbon and fluid types. A novel resistivity-saturation interpretation model, which is a function of saturation, for mature lacustrine shale (MLS) is developed based on lithologic distinctions and the influence of OM. This model identifies two primary conductive pathways: free water conduction in matrix pores and additional conduction from clay-bound water. The bound water cementation indexmwb and conductive bound water fraction Swb are introducedto reflect the impact of OM on clay additional conductivity. Compared with the Archie model developed for clay-free rockand the Indonesian model used for shales, the MLS model offers a more accurate calculation of oil saturation in MLS. Our approach makes a step ahead toward reducing uncertainty in  the evaluation of MLSs as potentially economic oil reservoirs. 

How to cite: Li, Z.: Conducting mechanism and saturation model of mature lacustrine shales: A case study of the first member of the Qingshankou Formation in the Changling Sag, Southern Songliao Basin , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1767, https://doi.org/10.5194/egusphere-egu26-1767, 2026.

Electrical imaging logging provides rich information on reservoir petrophysical properties and geological features. Fracture identification based on image logs is of great significance for accurate production prediction and reliable estimation of hydrocarbon reserves. However, in electrical imaging log images, fractures typically appear as elongated, low-contrast targets with strong sensitivity to structural continuity. Moreover, variations in formation conditions, imaging parameters, and noise characteristics across different wells pose substantial challenges to existing fracture identification methods, particularly in terms of fine-scale fracture continuity recognition and cross-well generalization. To address these challenges, this study proposes a dual-backbone deep learning framework, termed LGAF-FracNet, for fracture identification in electrical imaging logs. The proposed framework parallelly integrates a convolutional neural network and a Transformer architecture to model local texture features and global semantic relationships, respectively. Considering the circumferential structural characteristics of electrical imaging logs, a liquid ordinary differential equation–based dynamic feature evolution module, an adaptive graph fusion module, and a stripe-aware pooling strategy are incorporated to enhance the representation of elongated and subtle fracture geometries. In addition, a multi-decoder consistency supervision mechanism is introduced to improve cross-well generalization performance. The proposed method is evaluated on a dataset comprising approximately 3,000 electrical imaging log images collected from 21 wells in the Sichuan Basin, covering conductive fractures, resistive fractures, drilling-induced fractures, and bedding structures. A standardized dataset is constructed through manual annotation and data augmentation. Experimental results demonstrate that LGAF-FracNet consistently outperforms mainstream segmentation models in terms of mIoU, F1-score, and pixel accuracy, exhibiting significant advantages in fine-scale fracture continuity, morphological consistency, and cross-well adaptability. These results indicate that the proposed method provides a reliable technical solution for intelligent fracture identification and quantitative characterization in electrical imaging logging.

How to cite: Yang, H., Zhang, C., Xiong, W., and Zhang, J.: LGAF‑FracNet: A Dual‑Backbone Deep Learning Framework for Intelligent Fracture Identification in Electrical Imaging Logging, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1966, https://doi.org/10.5194/egusphere-egu26-1966, 2026.

The effective stress coefficient (ESC) is an important parameter in formation pressure prediction and formation stress estimation, which is usually obtained by experiments and by the empirical function formulas of ESC and porosity. However, the calculation accuracy of these empirical formulas is often affected by lithology and critical porosity, and their application in the whole area or multi-lithology formations is limited. In addition, shear wave velocity data is limited by cost and technical conditions in practical logging applications. Therefore, based on the Gassmann equation and the approximation of P-wave modulus and volume modulus, this study realizes a multi-lithology ESC estimation method using P-wave velocity, density, and porosity, and applies it to the logging of the study block. The dynamic ESC along the wellbore direction is obtained, and the logging dynamic ESC estimation model is corrected to verify the reliability of the method. The results show that the logging-derived ESC is mainly distributed in the range of 0.3~0.8, while the average ESC measured in the laboratory is between 0.5~0.6. The ESC of the sandstone layers with high porosity is relatively large, and that of the mudstone layers with low porosity is small. In the absence of shear wave velocity, this method can effectively estimate the ESC and further predict formation pressure, which plays an important role in oil exploration and development.

How to cite: Wang, X.: Research on the Calculation Method of Dynamic Effective Stress Coefficient Based on P-Wave Velocity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2114, https://doi.org/10.5194/egusphere-egu26-2114, 2026.

Metalliferous brine represents a globally significant strategic mineral resource. The western Qaidam Basin in Qinghai, China, harbours immense potential for deep metalliferous brine deposits, yet exploration remains limited due to technical challenges such as the difficulty in identifying deep brine-bearing reservoirs and inaccuracies in delineating their spatial distribution. This paper focuses on the core technological methodology of integrated well and seismic data for deep metalliferous brine detection and intelligent reservoir identification. It comprehensively utilises geological, seismic, and logging data from oil and gas exploration in the western Qaidam Basin to conduct research on multi-source geophysical data fusion and intelligent interpretation. A systematic analysis of brine layer response characteristics in logging (e.g., low resistivity, low natural gamma) and seismic data was conducted. For the first time, seismic forward modelling clearly defined the identification resolution limits of onshore seismic data for halite-bearing sand bodies at primary frequencies of 25–50 Hz: Effective identification requires sandbody interlayers ≥1 metre and single sandbody thickness ≥7–8 metres. Overlapping sandbodies with elevation differences <6 metres are prone to misinterpretation as single layers, while low-velocity interlayers may cause strong reflections to drown out brine-bearing layer signals. This provides crucial theoretical support for practical data interpretation. Building upon this, an innovative approach combining well-logging and seismic data inversion with intelligent recognition based on a deep neural network (UNet++) was proposed. By integrating the high resolution of logging with the lateral continuity advantage of seismic data, automated and high-precision identification of brine layers was achieved. The research successfully established a brine-bearing layer prediction model, enabling quantitative forecasting of the spatial distribution of metalliferous brine layers in the western Qaidam area. This provides scientific justification for subsequent drilling deployment in target zones. It significantly enhances the exploration efficiency and intelligence level for deep metalliferous brine, holding substantial scientific significance and practical value for advancing mineral resource reserves and production.

How to cite: Chen, C., Zhang, H., Fang, X., Zhou, Y., and Xia, Y.: Metalliferous Brine Exploration and Reservoir Intelligent Identification in Qaidam Basin, Northwestern China, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2181, https://doi.org/10.5194/egusphere-egu26-2181, 2026.

Water-based drilling fluid invasion-induced shale hydration is one of the key mechanisms leading to shale formation instability. Its essence lies in the microstructural damage caused by hydration, which further leads to the deterioration of mechanical and acoustic properties. To thoroughly investigate the intrinsic relationship of the evolutionary characteristics of “hydration–structural damage–mechanical weakening,” shale samples from the Da’anzhai Formation in the Sichuan Basin were selected. A series of comprehensive experiments were designed, covering different fluid systems, immersion pressures , and immersion times. By conducting simultaneous acoustic (velocity and attenuation), rock mechanical (triaxial), and CT scanning tests, the evolution trends of macro- and microstructures, mechanical properties, and acoustic characteristics of shale under water–rock interaction were quantitatively characterized. The response relationship between mechanical and acoustic parameters was clarified, and the influence of different drilling fluid systems on formation collapse pressure was compared, providing a quantitative basis for drilling fluid optimization. The main conclusions are as follows:
(1) Macro- and microstructural damage exhibits time- and pressure-dependent behavior. After immersion in drilling fluid, macroscopic fractures appeared on the shale surface. CT scanning indicated that hydration-induced fractures primarily formed during the initial immersion stage. With prolonged immersion time or increased pressure, existing fractures continued to expand, while the number of new fractures gradually decreased, reflecting irreversible cumulative hydration damage within the shale.
(2) Mechanical and acoustic parameters exhibit a synergistic deterioration trend. As immersion time and pressure increase, shale acoustic wave velocity, peak amplitude, compressive strength, and elastic modulus all decrease, while the acoustic attenuation coefficient increases. Microscopically, this is attributed to hydration-induced microcrack propagation and mineral interface weakening, which complicate wave propagation paths and intensify energy dissipation, leading to a simultaneous reduction in mechanical performance.
(3) Drilling fluid modification effectively inhibits hydration damage. Compared with the original water-based drilling fluid, adding 3% plugging agent increased shale longitudinal wave velocity and compressive strength by 8.2% and 10.2%, respectively. Using 50% organic salt as an inhibitor increased these values by 12.4% and 22.3%, respectively. When both were used in combination, the improvements further increased to 14.7% and 27.2%. This indicates that physical plugging and chemical inhibition can significantly mitigate structural damage and mechanical weakening.
(4) The acoustic attenuation coefficient is a sensitive indicator for evaluating structural damage. After water–rock interaction, the correlation between shale compressive strength, elastic modulus, and acoustic wave velocity is weaker, while the correlation with the acoustic attenuation coefficient is stronger. This suggests that the acoustic attenuation coefficient responds more sensitively to microstructural damage and can serve as a non-destructive evaluation method for optimizing drilling fluid systems.
(5) Drilling fluid optimization significantly reduces collapse pressure risk. As immersion time or pressure increases, the incremental collapse pressure of the shale formation gradually rises, reaching 0.248 g/cm³ and 0.201 g/cm³ under conditions of 15 days and 7 MPa, respectively. After adding 3% plugging agent, 50% organic salt, and their combination, the incremental collapse pressure decreased to 0.167, 0.151, and 0.113 g/cm³, respectively. This confirms that optimizing drilling fluids through physical–chemical synergy can effectively enhance wellbore stability.

How to cite: Xiong, J., Fang, H., Liu, X., Liang, L., and Ding, Y.: Mechanical and Acoustic Dynamic Evolution of Shale under Water-Shale Interaction, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2499, https://doi.org/10.5194/egusphere-egu26-2499, 2026.

Accurate evaluation of pore systems in shale reservoirs is critical for understanding fluid flow, gas storage capacity, and overall reservoir performance. Extensive research has been conducted on the micro- and nano-pore structure of shale reservoirs using various experimental methods and shale samples. However, discrepancies in sample preparation standards and experimental parameters—such as observation techniques and data interpretation—raise concerns about the reliability and comparability of these results across different shale reservoirs. This review systematically evaluates the current methods for characterizing shale pore systems, with particular emphasis on commonly used techniques such as scanning electron microscopy (SEM), gas adsorption, and CT scanning. Key challenges related to sample preparation (e.g., sample size) and experimental conditions (e.g., voltage and current) are discussed, as these factors can introduce significant inaccuracies into pore structure characterization, even for well-established methods. Additionally, we examine the difficulties in integrating these methods to achieve a comprehensive understanding of shale pore parameters, including the quantification of organic and inorganic porosity, full-scale pore size distribution, and pore connectivity. Addressing these challenges requires the establishment of standardized processing workflows to enhance the comparability of results and minimize experimental errors. We also highlight often-overlooked issues, such as the potential discrepancy between pore structures observed under laboratory conditions and those at in-situ depths. The review concludes with recommendations for future research, including the development of advanced experimental techniques and more efficient data processing strategies to improve pore characterization in shale reservoirs. This review provides a new perspective for future research and addresses a critical gap in understanding the impact of experimental conditions on pore structure characterization outcomes.

How to cite: Gou, Q., Xu, S., Xiong, Q., and Li, Z.: A multiple review on identification and fine evaluation of shale pores: Prospects and challenges, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2531, https://doi.org/10.5194/egusphere-egu26-2531, 2026.

The Ordos Basin is a core area for deep coalbed methane (CBM) accumulation and production in China. The No. 8 coal seam at the Benxi Formation top is a primary target due to its wide distribution, stable thickness, and good gas-bearing capacity. However, complex roof/floor lithology and coal heterogeneity lead to intricate petrophysical responses of mechanical parameters, affecting fracturing efficiency. Thus, an integrated system incorporating rock properties for mechanical parameter prediction, in-situ stress calculation, and fracturability classification is critical for deep CBM sweet spot identification.

Sixty core samples of major lithologies (limestone, mudstone, sandstone, coal rock) were collected from the central-eastern Ordos Basin. Comprehensive laboratory tests were performed, including X-ray diffraction (XRD), uniaxial/triaxial compression, Brazilian splitting, and basic physical tests (P-/S-wave velocity, density). For coal rock, an extended Gassmann-based petrophysical model was established via Biot’s porous medium theory, incorporating pore-fracture extrusion-spray flow effect to accurately predict cleat density. Pearson correlation analysis identified key factors governing mechanical parameters: acoustic velocity, density, cleat density for coal; acoustic velocity, density, shale content for roof/floor rocks. Multiple nonlinear regression models were built for their mechanical parameters, with R²>0.75 between predicted and measured values, ensuring high accuracy.

Using the predicted rock mechanical parameters, the combined spring model was employed to calculate the in-situ stress of the coal-bearing strata. The prediction results demonstrated excellent consistency with field measured data, with an average relative error of less than 10%. Focusing on CBM reservoirs, relevant parameters were extracted, including rock mechanical properties (USC, E, V……) and in-situ stress components (σ_H, σ_h……). The correlation between these parameters and single-well daily gas production was systematically analyzed. The XGBoost ensemble learning algorithm was utilized to screen key influencing parameters from high-dimensional data, identifying four critical factors: minimum horizontal stress difference between reservoir and roof, reservoir horizontal stress difference, reservoir tensile strength, and reservoir elastic modulus. A fracturability evaluation model (FI) was constructed based on these key factors, and clustering analysis was applied to classify reservoir fracturability through iterative updating of cluster centers. The classification results yielded three reservoir grades: Class Ⅰ (FI > 0.32) with excellent fracturability, facilitating the formation of complex fracture networks; Class Ⅱ (0.21 < FI ≤ 0.32) with moderate fracturability, tending to form relatively simple fracture networks; and Class Ⅲ (FI ≤ 0.21) with poor fracturability, for which fracturing stimulation is not recommended.

The results of this study show great potential in evaluating deep CBM in the basin. It significantly improves the accuracy of parameter prediction (R² > 0.75) and in-situ stress calculation (error < 10%). Meanwhile, the combination of the FI model and classification standard effectively enhances evaluation precision and decision-making efficiency, providing strong support for targeted fracturing and sustainable deep CBM development.

How to cite: Luo, J. and Xiong, J.: Prediction of Rock Mechanical Parameters in Deep Coal-Bearing Strata and Fracturability Classification Evaluation of Coalbed Methane Reservoirs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2550, https://doi.org/10.5194/egusphere-egu26-2550, 2026.

Recent shale gas drilling wells in southern China confirms significant resource potential within the Lower Carboniferous marine shale of the Yaziluo Rift Trough, yet exploration efficiency is hindered by poorly understood sedimentological heterogeneity and gas accumulation mechanisms. This study integrates major and trace elements, high-resolution field emission scanning electron microscopy, and adsorption analyses (low-pressure N₂/CO₂, high-pressure CH₄) to systematically characterize sedimentary facies, reservoir properties, and gas enrichment patterns. The trough exhibits an asymmetric "platform-basin" model with three distinct sedimentary facies. Basin facies comprise siliceous shale formed in deep-water anoxic settings, yielding the highest total organic carbon (TOC) content (average 3.63%) controlled by redox conditions and paleoproductivity. Lower slope facies consist of mixed shale in dysoxic environments, with moderate TOC (average 1.80%), dominated by redox conditions. Upper slope facies are calcareous shale in shallow, weakly reducing settings, showing the lowest TOC (average 0.99%), influenced by clay-mediated organic preservation. Reservoir analysis reveals that basin facies are dominated by organic pore, whereas lower slope facies display reduced organic pores but increased inorganic pores and micro-fractures, and upper slope facies shift predominantly to inorganic pores and microfractures. Moving from basinward to upper slope, increasing carbonate content expands dissolution pore networks, yet declining TOC diminishes organic pore development, promoting organic-clay complexes and weakening pore structure. Additionally, three shale associations classified by shale-to-argillaceous limestone ratios correspond to specific sedimentary facies. The lower slope shale association demonstrates optimal gas preservation due to high TOC, argillaceous limestone interlayers acting as direct caps, and fracture-enhanced porosity facilitating gas migration. The upper slope association shows promise for self-sealing bodies via acid fracturing despite lower TOC. In contrast, the basin facies shale association exhibits constrained gas retention capacity owing to clay-dominated mineralogy and absence of argillaceous limestone interlayers. This study emphasizes the critical role of lithofacies heterogeneity and integrated "source-reservoir-seal" configurations in evaluating of shale gas accumulation under "slope-basin" depositional architectures, providing a theoretical basis for reservoir development in analogous geological settings.

How to cite: Chen, X.: Sedimentology Dominated Accumulation Mechanism of Marine Shale Gas, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2639, https://doi.org/10.5194/egusphere-egu26-2639, 2026.

Abstract: Buried hill reservoirs in the eastern South China Sea exhibit highly heterogeneous mechanical properties and complex fracture networks that exert significant control on reservoir productivity, fluid pathways, and wellbore stability. Their tectonic evolution, multistage deformation, and lithological diversity make traditional fracture prediction methods insufficient for supporting safe and efficient offshore drilling operations. To address these challenges, this study develops a new integrated framework that combines geomechanical analysis, finite-element numerical simulation, and multi-source geological and geophysical data to characterize fracture attributes and predict fracture behavior under present-day stress conditions. The workflow incorporates acoustic and imaging logging data, high-resolution seismic attributes, lithology-based mechanical property modeling, and 3D Mohr circle stress analysis. These datasets are used to construct a heterogeneous geomechanical model that captures the spatial variability of elastic and strength parameters across the buried hill. Numerical simulations are performed to evaluate the distribution of stress perturbations associated with structural relief and lithological layering, and to assess the likelihood of fracture initiation, propagation, and reactivation under different stress regimes. The results demonstrate pronounced variations in fracture intensity and failure potential both laterally and vertically. Zones near faulted structural highs exhibit locally elevated differential stress and shear strain concentration, leading to enhanced fracture connectivity and increased reactivation probability. In contrast, massive crystalline units and mechanically strong lithologies show limited deformation and lower fracture susceptibility. By integrating the simulated stress field with observed fracture indicators, the study identifies high-risk intervals with elevated risks of borehole collapse, drilling fluid loss, or induced fracturing, as well as fracture-favorable sweet spots that may enhance reservoir penetration and productivity. Furthermore, the framework provides quantitative guidance for optimizing well trajectories, selecting safe drilling windows, and improving well placement strategies in offshore buried hill settings. The results highlight the importance of incorporating geomechanical constraints into fracture characterization workflows to reduce drilling uncertainty and improve the reliability of fracture prediction models. This integrated analytical–numerical approach offers a robust and transferable methodology for evaluating fracture development in complex tectonic reservoirs. It provides practical insights into wellbore stability management, reservoir development optimization, and risk mitigation in the challenging geological environment of the eastern South China Sea. The proposed workflow can also be adapted to similar buried hill or basement reservoirs worldwide.

Keywords: fracture characterization; geomechanical modeling; finite-element simulation; buried hill reservoirs; in-situ stress analysis; south China Sea

How to cite: Deng, C., Tan, J., and Yin, L.: An integrated geomechanical–numerical simulation framework for fracture characterization and prediction in buried hills of the eastern south China Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2650, https://doi.org/10.5194/egusphere-egu26-2650, 2026.

     In deep coal seams, the primary storage space for adsorbed methane resides in the micropores and associated adsorption surfaces of the coal matrix, whereas the fracture network provides the dominant pathway for gas–water transport. Fractures promote the migration and local enrichment of free gas, with their connectivity exerting a fundamental control on methane desorption efficiency and overall seepage capacity. Therefore, elucidating the pore‑scale mechanisms of gas‑driven water displacement is critical for understanding gas–water occurrence and flow dynamics, as well as for assessing the reservoir‑controlling role of different microstructures.In light of the pronounced heterogeneity of coal, this study constructs two pore‑scale digital core models—a matrix‑pore model and a pore–fracture model—based on micro‑CT imaging of low‑rank coal. Utilizing the Volume of Fluid (VOF) method within an immiscible two‑phase displacement framework, we simulate methane‑driven water displacement under reservoir‑formation conditions. The simulation results are used to systematically analyze how the contact angle (θ) and capillary number (Ca) govern displacement morphology, phase connectivity, and residual phase distribution. Furthermore, a capillary‑number–contact‑angle (Ca–θ) phase diagram characterizing the displacement process is established.

       Key findings are summarized as follows: (1) Under gas–water viscosity ratios representative of the reservoir‑formation stage, pore‑scale gas‑driven water displacement exhibits three distinct regimes: capillary fingering, viscous fingering, and a transitional capillary–viscous fingering regime. In the matrix‑pore model, displacement is dominated by capillary fingering due to pore‑throat constrictions and microstructural bottlenecks, resulting in a dispersed and fragmented displacement front. In contrast, displacement in the pore–fracture model is governed by an interconnected fracture network, where capillary forces are substantially weakened, leading to displacement patterns characteristic of viscous fingering. (2) At low Ca, displacement follows the capillary‑fingering regime, and the gas phase predominantly forms a connected flow network after displacement. At high Ca, viscous fingering dominates, generating numerous isolated gas bubbles and yielding poor gas‑phase connectivity. At intermediate Ca, a transitional regime emerges, combining features of both capillary and viscous fingering. (3) The influences of wettability, capillary number, and pore‑structure type on final gas saturation and gas‑phase connectivity differ markedly between the two models. Under identical displacement conditions, the pore–fracture model attains significantly higher gas saturation and superior gas‑phase connectivity compared to the matrix‑pore model. These insights advance the understanding of methane occurrence and migration in deep coal seams and provide a basis for optimizing coal reservoir development strategies.

Keywords：Deep coal;Gas-water two-phase flow;Displacement pattern;Wettability, Capillary number

How to cite: Jin, X. and Chen, Z.: Pore‑Scale Simulation of Methane‑Water Two‑Phase Flow in Deep Coal Seams Using the Volume of Fluid Method, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2800, https://doi.org/10.5194/egusphere-egu26-2800, 2026.

With the continuous advancement of oil and gas exploration and development, unconventional resources have become a crucial component of national energy strategies. The extraction of resources such as shale oil and gas relies on technologies like horizontal drilling and multi-stage fracturing, where accurate geomechanical parameters are essential for engineering design. Conventional core-based experiments and well-log inversion methods, though reliable, are often costly, time-consuming, and limited in representativeness. Recent progress in digital cutting analysis—using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS)—offers a fast, economical, and practical alternative. This study presents a workflow for predicting rock mechanical parameters from the mineral composition and pore structure of cuttings. Site-collected cuttings were characterized via SEM-EDS to analyze morphology, mineralogy, pore networks, and microfractures. Given the high heterogeneity and anisotropy of shale, a composite modeling approach integrating heterogeneous structure theory and equivalent medium models was applied. This included the Reuss and Voigt bounds, Voigt–Reuss–Hill average, Hashin–Shtrikman bounds, Kuster–Toksöz theory, and Gassmann fluid substitution to estimate equivalent static elastic parameters. These were then converted to dynamic parameters using linear regression to ensure consistency with logging data. Results show strong agreement between cutting-derived parameters and well-log inversions. Young’s modulus and Poisson’s ratio errors range from –12.62% to 4.03% and –10.18% to 10.47%, respectively, within acceptable limits. Although minor uncertainties arise from mineral identification and image segmentation, overall trends match well-log data closely. The introduction of a Weakness Index effectively highlights reservoir heterogeneity and correlates well with measured fracture pressures. A strong linear relationship between static and dynamic parameters, particularly Young’s modulus, supports the reliability of the regression-based conversion. This study confirms the feasibility and applicability of digital cuttings for building rock-physics models and predicting mechanical properties in unconventional reservoirs. The method not only aligns with well-log results but also better captures formation heterogeneity. More importantly, it enables real-time, cost-effective mechanical characterization from wellsite cuttings, offering a practical alternative to core- or log-dependent methods. This is particularly valuable in complex wells such as horizontals, where rapid formation evaluation and fracture design are critical. 

How to cite: Nie, X., Hou, Y., and Zhang, Z.: Rock Mechanical Parameters Prediction Based on Digital Drilling Cuttings Mineral Logging Data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2877, https://doi.org/10.5194/egusphere-egu26-2877, 2026.

Pyrite is widely developed in shale formations with diverse occurrence morphologies and relatively low contents. However, it exerts a significant impact on the electrical properties of reservoirs and severely restricts the accuracy of reservoir evaluation. Based on computed tomography (CT) scanning technology, combined with mathematical morphology methods and finite element simulation techniques, this study focuses on the coupling relationship between pyrite occurrence states and reservoir resistivity, along with the quantitative calculation method of pyrite content. The results indicate that: (1) The influence of pyrite occurrence morphologies on the electrical conductivity of shale varies remarkably. Dispersed pyrite particles exert a weak interference on reservoir resistivity, and the rock-electrical relationship conforms to Archie's law. In contrast, massive and banded pyrite have a more prominent impact on resistivity, their conductive contribution exceeds that of pore water. This renders the traditional Archie's law inapplicable. Furthermore, under the condition of the same content, banded pyrite exerts the most significant influence on reservoir resistivity. (2) Three parameters, namely the Pyrite Resistivity Ratio (PRR), the Conductive Path Influence Characterization (VPYRZ), and the Pyrite Occurrence Index (PYI), are constructed to quantitatively describe the nonlinear influence of pyrite with different occurrence morphologies on reservoir resistivity. Combined with scanning electron microscopy (SEM) data, the PYI thresholds for different occurrence morphologies are determined as follows: dispersed pyrite (< 7.8623), massive pyrite (7.8632–16.986), and banded pyrite (>16.986). On this basis, a quantitative calculation model for pyrite content is established. (3) The verification results of actual well logs show that the calculated results of the proposed model are in high agreement with the formation element logging data, and the accuracy meets the practical requirements of reservoir evaluation.

How to cite: Liu, Y.: A Study on the Coupling Relationship Between Pyrite Occurrence Morphologies in Shale and Resistivity, and the Quantitative Calculation Method of Its Content Based on Numerical Simulation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2946, https://doi.org/10.5194/egusphere-egu26-2946, 2026.

Mean grain size (Mz) is a key indicator of depositional processes and reservoir quality, yet its continuous characterization is commonly limited by the availability of core or cuttings data. This study presents a new method for predicting Mz from conventional well logs by integrating Complete Ensemble Empirical Mode Decomposition with Adaptive Noise (CEEMDAN) and Principal Component Analysis (PCA). A density–neutron separation parameter is first constructed to capture lithological and grain-framework variations. Multi-scale components sensitive to grain-size changes are then extracted from this parameter using CEEMDAN, and the dominant controlling features are identified through PCA. The resulting model enables continuous Mz prediction along the wellbore. Comparisons with measured data demonstrate that the proposed approach reliably captures vertical grain-size variations, providing a practical and robust solution for quantitative grain-size characterization and supporting detailed reservoir analysis and geological modeling.

How to cite: Shen, B., Wang, C., Ma, X., and Sun, K.: Quantitative Inversion of Mean Grain Size from Conventional Well Logs Using Complete Ensemble Empirical Mode Decomposition （CEEMDAN）and Principal Component Analysis（PCA）, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3232, https://doi.org/10.5194/egusphere-egu26-3232, 2026.

Nuclear magnetic resonance (NMR) T₂ spectra provide pore-scale information on fluid occurrence and mobility and are widely used for saturation evaluation. However, in shale and other unconventional reservoirs, strong heterogeneity, multi-fluid signal overlap, and weak/complex relaxation responses often undermine the reliability of conventional cutoff- and template-based saturation methods.

To address this challenge, we propose a data-driven workflow that directly predicts oil saturation from NMR T₂ spectra by integrating feature engineering, unsupervised decoupling, and a compact deep-learning regressor. The method first applies robust preprocessing to suppress abnormal values and outliers, followed by dimensionality reduction to extract the most informative latent features. To alleviate multi-component signal superposition, an unsupervised clustering step is introduced to partition spectral patterns into representative groups, providing a more stable feature basis for learning. Finally, a lightweight convolutional neural network (CNN) is employed as the regression model to map processed T₂ features to core-calibrated oil saturation, with standard strategies (normalization, dropout/regularization, and learning-rate scheduling) to improve generalization.

The workflow is validated using core-log paired datasets from a shale reservoir, showing that the predicted oil saturation agrees well with laboratory measurements and significantly improves stability compared with conventional interpretation in complex intervals. The proposed approach offers an efficient and scalable route for saturation evaluation in data-limited unconventional plays, supporting sweet-spot identification and development planning.

This research was supported by the National Oil & Gas Major Project (No. 2025ZD1400202) and Natural Science Foundation of Shandong Province, China (No. ZR2023YQ034).

How to cite: Li, Q., Wang, C., Ge, X., Chi, R., Wang, Y., Zhang, W., Miao, Q., and Zhang, J.: Direct Prediction of Oil Saturation from NMR T₂ Spectra Using Unsupervised Feature Decoupling and Deep Learning, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3276, https://doi.org/10.5194/egusphere-egu26-3276, 2026.

Electrical imaging logs are widely used for the quantitative characterization of unconventional reservoirs. However, their applicability is severely limited in pervasively fractured continental shale-oil reservoirs due to strong heterogeneity and complex resistivity responses.

To address this limitation, an electrical statistical framework is developed that integrates variogram-based attributes with smoothness functions to identify fracture occurrence and quantitatively characterize fracture development in shale reservoirs. Resistivity percentile curves derived from electrical imaging logs are analyzed using a moving-window scheme to extract multidimensional parameters, including variance, Shannon entropy, variogram metrics, and smoothness indices. These parameters are jointly used to construct a fracture development index. In addition, fracture-prone intervals are identified using an adaptive thresholding approach constrained by geological rules and resistivity separation characteristics. To suppress the influence of stratigraphic background trends, a local background-resistivity normalization is applied, enabling fracture classification based on resistivity ratios.

The method is validated using shale-oil reservoirs of the Shahejie Formation in the Dongying Sag, eastern China. The results demonstrate that, within a tolerance range of 0.125 m, fracture identification derived from electrical image logs achieves an 85.4% agreement with core descriptions. The identification accuracies for high-conductivity and high-resistivity fractures reach 90.41% and 80.65%, respectively, while approximately 47% of the identified fractures exhibit high conductivity.The proposed approach provides new insights into fracture-filling properties and their vertical distribution, highlighting its applicability to shale-oil reservoir characterization, sweet-spot evaluation, and hydraulic fracturing design. 

This research was supported by the National Oil & Gas Major Project (No. 2024ZD1405102).

Fig. 1. Electrical Imaging–Based Statistical Indicators for Fracture Identification in Well FY3

How to cite: Wang, Z., Ge, X., Jiang, L., Sun, H., Li, Z., Zhang, D., and Yang, D.: Electrical Imaging–Based Statistical Indicators for Fracture Identification in Shale-Oil Reservoirs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3394, https://doi.org/10.5194/egusphere-egu26-3394, 2026.

Extra-deep electromagnetic (EM) logging-while-drilling (LWD), with its deep investigation and sensitivity to resistivity distribution, is widely used in high-angle and horizontal wells. Due to the complex tool responses of extra-deep EM LWD in heterogeneous formations, eyeball evaluation is often insufficient for accurate bed boundary identification. Therefore, quantitative formation parameter estimation necessitates inversion, with multi-boundary inversion based on 1D models being the most prevalent approach. The inherent limitation of 1D-based inversion is its inability to accurately resolve structurally complex formations, frequently producing oversimplified results. 2D inversion methods alleviate this oversimplification, but their high computational cost limits applicability in real-time geosteering.

In paper, we propose a 1D-2D adaptive parametric inversion framework to combine the efficiency of 1D inversion with the accuracy of 2D inversion. First, we investigate the tool responses and parameter sensitivities in 2D formations using a 2.5D finite-difference algorithm. Then, a scenario-dependent adaptive parametric inversion strategy is developed for specific models based on the sensitivity analysis. For instance, in a fold model, we use a 1D horizontally layered inversion for gently dipping limb regions and a three-point parameterization scheme for the fold core. To improve global optimization, a probabilistic inversion method is established based on the PT-MCMC algorithm, incorporating multiple prior distributions and multiple joint constraints. Finally, the method is applied to the inversion of fold and fault models. Numerical experiments demonstrate that the proposed method accurately reconstructs fold cores and fault structures. Specifically, the relative errors of fault dip angle and fault throw are less than 10%, while the relative error of the fold core location is less than 8%.

Generally, the proposed 1D-2D adaptive parametric inversion framework provides an efficient and robust strategy for real-time geosteering in horizontal and highly deviated wells within complex reservoirs, showing potential for refined reservoir characterization.
We are indebted to the financial support from the National Natural Science Foundation of China (42304140).

How to cite: Chang, J. and Wu, Z.: An Efficient 1D-2D Adaptive Parametric Inversion Method for Extra-Deep Electromagnetic LWD in Complex Scenarios, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3483, https://doi.org/10.5194/egusphere-egu26-3483, 2026.

Shale microstructure is critical for predicting shale-oil “sweet spots” and improving reservoir evaluation. As a typical unconventional reservoir, shale exhibits ultra-low permeability, complex pore systems, strong heterogeneity, and diverse mineral compositions with highly uneven spatial distributions; pore morphology and connectivity, together with mineral assemblages, strongly control fluid migration, mechanical behaviour, and acoustic–electrical responses. However, conventional rock-physics experiments and image-analysis workflows are often time-consuming and insufficiently accurate for such complex materials. Here we develop a multi-scale, multi-modal segmentation workflow based on nnU-Net v2. Using paired 0.3 µm-resolution SEM and QEMSCAN images, we perform multi-class segmentation of pores, organic matter, clays, felsic minerals, carbonates, and heavy metal-bearing minerals, achieving a weighted Dice score of 0.95 and clearly outperforming threshold-based segmentation. The SEM-trained network is then transferred to 0.3 µm CT data to enable cross-modality prediction and reconstruct three-dimensional distributions of the segmented phases. We further extend the model to 4 nm-resolution CT images for cross-scale and cross-modality segmentation; three denoising filters are evaluated to suppress noise and improve nanoscale segmentation accuracy. Finally, we compare 3D digital rock volumes generated from single-direction inference with those obtained by tri-axial inference and fusion, highlighting differences in volumetric consistency and structural representation. This workflow provides a robust basis for future multi-scale digital rock construction and for simulations of porosity, permeability, and saturation, thereby supporting more comprehensive shale-oil reservoir assessment.

How to cite: Huang, X., Guo, Y., Li, X., and Li, Y.: AI-Driven Automatic Segmentation and Quantitative Characterization of Shale Microstructures, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3653, https://doi.org/10.5194/egusphere-egu26-3653, 2026.

The accurate quantification of karst volume is essential for evaluating storage capacity and fluid flow behavior in carbonate reservoirs, which are often highly heterogeneous due to complex diagenetic and karstification processes. However, conventional approaches to karst characterization frequently rely on qualitative descriptions or isolated datasets, lacking an integrated framework that effectively combines petrophysical properties with lithological controls. To address this gap, this study proposes a novel and practical workflow that systematically integrates routine well logs and core data to quantitatively estimate karst-dominated porosity in carbonate sequences. The methodology is designed to be both log-based and scalable, making it suitable for field-wide application even in data-constrained environments.

The proposed method is structured into four sequential and interpretative steps: (1) Data Integration and Quality Control: Multi-source data—including conventional well logs, core measurements, geological models, and lithology logs—are carefully depth-matched and subjected to rigorous quality checks to ensure consistency and reliability. Special attention is paid to correcting for borehole environmental effects and log normalization across multiple wells. (2) Definition of Karst Porosity Threshold: Based on systematic analysis of core-derived porosity-permeability cross-plots, a porosity value greater than 15% is established as a robust indicator for significant karst development. This threshold effectively distinguishes karst-related pore space from matrix porosity and is validated through thin-section and CT-scan observations. (3) Identification of Host Lithology: Lithofacies modeling and petrographic analysis are employed to confirm that karst features are predominantly hosted within dolomite intervals, highlighting the lithological control on karst distribution. Log-based facies classification is calibrated with core data to ensure accurate lithology discrimination in non-cored sections. (4) Calculation of Karst Volume Percentage: The proportion of karst volume is quantitatively computed as the ratio of high-porosity (>15%) dolomite volume to the total pore volume, which includes matrix pores, vugs, and fractures. This volumetric approach enables a more realistic representation of karst contribution to total porosity and reservoir performance.

This workflow was applied to a carbonate reservoir in the Pre-Caspian Basin, where it yielded a karst volume proportion of 11.1%. This result aligns closely with independent core statistics and regional geological understanding, validating the method’s accuracy and applicability. Sensitivity analyses were conducted on the porosity threshold, confirming that the selected 15% cutoff optimally balances discrimination capability and geological plausibility. The integrated approach not only enhances the reliability of karst assessment but also offers a scalable and reproducible tool for reservoir characterization. It supports improved geological modeling, reservoir performance prediction, and development planning in complex carbonate settings, ultimately contributing to more efficient hydrocarbon recovery. Future work will focus on extending the workflow to incorporate seismic attributes and dynamic production data for enhanced 3D karst modeling.

Keywords: Karst Volume; Carbonate; Well Logging; Porosity; Integrated Workflow

How to cite: Hou, J. and Yang, Y.: An Integrated Method for Quantifying Karst Volume in Carbonate Reservoirs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3739, https://doi.org/10.5194/egusphere-egu26-3739, 2026.

Acoustic Remote Detection Logging can extend the investigation range of conventional borehole logging from the immediate vicinity of the borehole to several meters and even tens of meters, providing complementary information for formation evaluation in reservoirs with heterogeneity such as fracture–cavity systems and thin interbeds. Such complex anomalous bodies may reduce the effectiveness of conventional rock-physics log interpretation. Overall, the current work still faces two main issues: the lack of geology-constrained 3D models and the lack of more efficient and transferable interpretation tools, which makes it difficult to establish a stable relationship between complicated reflected wavefields and subsurface heterogeneity.

This study focuses on Acoustic Remote Detection Logging and develops geology-constrained 3D heterogeneous modeling and response analysis. First, a 3D stochastic medium is generated using the spectral synthesis method to produce correlated random fields, which are then mapped to elastic-parameter perturbation fields. Then, a bisection-based thresholding scheme is applied to satisfy a target volume fraction (or porosity), and local-maximum detection is used to determine seed points and spatial distributions for cavities, enabling 3D construction of fracture–cavity heterogeneity. Cavity geometry and scale parameters are constrained by statistical characteristics derived from existing geological and logging data. Finally, CUDA-based parallelization is introduced to improve the efficiency of full 3D model generation and support rapid construction of high-resolution reservoir models.

Based on forward simulations of Acoustic Remote Detection Logging responses excited by dipole shear-wave sources, we design parameter combinations covering formation porosity and fracture–cavity geometric parameters, and summarize the results into response charts/templates for interpretation. These charts provide quantitative relationships between fracture parameters and key waveform features, supporting comparative identification and interpretation of fracture development, orientation, and spatial position under different geological scenarios. In addition, we explore using a model for feature discrimination under chart-based constraints, thereby providing auxiliary support for comparative evaluation and interpretation of fracture parameters.

How to cite: Wang, H., Guo, Y., Zhang, Z., Ye, Z., and Sun, H.: Acoustic Remote Detection Logging for Heterogeneous Reservoirs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4177, https://doi.org/10.5194/egusphere-egu26-4177, 2026.

Accurate estimation of hydrocarbon content is a critical component of shale reservoir evaluation. Although nuclear magnetic resonance (NMR) T₁-T₂ measurements are highly sensitive to fluid properties, conventional assessments of hydrocarbon content typically rely on empirical interpretation charts or supplementary experiments, which limit their quantitative reliability and practical applicability. In this study, we propose a novel fractal characterization method based on NMR T₁-T₂ measurements for quantitative evaluation of hydrocarbon content in shale reservoirs. To mitigate the uncertainty introduced by T₁-T₂ spectral inversion, fractal parameters are directly extracted from the original NMR echo train data, bypassing the inversion process entirely. Numerical simulations demonstrate that the echo-based fractal parameters exhibit significantly enhanced sensitivity and discrimination capability with respect to hydrocarbon content when compared with fractal parameters derived from inverted T₁-T₂ spectra. Core-scale experiments further validate that the proposed fractal dimension effectively differentiates movable hydrocarbons from pyrolytic hydrocarbons in shale formations. The proposed method provides a robust, efficient, and inversion-independent approach for shale hydrocarbon content evaluation, offering strong potential for both laboratory studies and field-scale NMR applications.

How to cite: Gu, M., Wang, L., Shi, P., Li, G., and Ai, Y.: Direct Fractal Characterization of Shale Hydrocarbon Content from NMR T1-T2 Echo Train Data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4225, https://doi.org/10.5194/egusphere-egu26-4225, 2026.

Tight sandstone reservoirs are generally characterized by low porosity and permeability, complex pore structures, and ambiguous electrical responses, which significantly limit the applicability of conventional water saturation evaluation models. To address these challenges, this study proposes a physically constrained reinforcement learning–based symbolic regression framework that integrates nuclear magnetic resonance (NMR)–derived pore size distribution information to automatically derive a dynamic Archie water saturation model with explicit physical interpretability.In the proposed approach, pore size distribution features are embedded into the formation factor formulation, enabling a dynamic correction of the classical Archie equation. A policy neural network combined with reinforcement learning is employed to jointly optimize the model structure and parameters, while explicitly enforcing physical constraints such as monotonicity and nonlinear electrical response behavior.Experimental results demonstrate that, compared with the conventional Archie model, the proposed dynamic model achieves a significant improvement in water saturation prediction accuracy for tight sandstone reservoirs, reducing the mean absolute error by approximately 11%. Moreover, the model more effectively captures the influence of pore structure heterogeneity on the relationship between electrical resistivity and water saturation.This study provides a novel water saturation evaluation methodology that combines physical interpretability with data-driven adaptability for tight sandstone reservoirs and offers valuable insights into the intelligent construction of logging interpretation models for complex reservoirs.

How to cite: Liao, W. and Zhao, B.: A Physically Constrained Dynamically Corrected Archie Saturation Model Based on Pore Size Distribution and Its Application to Tight Sandstone Reservoir Evaluation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4245, https://doi.org/10.5194/egusphere-egu26-4245, 2026.

Carbonate reservoirs host substantial hydrocarbon resources; however, their characterization remains challenging due to strong heterogeneity and complex pore systems, which often produce asymmetric and highly multimodal NMR T₂ distributions. These characteristics undermine the applicability and robustness of conventional pore-structure interpretation and permeability models. To overcome these limitations, we propose a novel nuclear magnetic resonance (NMR)-based method that employs an Exponentially Modified Gaussian (EMG) model to quantitatively characterize pore structure and improve permeability estimation. First, the EMG function is used to decompose the measured T₂ distribution into multiple components with clear physical implications, enabling separation and quantification of pore contributions across scales. Second, the EMG-derived characteristic parameters are subsequently incorporated to refine the conventional Schlumberger-Doll Research (SDR) permeability model, thereby accounting for the impact of complex pore geometry on flow capacity. Validation using diverse carbonate samples demonstrates that the EMG model provides accurate and stable fitting of T₂ distributions across varying pore complexity, ranging from unimodal to highly multimodal distributions. Moreover, EMG-informed permeability estimation yields significantly improved accuracy and robustness compared with conventional methods. Overall, the proposed NMR EMG-based method offers a reliable solution for pore-structure characterization and permeability evaluation in complex carbonate reservoirs.

How to cite: Li, G., Wang, L., Gu, M., and Ai, Y.: An NMR EMG-Based Method for Pore Structure Characterization and Permeability Prediction in Carbonate Reservoirs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4329, https://doi.org/10.5194/egusphere-egu26-4329, 2026.

Due to the mutual flow between drilling mud and formation fluid, the pore pressure in the surrounding rock of the wellbore directly affects the rock’s mechanical properties. Since the properties and density of drilling mud are adjusted in real time during drilling operations, wellbore stability is inherently a dynamic process. This study aims to enhance wellbore integrity and reduce drilling accidents by investigating the dynamic characteristics of wellbore stability.

Rock mechanics experiments were conducted to simulate the mechanical properties of rocks under varying pore pressure conditions, yielding the quantitative relationship between pore pressure and rock mechanical behaviors. In conjunction with the radial seepage characteristics around the wellbore at different drilling stages, dynamic fluid-solid coupling simulations of the wellbore were performed based on geomechanical principles, enabling the development of a dynamic wellbore stability prediction method. This method further facilitates the prediction of key parameters such as wellbore collapse period and radial collapse depth.

This dynamic wellbore stability prediction technology was applied to three wells drilled in sandstone formations. It successfully predicted the collapse depth and period under different drilling mud densities and properties, significantly improving drilling efficiency while ensuring wellbore integrity. Compared with static wellbore stability prediction techniques, this technology provides drilling engineers with a richer set of drilling parameters and defines a clear wellbore collapse period, thereby effectively preventing stuck pipe accidents. Comparative drilling tests in the same block and formation layer showed that drilling efficiency increased by 12%, and the stuck pipe accident rate decreased by 27.3%.

This exploratory research demonstrates that wellbore stability in permeable formations during drilling is indeed a dynamic equilibrium process. As drilling mud properties, time, and stress conditions change, wellbore stability evolves accordingly. Predicting dynamic parameters can provide valuable references for drilling design and optimization, thereby enhancing wellbore integrity and minimizing drilling accidents.

How to cite: Liu, X. and Xiong, J.: Research on Dynamic Wellbore Stability Based on Wellbore Seepage, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4382, https://doi.org/10.5194/egusphere-egu26-4382, 2026.

Identification of fluid components and reliable saturation evaluation remain critical challenges in shale exploration and development. Conventional core experimental methods are often limited by high costs and long cycle times, while laboratory nuclear magnetic resonance (NMR) techniques fail to perform rapid and continuous measurements.  In this study, we propose a rock-constrained multilevel Gaussian mixture model (GMM) approach for the quantitative evaluation of multiple fluids in shale using wellsite NMR data. The proposed workflow begins with normalization and thresholding of the T₁–T₂ spectra. A rock-physics constraint is then incorporated to partition the relaxation domain and distinguish movable fluids from bound fluids. GMM clustering is first applied independently within each relaxation region to extract representative fluid signatures. These characteristic signatures are subsequently integrated into a second-stage GMM analysis, enabling robust identification and quantitative evaluation of individual fluid components. Application to representative shale NMR datasets, coupled with multi-state core experiments, demonstrates that the proposed method effectively identifies multiple fluid components, including heavy components, bound oil, and movable oil. Furthermore, the saturation of various hydrocarbon components can be quantitatively predicted, demonstrating strong agreement with Rock-Eval measurements. The proposed approach provides a practical and reliable solution for rapid downhole fluid identification and saturation evaluation in shale reservoirs.

How to cite: Shi, P., Wang, L., and Gu, M.: A Rock-Constrained Multilevel GMM Approach for Wellsite NMR Multifluid Quantitative Evaluation in Shale, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4645, https://doi.org/10.5194/egusphere-egu26-4645, 2026.

    Reservoir porosity, permeability, and saturation are regarded as the core parameters in oil and gas exploration. The prediction of reservoir productivity and the analysis of fluid transport behavior in porous media are directly influenced by these parameters. Nuclear magnetic resonance (NMR) is recognized as a non-invasive, non-destructive, and highly quantitative technique. Typically, echo signals are inverted to obtain relaxation spectra, which is then used to calculate rock parameters.

    In the existing methods, reservoir petrophysical parameters are usually calculated based on a relaxation spectrum obtained from a single inversion. However, the inversion of NMR relaxation spectra is inherently an ill-posed problem. Its solutions are characterized by non-uniqueness and high sensitivity to noise. In the traditional interpretation workflows, the single relaxation spectrum obtained from inversion is often assumed to be accurate and deterministic. Consequently, the propagation effect of inversion errors in the calculation of petrophysical parameters is ignored. This leads to a lack of credibility assessment for results in evaluations involving low signal-to-noise ratio (SNR) or unconventional reservoirs.

    To address this challenge, an improved Bootstrap resampling method is proposed in this study. It aims to achieve uncertainty quantification from NMR relaxation spectra to petrophysical parameters. The traditional approach of seeking a single solution is abandoned. Instead, single-measurement echo data are resampled multiple times to generate statistically significant pseudo-sample sets by fully mining the statistical fluctuation information hidden within the single measurement. Subsequently, each set of samples is inverted independently to construct a distribution set of relaxation spectra.

    By performing independent parameter calculations on the relaxation spectrum set, the distribution range, central tendency, and dispersion of each parameter can be obtained. Thus, the impact of data noise and pore size distribution differences on parameter estimation is revealed. On this basis, an error propagation model from the spectral domain to the parameter domain is established. Confidence intervals (CI) and prediction intervals (PI) for porosity, permeability, and saturation are calculated simultaneously. Specifically, model uncertainty caused by the non-uniqueness of the inversion algorithm is primarily quantified by the CI. Meanwhile, data uncertainty resulting from measurement noise is further incorporated into the PI, which provides a broader parameter interval range. A transition from "point estimation" to "interval estimation" for key petrophysical parameters is achieved by this method. Consequently, the robustness and credibility of parameter evaluation under complex reservoir conditions are significantly enhanced.

    This work was supported by the National Natural Science Foundation of China (42304118), the Frontier Interdisciplinary Exploration Research Program of China University of Petroleum Beijing (2462024XKQY009), the Young Elite Scientist Sponsorship Program by BAST (BYESS2023027).

How to cite: Zhao, Y., Guo, J., Wan, Q., and Xie, R.: A New Uncertainty Quantification Method for Petrophysical Parameters Based on NMR Relaxation Spectra , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4758, https://doi.org/10.5194/egusphere-egu26-4758, 2026.

To address the high cost and low efficiency of lithology identification for intrusive rocks in oil and gas exploration, this study proposes a hierarchical identification method based on petrochemical big data and machine learning. By integrating global geochemical databases, a standardized sample set covering ultramafic to felsic intrusive rocks was constructed, and a three-level classification system (“group–subgroup–specific lithology”) was established. Visual discrimination charts and an automatic identification model were developed using Linear Discriminant Analysis and Multilayer Perceptron, respectively. The results show that the method achieves over 90% accuracy in the first- and second-level classifications, effectively identifying major rock types such as gabbro, diorite, and granite. Although the accuracy fluctuates in the third-level classification due to compositional overlap and data quality issues, the method exhibits good interpretability and generalizability. This approach provides a low-cost and efficient technical solution for rapid lithology identification and reservoir evaluation, demonstrating potential application in deep and unconventional hydrocarbon exploration.

How to cite: Quan, Z. and the Quan Zhou: Lithology identification of intrusive rocks based on petrochemical big data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6040, https://doi.org/10.5194/egusphere-egu26-6040, 2026.

The accurate petrophysical characterization of unconventional reservoirs, particularly deeply buried fractured–vuggy carbonate systems, remains challenging due to extreme heterogeneity, ultra-low permeability, and strong scale dependency of reservoir properties. Conventional formation evaluation workflows often fail to reconcile geological observations at the core scale with log- and seismic-scale petrophysical interpretations in a physically consistent manner.In this study, we propose an integrated geological–geophysical petrophysical characterization workflow that links core and thin-section observations, electrofacies classification, and seismic data through an electrofacies-constrained seismic waveform-guided inversion framework. Rock types identified from core and petrographic analyses are translated into electrofacies at the log scale to represent pore-structure variability. Electrofacies are incorporated as conditional constraints in a Bayesian seismic waveform-guided porosity inversion framework. Electrofacies-dependent porosity priors derived from well-log statistics restrict the admissible pore-structure space, while seismic waveform similarity controls the lateral propagation of high-frequency porosity features, implicitly embedding seismic facies within the inversion.The resulting porosity volume exhibits enhanced vertical resolution and improved lateral continuity, allowing thin-layer and non-layered heterogeneities to be resolved beyond the limitations of conventional impedance-based inversion. Recognizing that permeability is not directly observable at the seismic scale, permeability is subsequently derived from the inverted porosity volume under electrofacies control, ensuring that pore connectivity and flow characteristics are consistently represented at the appropriate scale.This workflow establishes a causally consistent and scalable solution for advanced petrophysical characterization and formation evaluation of heterogeneous unconventional reservoirs. By integrating geological constraints with seismic waveform-driven inversion, the proposed method effectively bridges core-, log-, and seismic-scale information and demonstrates strong potential for application in complex carbonate systems as well as other unconventional plays.

How to cite: Zhao, A., Tian, F., and Cao, W.: Multi-Scale Petrophysical Modeling and Characterization of Heterogeneous Carbonate Reservoirs Based on Facies-Constrained Seismic Waveform Integration, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6054, https://doi.org/10.5194/egusphere-egu26-6054, 2026.

Fracture parameters play a crucial role in productivity prediction, reservoir evaluation and fracturing production of buried-hill reservoirs and carbonate reservoirs. The main method for calculating fracture parameters is electrical logging based on rock-electric experiments. However, due to significant differences in observation systems between rock-electrical experiments and various electrical logging methods, directly calibrating electrical logging data with cores in different fractured formations will lead to large errors. Based on the finite element method calibrated with core samples, we established micro-fractured formation models and conducted fracture parameter simulation experiments for plunger core samples, full-diameter core samples, electrical imaging logging, micro-spherical focusing logging, shallow lateral logging, and deep lateral logging, respectively, aiming at the influence of multiple fracture parameters. A comparison of the measurement results of the six models for the same fractured formation showed that the fracture-induced resistivity reduction rates were ranked in descending order as follows: electrical imaging logging, plunger core testing, micro-spherical focusing logging, full-diameter core testing, shallow lateral logging, and deep lateral logging, with the maximum discrepancy in resistivity reduction rates across these models reaching a factor of 45. Specifically, the resistivity reduction rate of plunger cores was 2.7 times higher than that of full-diameter cores, and the rate of electrical imaging logging was 11.8 times higher than that of micro-spherical focusing logging, whereas the values for shallow lateral logging and deep lateral logging were identical. Finally, this study proposed a correction rate required for fracture core calibration, which could comprehensively optimize the interpretation range of various resistivity logging methods and effectively improve the interpretation accuracy of reservoirs.

How to cite: Fang, S., Zhang, Z., Zhang, C., Nie, X., Song, H., and Zhao, B.: Response Mechanism of Multi-scale Electrical Logging in Fractured Formations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6308, https://doi.org/10.5194/egusphere-egu26-6308, 2026.

Fluids in porous rocks can be divided into two categories according to their distribution patterns: irreducible fluids and movable fluids. Due to the influence of various geological processes, porous rocks exhibit a wide range of pore size distributions, leading to complex fluid distributions in pores of different sizes. The microscopic distributions of irreducible and movable fluids, namely the contents of irreducible and movable fluids in rock pores of varying sizes, can directly reflect the petrophysical properties of rocks, such as microscopic pore structure characteristics and seepage capacity. In the exploration and development of oil and gas resources, the quantitative characterization of the distributions of irreducible and movable fluids in reservoirs—especially the characterization of movable fluid distributions—is of great significance for reservoir evaluation, productivity prediction, and efficient reservoir development. However, owing to the limitations of actual core data, traditional modeling methods face bottlenecks at the data-driven level, posing challenges to the establishment of accurate fluid distribution characterization models. In this study, the fluid distribution laws in tight sandstones were first analyzed based on core experimental data. Then, the Generative Adversarial Networks (GAN) were used to expand the core dataset. The results of core data processing indicated that the fluid distribution laws of the generated data were consistent with those of the original data, which verified the effectiveness of the adopted data expansion method. Finally, the fluid distribution prediction model were established based on a Multilayer Perceptron (MLP) and realized the accurate characterization of the distributions of irreducible and movable fluids in tight sandstone reservoirs through core experiments and logging data processing.

How to cite: Jin, G., Qin, S., Ma, Y., and Jin, B.: Research on Fluid Distribution Characterization Method of Tight Sandstone Reservoirs Based on Machine Learning Using Nuclear Magnetic Resonance Logging, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6375, https://doi.org/10.5194/egusphere-egu26-6375, 2026.

The fractures in the deep shale reservoirs in southern Sichuan are one of the main factors affecting the enrichment of shale gas, fracturing design, and development effectiveness. Therefore, it is necessary to carry out multi-scale joint inversion methods to identify and evaluate fractures at different scales to improve the accuracy of their evaluation and prediction.

To address the above problems, four different scale fracture evaluation methods and their consistency studies have been conducted, including core, logging, remote detection, and seismic. Through the analysis of geological characteristics, core observation, CT scanning and experiments on acoustic anisotropy, the intervals of shale fractures and their anisotropic characteristics are determined. The anisotropy coefficient of shale reservoirs is calculated by dipole cross-wave logging, determining the longitudinal development degree and characteristics of shale reservoir fractures. The acoustic field imaging is used to extract the reflection coefficients of high and low frequency Stoneley wave and equivalent anisotropic coefficients of shear reflection waves, which allowed for the identification and evaluation of the development characteristics and effectiveness of fractures near borehole at a remote acoustic detection scale. Based on high-resolution seismic data with wide azimuth vector offset, the pre-stack seismic anisotropy coefficient is constructed, clarifying the intrinsic relationships between the fracture medium reservoir parameters described at four different scales.

The study shows that the deep shale reservoir fractures of Wufeng-Longmaxi Formation in southern Sichuan are mainly bedding fractures, mostly in a closed state, followed by structural fractures, most of which are filled with calcite. The high-and low-frequency Stoneley wave reflection coefficients in the formations Wufeng-Long11 of the first deep shale gas evaluation well Lu203 in southern Sichuan, show a significant difference, with fracture development and good reservoir connectivity, with an initial production of 1.38×106m3/d. Determining the acoustic anisotropy coefficient is a common parameter for evaluating shale reservoir fractures from core scale to seismic scale, and innovating the acoustic remote detection of shear wave reflection waves equivalent anisotropic coefficients fills the gap in detection range and resolution from logging to seismic scale, achieving high precision prediction of small-scale fractures of 3-20m. The results are generally consistent with core, logging, acoustic remote detection imaging, gas testing results, and production performance, proving the effectiveness of the multi-scale fracture evaluation method of shale reservoirs using cross-scale constraints. Further predictions indicate that the well area Lu 203 is characterized by the development of small fractures/bedding fractures on the basis of relatively stable structures, while the well area Yang101 is the development of structural fractures on the basis of large fault, with multiple calcite fillings, which is one of the main factors contributing to the significant differences in shale gas production between the above two areas. The proposed multi-scale fracture evaluation method based on acoustic anisotropy coefficients provides significant reference value for the comprehensive evaluation of unconventional reservoir fractures.

This research was supported by the National Oil & Gas Major Project (No. 2025ZD1403902) and the CNPC International Science and Technology Cooperation Project (No. 2023DQ0422).

Fig. 1  AVAZ inversion plan along the O₃w layer in L203 Block in Southern Sichuan

How to cite: Xiao, Y., Wang, H., Ge, X., Xiao, G., Chen, S., Wei, Z., and Jiang, Z.: Multi-scale Fracture Evaluation Method for Shale Reservoirs in Deep Formation Wufeng-Longmaxi in Southern Sichuan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6994, https://doi.org/10.5194/egusphere-egu26-6994, 2026.

The construction of three-dimensional multi-mineral digital rock cores is essential for the fine characterization of reservoir pore–throat structures and for the quantitative evaluation of reservoir electrical, acoustic, and mechanical properties.Existing digital rock core modeling approaches can be broadly classified into physical experimental methods and numerical reconstruction methods.Physical experimental methods can produce relatively realistic three-dimensional digital rock cores; however, they are costly and struggle to achieve fine mineral discrimination.Numerical reconstruction methods offer advantages such as low cost and high efficiency; however, most high-fidelity approaches remain limited to single-mineral digital rock cores, whereas multi-mineral modeling methods often rely on idealized assumptions.To address these limitations, this study proposes a 2D–3DGAN-based deep learning algorithm capable of generating three-dimensional multi-mineral digital rock cores from a single AMICS image.Using the “AMICS + 2D–3DGAN” modeling framework, three-dimensional multi-mineral digital rock cores are constructed with high accuracy, efficiency, and realistic multi-mineral representation.The accuracy of the generated results is systematically evaluated by analyzing diagenetic characteristics and by comparing the generated cores with training images in terms of mineral content, pore size distribution, and two-point correlation functions.The results demonstrate that the proposed method significantly enhances reconstruction accuracy and generation efficiency while maintaining economic feasibility, thereby providing a solid foundation for subsequent simulations of multi-mineral diagenetic evolution and reservoir property analysis.Previous studies on diagenesis have largely relied on qualitative approaches, such as identifying diagenetic evolution sequences and constructing two-dimensional schematic representations.Based on the generated three-dimensional multi-mineral digital rock cores, this study proposes a “nucleation–growth” algorithm for numerically simulating diagenetic processes, enabling quantitative modeling of diverse cementation morphologies that closely reflect geological conditions.Meanwhile, the four-parameter structure method is improved to quantitatively simulate dissolution and replacement processes with varying morphologies and to control the precipitation location of dissolution by-products, either inside or outside dissolution pores.Ultimately, a three-dimensional digital rock core diagenetic evolution model based on diagenetic sequences is established, enabling analysis of the evolution of reservoir properties—such as porosity, permeability, tortuosity, and coordination number—through diagenetic processes.

How to cite: Lu, Z., Xi, K., and Wu, Y.: Deep Learning–Based 3D Multi-Mineral Digital Rock Modeling and Diagenetic Simulation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7042, https://doi.org/10.5194/egusphere-egu26-7042, 2026.

The physical characterization of marine sediment cores is pivotal for the exploration of shallow seafloor resources, such as gas hydrates and shallow gas. While non-intrusive imaging is essential for preserving sample integrity, conventional detection methods face significant limitations. Although X-ray CT offers high-resolution imaging, its application is constrained by bulky instrumentation and time-consuming scanning processes, rendering it impractical for rapid, on-site evaluation of sediment cores. Conversely, Electrical Resistivity Tomography (ERT) offers high sensitivity to electrical properties but relies on contact measurements. This inevitably disturbs the original structure of unconsolidated sediments and suffers from electrode polarization, thereby degrading imaging quality.

To address these challenges, this study investigates a non-intrusive imaging technique based on Magnetic Induction Tomography (MIT) to probe the internal electrical conductivity structure of sediments. In this method, excitation coils surrounding the core are energized with time-harmonic alternating current to generate a primary magnetic field. According to the principle of electromagnetic induction, eddy currents are induced within the conductive sediment, subsequently generating a secondary magnetic field that opposes the primary one. Since the intensity and path of these eddy currents are strictly governed by the spatial distribution of conductivity within the core, the internal structural information can be retrieved by detecting the perturbed total magnetic field via an array of receiver coils.

The feasibility of this proposed method was validated through forward modeling based on the Finite Element Method. To reconstruct the conductivity distribution from the magnetic measurements, the inverse problem was solved using the Gauss-Newton method. Preliminary simulation results demonstrate that the magnetic induction-based approach can effectively recover the internal electrical structure of the target, confirming its potential as a compact and efficient tool for marine sediment characterization.

This work was supported by the National Natural Science Foundation of China, Grant No. 42274232.

How to cite: Yuan, C., Zou, C., and Peng, C.: Research on Non-intrusive Detection of Marine Sediment Cores Using Magnetic Induction Tomography, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7276, https://doi.org/10.5194/egusphere-egu26-7276, 2026.

Electromagnetic logging while drilling (EM LWD) provides unique capabilities for both look-around and look-ahead detection, making it a foundational technology for identifying formation interfaces in complex horizontal and ultra-deep well applications. Over recent decades, its detection depth has advanced from a few meters to a remarkable range of several tens of meters. Nevertheless, directional EM LWD remains the primary geosteering method in horizontal wells, largely due to its sensitivity to formation boundaries and relatively low operational cost. In early directional EM LWD applications, where the maximum detection range was generally below 5 meters, one-dimensional (1D) formation models were widely adopted for forward simulation and inversion. However, current-generation tools can achieve detection ranges approaching ten meters, meaning that formation influences now extend across a significantly larger volume. Continuing to use conventional 1D models under such conditions may introduce substantial errors. While transitioning to two-dimensional (2D) models preserves accuracy, the increased model dimensionality substantially reduces computational speed, hindering real-time inversion.

In this paper, we propose an approximate method for simulating directional EM LWD responses in 2D models, designed to address this inherent trade-off between computational efficiency and accuracy. The core concept involves reducing complex 2D geological models into a series of 1D representations, which are then solved using a pseudo-analytical algorithm. First, the computational domain is divided into sliding windows of varying widths, defined with reference to the tool’s depth of investigation. Within each window, curved formation interfaces are approximated by straight lines to construct a 1D model, from which the tool response is computed. The series of 1D responses is then synthesized to obtain an approximate response for the original 2D model. To ensure both efficiency and reliability, an automated workflow integrating progressive model reduction with real-time quality control is implemented. The process begins with a coarse 1D approximation (typically 3–5 models). A divergence metric quantifies its representativeness; if below a preset threshold, the result is accepted. Otherwise, an iterative refinement phase is activated, dynamically increasing the number of 1D models only where necessary until the synthesized response stabilizes. The algorithm was validated on representative fault and fold models. Numerical results demonstrate that the proposed method successfully avoids the oversimplification inherent in conventional 1D modeling while achieving a computational speedup of more than 10 times compared to conventional full 2D numerical simulations.

We are indebted to the financial support from the National Natural Science Foundation of China (42304140).

How to cite: Wu, Z. and Yue, X.: An Efficient Method for Approximating Directional Electromagnetic LWD Responses in Complex 2D Formation Models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8555, https://doi.org/10.5194/egusphere-egu26-8555, 2026.

Micro-resistivity imaging logging is one of the primary techniques for subsurface fracture identification. However, conventional manual interpretation is time-consuming and highly subjective, while existing deep learning-based methods generally require large-scale, well-annotated datasets, resulting in substantial labeling costs and limited applicability in data-scarce scenarios. To address these challenges, this study proposes a fracture identification method for electrical image logs under limited-sample conditions by incorporating prior knowledge derived from outcrop fractures. Leveraging the morphological and statistical similarities between surface outcrop fractures and subsurface electrical image responses, a two-stage training strategy based on a Multi-scale Attention Network (MANet) backbone is developed. In the first stage, optical images of outcrop fractures are preprocessed through grayscale transformation and noise injection to approximate the feature distribution of electrical image logs, enabling the network to learn generalizable edge and texture features. In the second stage, outcrop–electrical image pairs with similar fracture morphology and texture characteristics are generated through similarity matching, and the model is fine-tuned using a composite loss function incorporating Correlation Alignment (CORAL), thereby accelerating domain adaptation to subsurface logging environments. Experimental results from basement reservoirs in the Dongping area of the Qaidam Basin demonstrate that the proposed method significantly improves fracture identification performance under limited-sample conditions. Compared with baseline models, the proposed approach achieves improvements of 13.05% in accuracy and 12.88% in Intersection over Union (IoU), reaching 81.13% and 75.74%, respectively. These results indicate that the proposed method effectively alleviates data scarcity issues in electrical image log interpretation and provides robust technical support for fracture characterization and hydrocarbon resource evaluation.

How to cite: Ai, Y., Wang, L., Gu, M., Gang, L., Shi, P., and Zhao, Z.: Fracture Identification in Electrical Image Logs with Limited Samples by Incorporating Outcrop Priors, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8628, https://doi.org/10.5194/egusphere-egu26-8628, 2026.

As the fundamental constituent units of hybrid shale oil reservoirs, laminae exhibit complex structures and diverse combination types, leading to strong reservoir heterogeneity. Therefore, revealing the structural characteristics of laminae and their controlling mechanisms is crucial for understanding the storage and occurrence properties of such reservoirs. This study focuses on the shale reservoirs of the Fengcheng Formation in Mabei, Xinjiang, integrating methods such as cast thin sections, field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and nitrogen adsorption to characterize the storage properties of laminae. On this basis, laser scanning confocal microscopy was used to accurately analyze the oil-bearing properties and occurrence differences within individual laminae. Furthermore, nuclear magnetic resonance (NMR) technology was applied to elucidate the controlling effects of complex laminae combinations on storage and occurrence. The results indicate that: (1) The Fengcheng Formation contains diverse laminae types, including felsic clastic laminae, tuff laminae, dolomite laminae, limestone laminae, siliceous laminae, borax laminae, and organic matter laminae. (2) The primary storage spaces consist of dissolution pores in felsic clastic/tuff laminae, intragranular and intercrystalline pores in dolomite laminae, and intercrystalline pores in limestone/siliceous/borax laminae. Strong dissolution, conducive to the development of high-quality reservoirs, occurs when the proportion of felsic clastic laminae is 50%-60% or dolomite laminae is 20%-40%. (3) Confocal microscopy characterization reveals that dolomite, micritic siliceous, and felsic clastic laminae exhibit superior oil-bearing properties. The differences in oil-bearing properties are mainly controlled by two key factors: ① The interbedded distribution of source and reservoir rocks without barriers; the closer a lamina is to the source, the better its oil-bearing properties. ② A coordinated source-to-reservoir ratio; the optimal oil-bearing properties occur when the ratio of source laminae to reservoir laminae is 1:3 to 2:1, with the poorest properties when the ratio exceeds 6:1. NMR core-scale testing further validates these controlling patterns. The conclusion is that the key to efficient storage and occurrence of shale oil lies in the optimal spatial configuration and coordinated proportion between high-quality source rocks and effective reservoirs.

How to cite: Wang, H.: Research on the Control Mechanism of Mixed-Type Shale Laminar Structure on Shale Oil Reservoir and Storage Properties, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8636, https://doi.org/10.5194/egusphere-egu26-8636, 2026.

Marine shales of X-block are featured with poor organic matter, over maturity and complex mineralogical assemblages, which collectively result in weak organic-related logging responses. As a consequence, conventional total organic carbon (TOC) evaluation methods exhibit substantial uncertainties associated with baseline calibration and parameter generalization, thereby limiting prediction accuracy and robustness.

To overcome these limitations, this study develops a physics-constrained deep learning framework for TOC prediction that integrates multi-source logging data using a hybrid DNN–LSTM–GPC architecture. High-resolution nuclear magnetic resonance (NMR) and electrical imaging logs are incorporated as primary data sources to extract multi-scale, organic-matter-sensitive features. These features are further integrated with conventional well logs to construct a comprehensive feature space that captures organic matter distribution, pore structure characteristics, and lithological variability. In addition, an improved ΔLogR model and region-specific rock-physics constraints are embedded within the deep learning framework to ensure physical consistency and geological interpretability.

Application results demonstrate that the proposed method achieves superior prediction performance in low–organic-matter marine shales, yielding a root mean square error of 0.08% and a coefficient of determination (R²) of 0.95. The model consistently outperforms multivariate regression, uranium-based approaches, and porosity-difference methods, while maintaining stable predictive capability in intervals exhibiting pronounced TOC heterogeneity. These results indicate that physics-constrained deep learning integrated with multi-source logging data provides a reliable and effective approach for micro-scale TOC evaluation and favorable reservoir identification in low–organic-matter marine shale systems.

This research was supported by the Natural Science Foundation of Shandong Province of China (ZR2023YQ034) and Shida Jingwei Industry-Education Integration Research Institute Project (15572259-25-ZC0607-0011).

Fig. 1. Comparison of total organic carbon (TOC) prediction results for marine shale in Well JY6, X Block, obtained from different methods.

How to cite: Ge, X., Wang, Z., He, C., Ge, X., Fan, J., Wu, X., Yang, D., and Zhai, C.: A DNN–LSTM–GPC framework for TOC prediction in marine shales of the X Block using multi-source logging data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8697, https://doi.org/10.5194/egusphere-egu26-8697, 2026.

The fluid inclusion homogenization temperature method is the most widely used approach for reconstructing hydrocarbon accumulation history in deep to ultra-deep carbonate rocks. However, there are three major limitations: (1) The difficulty in restoring the eroded thickness of target strata, leading to uncertainty in the reconstruction of tectonic and burial history. (2) It is common to use the brine inclusion homogenization temperature as a substitute for hydrocarbon inclusion trapping temperature. However, it is often challenging to find coexisting hydrocarbon and brine inclusions within the same host mineral. (3) It is difficult to determine the trapping time based on the trapping temperature. As in multi-cycle superimposed basins, the same trapping temperature may correspond to multiple ages on the tectonic-burial history curve, resulting in non-unique solutions.

The application of laser U-Pb isotopic dating of carbonate minerals and clumped isotope thermometry has addressed these limitations and led to the development of a new method for reconstructing hydrocarbon accumulation history in deep to ultra-deep carbonate reservoirs. (1) Restoration of the eroded thickness of strata is achieved through multi-phase diagenetic mineral age-temperature constraints, effectively resolving the uncertainty in the tectonic-burial history reconstruction. (2) Direct measurement of hydrocarbon inclusion trapping temperature, overcoming the challenge of determining trapping temperature when no coexisting brine inclusions are present in the host mineral. (3) Direct measurement of hydrocarbon inclusion trapping time, addressing the issue of non-uniqueness in accumulation ages for multi-cycle superimposed basins.

This method has been applied to reconstruct the accumulation history of natural gas reservoirs in the Dengying Formation of the Sichuan Basin. It has significantly higher accuracy and success rates compared to the fluid inclusion homogenization temperature method.

How to cite: Anping, H. and Anjiang, S.: A New Method for Reconstructing Hydrocarbon Accumulation History of Deep to Ultra-Deep Carbonate Reservoirs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8804, https://doi.org/10.5194/egusphere-egu26-8804, 2026.

Shale oil and gas, as typical unconventional resources, have become crucial for stabilizing oil and gas production in China. The calculation of shale reservoir fracturability index is a core step in evaluating engineering sweet spots. However, acoustic logging data of shale formations are susceptible to geological interfaces, borehole deviation, and particularly the bedded structure-induced anisotropy. Such anisotropy causes the measured acoustic interval transit time to deviate from the true formation value, leading to inaccurate in-situ stress calculation and thus compromising the reliability of fracturability evaluation. Accurate acquisition of formation AIT is essential for optimizing fracturing schemes and analyzing borehole stability.

To address this issue, a correction method for acoustic anisotropy of bedded shale in horizontal wells was established. Based on the bedded shale medium model assumption, the formation stiffness coefficients and elastic wave velocities were derived using the equivalent medium theory and elastic wave equation. Shale models with varying bedding angles were constructed to analyze the functional relationships between acoustic velocity, acoustic anisotropy coefficient, and bedding angle.

Numerical simulation results demonstrate that the acoustic interval transit time of bedded shale exhibits significant anisotropic characteristics: both acoustic interval transit time and acoustic anisotropy coefficient increase with the increase of bedding angle, and the acoustic anisotropy coefficient has a good power exponential relationship with the cosine of the bedding angle. A correction model for acoustic anisotropy of bedded shale was thus developed. Field application in horizontal wells of Block L showed that after correction, the peak of the normal distribution curve of acoustic interval transit time in the target reservoir of horizontal wells was highly consistent with that of pilot wells. The corrected acoustic interval transit time was used to calculate Poisson's ratio, Young's modulus, horizontal principal stress difference, and fracture pressure, further deriving the fracturability index. Comparison with the fracturability index calculated from core experimental data indicated that the average relative error of the results was reduced by 9.28%.

This study provides a reliable acoustic anisotropy correction method for bedded shale, which is of great significance for improving the accuracy of shale reservoir fracturability evaluation and guiding the identification of engineering sweet spots.

How to cite: Wu, B. and Zhao, J.: Correction of Acoustic Anisotropy of Bedded Shale and Its Application in Fracturability Evaluation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9050, https://doi.org/10.5194/egusphere-egu26-9050, 2026.

The effective development of unconventional shale oil reservoirs depends on the accurate prediction of seepage pathways. In the lacustrine shales of the upper submember of the fourth member of the Shahejie Formation (Es4U) in the Dongying Sag, bedding fractures serve as both primary storage spaces and critical fluid conduits. However, intense reservoir heterogeneity makes it challenging to reliably identify fracture-prone intervals using well-log responses alone.

Based on systematic core descriptions and fracture statistics from research wells, this study integrates thin-section petrography, X-ray diffraction (XRD), total organic carbon (TOC) analysis, and logging data to perform a multi-scale correlation of bedding fracture occurrences.

The results demonstrate that: (1) bedding fractures are diagenetic products triggered by clay mineral dehydration/transformation and hydrocarbon-induced overpressure; (2) the development of these fractures is synergistically governed by carbonate and clay mineral contents, organic matter abundance, and lamina density, with laminated clay-rich lithofacies identified as the dominant zones for fracture occurrence.Building upon these geological insights, a vertical development model for bedding fractures was established to provide quantitative geological constraints for logging evaluation. By utilizing conventional logging suites (e.g., integrating AC, GR, and DEN curves), intervals characterized by carbonate content of 20%–60%, clay content of 20%–35%, TOC > 2%, and well-developed laminated structures are identified as potential fracture-bearing zones.

This model effectively bridges geological genesis with logging attributes, providing a direct geological foundation for the prediction of favorable seepage pathways in shale reservoirs using borehole geophysical data.

How to cite: Liu, C., Wang, G., and Yin, Z.: Geological Patterns and Conventional Well-log Prediction of Bedding Fractures in Lacustrine Shales: A Case Study from the Upper Submember of the 4th Member of the Shahejie Formation, Dongying Sag, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10363, https://doi.org/10.5194/egusphere-egu26-10363, 2026.

Carbonate reservoirs possess substantial hydrocarbon resource potential and have become a major focus of exploration in recent years. However, the complex geological settings, pronounced lithological heterogeneity, and strong vertical variability of carbonate reservoirs pose significant challenges to conventional lithofacies identification methods. Such approaches are often time-consuming, highly subjective, and limited in their ability to accurately discriminate complex lithofacies assemblages, thereby constraining the efficient development of carbonate reservoirs. Geophysical well logging offers advantages such as low acquisition cost, continuous coverage, and high vertical resolution, making it a fundamental dataset for continuous lithofacies characterization. In this study, integrated geological information, including whole-rock analysis, petrographic thin sections, and scanning electron microscopy, is employed to systematically investigate the mineral compositions and pore structure characteristics of different lithofacies, and the corresponding logging response mechanisms are quantitatively investigated. Five conventional logging curves—gamma ray, acoustic, neutron porosity, bulk density, and resistivity—are selected to construct a multidimensional feature parameter set. Considering the differences in numerical ranges among logging curves and the imbalance in lithofacies sample distributions, data normalization and class imbalance correction are performed prior to model training. Subsequently, a high-precision lithofacies identification model based on conventional well logs is developed by integrating Bayesian optimization with the Extreme Gradient Boosting (XGBoost) algorithm. The results indicate that the predicted lithofacies are highly consistent with geological interpretations, core observations, and thin-section identifications, demonstrating that the Bayesian-optimized XGBoost model exhibits robust classification performance and effectively captures the complex nonlinear relationships between lithofacies and logging responses. Compared with traditional machine learning methods, the proposed approach achieves significantly improved identification accuracy, providing robust technical support and a reliable theoretical foundation for carbonate reservoir evaluation and hydrocarbon exploration.

How to cite: Yang, J., Wang, L., Gu, M., Ai, Y., and Li, G.: Bayesian-Optimized XGBoost for High-Precision Lithofacies Identification in Carbonate Reservoirs Using Geophysical Well Logs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11545, https://doi.org/10.5194/egusphere-egu26-11545, 2026.

Shales exhibit significant seismic anisotropy, which can cause errors in seismic imaging, seismic attribute analyses and reservoir characterization. During deposition and burial, platy clay mineral particles tend to align along bedding planes, creating distinct direction-dependent seismic properties — a primary cause of anisotropy. Therefore, investigating the influence of clay on the evolution of shale anisotropy during compaction is crucial for enhancing seismic interpretation accuracy. However, quartz, serving as the primary rigid grain in shales, complicates this process. It forms a resisting framework that interferes with the preferred orientation of clay platelets. Hence, solely studying pure clay is insufficient, and investigating the compaction behavior of clay-quartz mixtures is of great value to accurately characterize the evolution of seismic anisotropy in realistic shale environments. Given their dominance in shale mineralogy, kaolinite and illite were chosen as the representative clay end-members for our study. We prepared eight clay-quartz mixtures (4 kaolinite-quartz mixtures and 4 illite-quartz mixtures) by mixing different amounts of clay and quartz: 100%, 80%, 60% and 40% of clay by weight. To simulate the natural burial process, these water-saturated loose sediments were subjected to uniaxial mechanical compaction experiments.

Experimental results indicate that for both groups, porosity decreases monotonically while seismic anisotropy increases with increasing compaction stress. However, the influence of quartz content differs significantly between the two mineral systems. In the kaolinite group, Thomsen parameters ε and γ show a negative correlation with quartz content at identical pressure levels. The illite group, conversely, exhibits more complex behavior: while γ remains negatively correlated with quartz fraction, ε displays a non-monotonic trend—initially decreasing, then increasing, and finally decreasing again as quartz content rises. Furthermore, the parameter δ serves as a distinct discriminator: kaolinite mixtures exhibit suppressed δ values (near zero or negative), whereas illite mixtures consistently display positive δ values (> 0.10), indicative of well-stratified compliant bedding.

These findings underscore that the "bridging effect" of quartz and the resulting anisotropy are strictly controlled by clay mineralogy. Specifically, the high positive δ of illite implies that conventional elliptical assumptions may cause significant errors in processing. Therefore, accurate seismic interpretation requires mineral-specific anisotropic models that account for the distinct structural evolution of kaolinite and illite during compaction.

Fig. 1 Thomsen’s anisotropy parameters ε, γ, and δ versus porosity for the K-series and I-series samples.

How to cite: Guo, X. and Han, T.: Evolution of Anisotropic Acoustic Properties in Clay-Quartz Mixtures during Mechanical Compaction: Implications for Shale Burial, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11711, https://doi.org/10.5194/egusphere-egu26-11711, 2026.

Aiming at the challenge of achieving full-wellbore continuous monitoring and quantitative evaluation of shale gas well production profiles, this study develops an evaluation method based on distributed fiber-optic temperature sensing (DTS). By comprehensively considering the effects of fluid thermal convection in the wellbore, wellbore-formation heat exchange, and the Joule-Thomson effect on wellbore temperature distribution, a forward model of wellbore fluid temperature is established to accurately characterize the depth-dependent temperature distribution of the wellbore. On this basis, with model parameters as inversion variables, a squared error objective function between DTS-measured temperatures and forward-calculated temperatures is constructed, and the Bayesian inversion method is employed to solve the key parameters of the production profile, realizing the quantitative inversion of gas-water two-phase production for each producing layer. Field verification is conducted using an actual shale gas well: results show that the average absolute error between forward-calculated temperatures and DTS-measured temperatures is 0.05°C, indicating a high overall profile agreement; the gas and water two-phase production obtained by inversion is consistent with the on-site production dynamic recognition law, verifying the accuracy and reliability of the proposed method. This research provides effective technical support for the engineering application of DTS technology in production dynamic monitoring and development optimization of shale gas wells.

How to cite: Song, H., Xia, T., Wang, M., Huang, H., Zhang, Z., Fang, S., Nie, X., Zhao, B., and Zhang, C.: esearch on roduction rofile evaluation method for shale gas wells based on distributed Fiber-Optic Temperature Sensing (DTS), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15374, https://doi.org/10.5194/egusphere-egu26-15374, 2026.

Unconventional reservoirs are confronted with numerous challenges due to their complex pore structures and diverse fluid occurrence states; therefore, the accurate acquisition of pore structure characteristics and fluid property parameters has become the key factor restricting development efficiency. 2D nuclear magnetic resonance (NMR) logging obtains more observation information by introducing longitudinal relaxation time (T₁), and it exhibits good application effects in wettability evaluation, reservoir parameter calculation and fluid property identification .The factors affecting NMR relaxation signals are multivariate, mainly including pore fluid properties (viscosity, density, etc.), pore structure characteristics (pore size and distribution, pore connectivity, etc.), magnetic susceptibility differences between rocks and fluids, and the content and distribution of paramagnetic minerals. The coupling of these factors complicates the mapping relationship between relaxation signals and reservoir properties, which urgently requires in-depth analysis.In the field of shale oil exploration and development, there are various types of 2D NMR instruments, and significant differences exist in their observation methods, measurement sequences and acquisition parameters, which further pose challenges to the cross-scale characterization and comprehensive interpretation of reservoir parameters. Therefore, this study analyzes the influencing factors of acquisition parameters on NMR from both experimental and numerical simulation scales.The influence of instrument frequency on 2D NMR is investigated through experiments, and the effects of echo spacing, waiting time and number of echoes on NMR measurement and inversion results are studied via numerical simulation. This research further clarifies the influence of acquisition parameters on fluid distribution characterized by 2D NMR, and verifies the reliability of NMR logging interpretation data in shale reservoirs.

How to cite: Zhao, J., Xie, B., Wang, Y., and Bai, L.: Research on the Influence of NMR Acquisition Parameters on Fluid Distribution in Shale Reservoirs, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15411, https://doi.org/10.5194/egusphere-egu26-15411, 2026.

Currently, researchers at home and abroad mainly study the pore space of marine shale, so the research on the pore space of marine-transitional shale is still relatively small. Based on the research findings, marine-continental transitional shale exhibits wide distribution, significant resource potential, and promising exploration and development prospects. This paper addressed this gap by analyzing the geochemical characteristics, petrological features, and pore characteristics of shale in the Upper Permian Leping Formation of the Sichuan Basin. It has been obtained through experimental analysis that the shale's reservoir storage space primarily consisted of organic pores, providing significant potential for shale gas accumulation. The gas adsorption experiments demonstrated the development of micropores, which contributed substantially to the pore specific surface area and volume in Leping shales. Additionally, the TOC content and clay minerals in shale samples have an influence on the storage space of shale. The TOC content had a more significant correlation with micropore volume and specific surface area, and clay mineral content had a better correlation with pore volume and specific surface area of mesopores and macropores. Furthermore, the calculated fractal dimensions (D₁, D₂, and D₃) for the Leping Formation ranged from 2.3616 to 2.5832 (average: 2.4835), 2.7698 to 2.8654 (average: 2.8172), and 2.3274 to 2.5161 (average: 2.4287), respectively. These values indicate that the pores have a complexity and heterogeneous. D₁ and D₂ are mainly affected by clay minerals, indicating that the complexity of mesopores is mainly controlled by clay minerals; D₃ is mainly controlled by TOC content, indicating that the complexity of micropores is mainly controlled by TOC content.D₃ has a good positive correlation with pore characterization parameter, indicating that the pore will be more complex with the increase of micropores' percentage and its volume and surface area.The findings from this study provide valuable insights into the pore characteristics of marine-continental transitional shale and offer potential implications for exploration and development prospects in the area.

How to cite: xiao, S. and Fu, X.: The pore characteristics and fractal characteristics of Permian marine-continental transitional shales in the Sichuan Basin, southwestern China, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15446, https://doi.org/10.5194/egusphere-egu26-15446, 2026.

Look-ahead electromagnetic logging while drilling (EM LWD) technology has been widely used in geostopping due to its capability to characterize resistivity distributions ahead of the bit. However, a complex nonlinear relationship between tool responses and rock properties makes it difficult to image the geological structure. Therefore, inversion is essential to reconstruct resistivity distributions and determine formation boundary positions. Currently, gradient-based and artificial intelligence algorithms are commonly used for look-ahead inversion and have shown considerable potential. However, most related studies have focused on the analysis of tool detection capabilities, while research regarding inversion applicability and thin-layer formations remain insufficiently addressed. Additionally, due to the weak signal contribution from the area ahead of the drill bit, challenges still remain, such as the high tendency of inversion to fall into local optima and strong dependence on tool configuration.

In this paper, a look-ahead inversion framework based on the Levenberg-Marquardt (LM) algorithm is proposed, and its applicability to layered formation inversion is investigated. To prevent the inversion from being trapped in local optima, a continuous random multi-initial-value search strategy is proposed. Specifically, formation resistivity from look-around detection is used as a constraint, and random perturbations are applied to parameters to be inverted to select multiple initial values. Furthermore, the look-ahead signal decay is closely associated with tool configuration. By optimizing the selection of response curves through the adjustment of coil frequencies and transmitter-receiver spacings, the accuracy of the look-ahead inversion is further improved. Results demonstrate that the proposed inversion framework achieves an accuracy of up to 80% in the inversion of the distance to the nearest boundary for layered formations and yields favorable results in thin and multi-layer formations. It also provides algorithmic support for look-ahead EM LWD inversion and lays a theoretical foundation for further research on look-ahead inversion in formations with complex structures.

We are indebted to the financial support from the National Natural Science Foundation of China (42304140).

How to cite: Xiao, H., Cheng, C., and Wu, Z.: An LM Algorithm-Based Inversion and Applicability Analysis for Look-Ahead Electromagnetic Logging While Drilling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15621, https://doi.org/10.5194/egusphere-egu26-15621, 2026.

Tight sandstone reservoirs are generally characterized by low porosity, low permeability, and complex pore structures, which pose significant challenges to reservoir evaluation. Nuclear Magnetic Resonance (NMR) T₂ spectra can characterize pore structures and fluid states in porous media. Therefore, gas-displacing-water petrophysical experiments were carried out to investigate the variations of T₂ spectra under different saturation states.The study finds that the T₂ spectrum in the original state presents a multi-peak distribution dominated by long relaxation components. With the increase of gas saturation, the macropore peak value decreases and the porosity tends to decrease. Meanwhile, compared with the gas-bearing state, the macropore peak widens when the core is water-saturated. Analysis shows that this phenomenon is caused by the low hydrogen index of gas.Thus, a spectrum correction model for gas-bearing sandstone reservoirs was established based on the Gaussian distribution, which corrects the T₂ spectrum morphology and porosity components to their original states. After gas saturation correction, the T₂ spectrum mainly presents a bimodal distribution dominated by short relaxation components. The irreducible water saturation calculated from the corrected T₂ spectrum is more consistent with the core measurement results.Combined with NMR experiments, this study clarifies the NMR response mechanism of reservoirs under different saturation states, establishes a Gaussian distribution model for T₂ spectrum correction of gas-bearing sandstone, and ultimately achieves accurate characterization of pore structures.

How to cite: Xie, B., zhang, B., tang, Q., zhu, X., zhao, J., wang, Y., bai, L., and lai, Q.: A Method for Characterizing Pore Structure of Gas-Bearing Sandstone Based on Nuclear Magnetic Resonance, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15708, https://doi.org/10.5194/egusphere-egu26-15708, 2026.

Due to its favorable pore-permeability characteristics, sandstone serves a crucial role in applications such as oil and gas production, freshwater extraction, and underground CO₂ storage. As the primary reservoir space and migration pathway for hydrocarbons, accurate porosity data are essential for seismic exploration and reservoir development. In unconventional reservoirs, establishing an appropriate tight sandstone rock physics model is key to understanding how petrophysical parameters influence porosity and permeability. However, conventional models often fail to dequately represent both the three-dimensional irregularity of pore geometries and the spatial distribution of clay minerals.

To address these limitations, this study develops a novel rock physics model that incorporates superspherical pores and clay distribution to characterize argillaceous sandstone. Clay is categorized into structural clay (within the matrix) and dispersed clay (within pores). Following a sequential approach that reflects natural sandstone grain stacking, the proposed model is constructed by employing Voigt-Reuss-Hill averaging methods to incorporate structural clay, coupling the superspherical pores model, and applying solid substitution equations to determine the saturated rock modulus, which contains dispersed clay. This framework allows quantitative analysis of how pore geometry and clay occurrence states affect rock elastic properties.

However, in practical settings, the high cost of well logging often prevents direct measurement of these parameters. Therefore, a simulated annealing algorithm is employed to inversely determine pore characteristics and clay content in different occurrence states at each sampling point along the wellbore.

The validity and practical applicability of the proposed model are demonstrated through comprehensive sensitivity analyses and real-data applications.

How to cite: Wang, B., Yin, X., Ma, Z., and Li, K.: Development and Application of a Rock Physics Model to Infer Pore-Geometry and Clay-Distribution in Argillaceous Sandstone, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16220, https://doi.org/10.5194/egusphere-egu26-16220, 2026.

Detecting fluid contacts, such as oil-water interfaces, remains a significant challenge for conventional seismic exploration when the acoustic impedance contrast is weak. However, the seismoelectric effect, which depends on electrical conductivity and electrokinetic coupling, provides a promising method for interface detection. In this study, we investigate the radiating seismoelectric electromagnetic (EM) reflection and transmission coefficients generated at an interface between two fluid-saturated porous media under fast P-wave incidence.

We analytically derive the reflection and transmission coefficients for all generated wave modes (fast P, slow P, SV, and EM waves) using Pride’s theory and Helmholtz decomposition. To verify the validity of our derivation, we calculate the energy fluxes of the wavefields using the electrokinetic Poynting vector and confirm that energy conservation is strictly satisfied.

Analyzing the frequency- and angle-dependent behavior of the reflection and transmission coefficients, we show that seismoelectric conversion is directly governed by the contrast in elastic and electric properties of the reservoir and the fluids therein, at the interface. Building on this, we compare the response of an "elastic interface" (strong mechanical contrast) with an "electric interface" (strong conductivity contrast but weak mechanical contrast, representing an oil-water contact). The analysis shows that while the seismic reflection energy at the electric interface is negligible, the reflected seismoelectric EM wave energy is comparable to the seismic signal generated at a strong lithological interface. These findings suggest that reflected seismoelectric waves are a reliable tool for identifying oil-water interfaces that are effectively invisible to conventional seismic surveys.

How to cite: Liu, Y. and Smeulders, D.: Seismoelectric Radiating Reflection and Transmission at Porous–Porous Interfaces: Angle- and Frequency-Dependent Coefficients, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17981, https://doi.org/10.5194/egusphere-egu26-17981, 2026.

Continental shale oil reservoirs in China are abundant in Solid Organic Matter (SOM), which represents a significant potential hydrocarbon resource. Nuclear Magnetic Resonance (NMR) is an effective tool for evaluating shale oil reservoirs. However, restricted by instrument dead time, it is difficult for the conventional CPMG acquisition mode to capture signals from ultra-short relaxation components such as SOM.To address this, this work introduces the Free Induction Decay (FID) acquisition mode, proposing two novel quantification workflows based on FID signal integration and a direct T₁-T₂^* spectrum, respectively. These approaches are designed to effectively capture the ultra-short components that are typically missed by standard CPMG sequences.The proposed methodology was validated against comprehensive geochemical benchmarks. The results demonstrate that the FID-driven approach yields a superior correlation with geochemical data compared to conventional methods. It exhibits high statistical consistency with Step-by-Step (SBS) Rock-Eval pyrolysis data, proving its capability to accurately quantify total solid hydrogen content, including solid petroleum hydrocarbons, bitumen, and kerogen. This work provides a non-destructive and high-precision means for evaluating the resource potential of shale oil reservoirs.

How to cite: Xu, C., Xie, R., Guo, J., Wang, X., and Zhang, G.: A novel method for quantitative characterization of organic matter based on NMR FID acquisition mode, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21370, https://doi.org/10.5194/egusphere-egu26-21370, 2026.

Slab tears are observed around the globe with increasing frequency as datasets and imaging methodologies improve, though the interactions between slab tears and the surrounding mantle flow remain enigmatic. Constraints on mantle flow around and through slab tears are crucial to a comprehensive understanding of slab-mantle interactions, as (1) cross-tear flow may allow mixing between upper mantle reservoirs otherwise separated by subducting slabs, and (2) cross-tear flow may impact the strength of nearby corner flow and therefore influence regional dynamic topography. However, the ability to study slab tears and their impact on mantle flow is limited by the number of slabs with clearly observed tearing and sufficient measurement density to capture lateral variations in mantle flow across the tear. On both counts, the Colombian Andes serve as an ideal region to study the interplay between slab tears and mantle dynamics.

The Colombian Andes are shaped by complex interactions between the subducting Nazca and Caribbean plates, as most clearly manifested by the Caldas Tear—a sharp lateral offset in slab seismicity near 5.5°N spanning more than 150 km. Using data collected across this boundary during the recent MUSICA (Modeling, Uplift, Seismicity, and Igneous geochemistry of the Colombian Andes) broadband seismic deployment, we investigate lateral variations in seismic anisotropy across the Colombian Andes by measuring shear-wave splitting of SKS and SKKS phases from teleseismic earthquakes.

Measurements of shear-wave splitting offer direct observational constraints on seismic anisotropy. Seismic anisotropy in the upper mantle forms primarily from the deformation-induced alignment of intrinsically anisotropic olivine crystals. Under various ambient stress and hydration conditions, different olivine petrofabrics develop that relate the bulk anisotropic fast direction to the orientation of maximum extensional strain. By inferring petrofabric type, shear-wave splitting measurements can directly constrain the geometry of deformation in the upper mantle and thus provide insight into the impact of complex slab geometry on mantle dynamics.

Our findings detail a complex regional pattern of mantle flow as the result of three interacting flow components: (1) entrained trench-perpendicular corner flow in the mantle wedges above sinking plates, (2) mantle flow through the Caldas Tear, and (3) trench-parallel flow far east of both subducting plates. Measured splitting delay times far exceed those expected from lithospheric anisotropy alone and thus support a deeper anisotropic source. Additionally, we observe strong back azimuthal variations in splitting measurement quality and quantity that demand further investigation. Future work will explore constraining lateral and vertical anisotropic complexity simultaneously using splitting intensity tomography.

How to cite: Carchedi, C., Wagner, L., and Monsalve, G.: Flow through a slab tear? Lateral variations in seismic anisotropy beneath the Colombian Andes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1431, https://doi.org/10.5194/egusphere-egu26-1431, 2026.

Azimuthal seismic anisotropy in the upper mantle is crucial for understanding the spatial patterns of past and present upper mantle deformation. Traditional interpretation of such anisotropy attributes to relative shear between surface plates and mantle. This requires the orientation of anisotropy azimuths to remain constant with depth. However, inferences of azimuthal anisotropy based on surface wave tomographic models often reveals depth-dependent azimuths. To this end, the existence of mechanically weak, thin asthenosphere beneath the lithosphere facilitates the channelization of plate-driven Couette flow and pressure-driven Poiseuille flow. The combination of two flows, especially when misaligned, yields depth rotations of asthenospheric shear. This provides a geodynamically plausible link between asthenospheric flow properties and depth rotations of azimuthal seismic anisotropy. In this submission, we utilize publicly available azimuthal seismic anisotropy models together with predictions from a global mantle flow model that incorporates Couette/Poiseuille flow. We find that Poiseuille flow profoundly affects depth rotations of seismically inferred azimuthal anisotropy. Prominent depth rotations are under the Atlantic basin and the Nazca plate, where Poiseuille flow dominates the modeled asthenospheric flow regime. Significant Poiseuille flow may exist beneath the Indian basin, yet with small depth rotation, probably because of its directional alignment with Couette flow. Our results indicate that interpretation of azimuthal seismic anisotropy cannot be simply tied to relative shearing between plates and mantle. Instead, the relative importance of Couette and Poiseuille flows must be taken into account.

How to cite: Wang, Z. R., Conrad, C. P., Lebedev, S., Iaffaldano, G., and Hopper, J. R.: Depth rotations of azimuthal seismic anisotropy associated with relative importance of Couette/Poiseuille flow in the asthenosphere, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2307, https://doi.org/10.5194/egusphere-egu26-2307, 2026.

The central Mediterranean area is a place where seismic anisotropy measurements have been collected for years, mainly along the Italian peninsula. Several different techniques have been applied to obtain this information, that carries relevant indications on the state of deformation at depth, both in the Earth’s crust and in the mantle. The most common type of measurements comes from the analysis of shear wave splitting of core phases (*KS), and from splitting intensity measurements. Seismic anisotropy patterns are a major support in answering questions such as where tectonic plates are actively deforming and which processes drive plate deformation. These are some of the questions addressed by the AdriaArray project. The seismic experiment AdriaArray aimed to densify the collection of seismographic data to the east with respect to the Adriatic microplate. The area covered by the project spans from southern France to the Black Sea in longitude and from Central Europe to the central Mediterranean in latitude, reaching the Sicily channel and the Hellenic Trench. Part of this wide area is already well studied for seismic anisotropy, as previously obtained data show. However, AdriaArray acquired data from 950 permanent and temporary broad-band stations thanks to the cooperation of nearly 40 institutions (https://orfeus.readthedocs.io/en/latest/adria_array_main.html) and most of them, located in the eastern part of AdriaArray study region, are now under analysis. Within the project, a Collaborative Research Group dedicated to seismic anisotropy has been created. It is working on building a dataset of shear-wave splitting measurements by improving already produced results with new data. The same has been done with splitting intensity measurements, with ongoing analyses for regions such as Sardinia, the eastern Adriatic coast and further east.

How to cite: Pondrelli, S., Rewers, J., Środa, P., Zailac, K., Stipčević, J., and Salimbeni, S.: Seismic anisotropy measurements within AdriaArray: a review of previous and new data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3092, https://doi.org/10.5194/egusphere-egu26-3092, 2026.

The Slave craton in northwest Canada is characterized by thick, cold, depleted lithosphere, and its surface geology includes some of the oldest rocks on the planet. In addition, the central Slave has proven economic importance, with a thriving diamond industry fed by suites of Miocene kimberlites. Previous studies of Slave craton architecture and anisotropy suggested a stratified lithosphere within the central Slave, likely associated with craton formation processes and subsequent metasomatism.

Since the original shear-wave splitting studies carried out in the central Slave craton, new seismograph installations have been carried out which permit an expanded view of the architecture of the craton as a whole, as well as its margins. Here we measure (or remeasure) shear wave splitting parameters for the complete dataset, which spans up to three decades for the longest-running stations. This systematic approach allows for a comprehensive comparison of anisotropic parameters across the craton.

Preliminary results suggest that NE-SW fast-polarization orientations dominate the craton at a large scale, though with significant local variability in the central Slave, suggesting lateral variability in lithospheric properties. We look for azimuthal variations in splitting measurements that may indicate stratified anisotropy, and we compare the results with models of azimuthal anisotropy from recent surface wave tomography studies.

How to cite: Darbyshire, F. and Dave, R.: Seismic anisotropy in the Slave craton, northern Canada: inferences from a new shear-wave splitting compilation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3197, https://doi.org/10.5194/egusphere-egu26-3197, 2026.

Abstract

Deformation in northern (NE) Tibet is essential for understanding the geodynamic processes of crustal thickening and outward growth associated with the Indo-Asian collision. We analyze receiver function data recorded by the regional seismic array of ChinArray-Ⅱ and permanent stations of the study region. The crustal thickness and Vp/Vs ratio are estimated using the H–κ grid searching technique (Zhu et al., 2000) . We also perform a joint analysis of Pms from radial and tangential receiver functions to measure fast polarization direction and splitting time (Liu et al., 2012). The harmonic analysis is adopted to obtain reliable crustal azimuthal anisotropy (Sun et al., 2012). Our result shows that the crust is significantly thickened (≥50 km) west of ~103°, while the Vp/Vs ratio is relatively low (~1.73) beneath the Qilian orogeny. We also found measurements of crustal azimuthal anisotropy beneath 71 stations in which the Pms arrivals with a dominant degree-2 back azimuth variations. The crustal anisotropy shows a north-south change across the Longshoushan fault. The measured splitting time in the region south of the Longshoushan fault is 0.22-1.02 s (with an average of 0.46 s). The fast direction mainly along the NW direction is roughly close to the fast polarization from XKS (Chang et al., 2021), which is parallel to the trending of the Qian orogenic belt, indicating a vertically coherent lithospheric deformation beneath the NE Tibet. To the north, the Alxa block exhibits a NNE-NE fast polarization with an average delay time of ~0.47s. Such observation differs from the fast-axis direction of mantle anisotropy, indicating that the crust and lithospheric mantle are decoupled. The north-south variation in crustal anisotropy of our study area may suggest that the growth front of the northeastern Tibetan Plateau may have extended to the Longshoushan fault.

Acknowledgments

Seismic data of the ChinArray was provided by the International Earthquake Science Data Center at Institute of Geophysics, China Earthquake Administration (https://doi.org/10.11998/ IESDC). Seismic waveforms recorded by the permanent stations of the China National Seismic Networks can be downloaded from the National Earthquake Data Center, China Earthquake Administration (https://data.earthquake.cn/) (Zheng et al., 2010). This research was supported by the NSF of China (42030310, 42474133).

References

Zhu, L., & Kanamori, H. 2000. Moho depth variation in southern California from teleseismic receiver functions. J. Geophys. Res., 105(B2), 2969–2980.

Liu H., Niu F., 2012. Estimating crustal seismic anisotropy with a joint analysis of radial and transverse receiver function data. Geophys. J. Int., 188: 144–164.

Sun Y., Niu F., Liu H., et al. 2012. Crustal structure and deformation of the SE Tibetan plateau revealed by receiver function data. Earth Planet. Sci. Lett., 349-350: 186–197.

Chang L., Ding Z., Wang C., 2021. Upper mantle anisotropy and implications beneath the central and western North China and the NE margin of Tibetan Plateau. Chin. J. Geophys. (in Chinese), 64(1):114-130.

How to cite: Zhang, R. and Hua, Y.: Crustal structural and anisotropy in northeastern Tibetan Plateau from receiver functions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3264, https://doi.org/10.5194/egusphere-egu26-3264, 2026.

The slowness surfaces for P, S1 and S2 waves in anisotropic medium are defined by solving the Christoffel equation. The regular point on the slowness surface can be mapped on corresponding group velocity surface. The irregular (singularity) point on the slowness surface results in the plane curve in the group velocity domain (Stovas et al., 2024).

The characteristic equations for double and triple singularity points define the tangent cone of second and third order, respectively. If the plane wave passes through singularity points in some layers of multilayered model, the effective characteristic equation has order given by product of orders of characteristic equations from individual layers. Therefore, the order of effective characteristic equation can be computed as N=2^K3^L, where K and L are the number of layers with double and triple singularity points, respectively. The effective characteristic polynomial F_N(Δp₁,Δp₂,Δp₃)  (the N-th order tangential cone) for multi-layered model can be computed by resultant of individual characteristic polynomials,where Δp_j,j=1,2,3, are the increments in slowness projections.The number of individual branches is given by J=Floor[(N+1)/2]. The dual curve Φ_M (V₁,V₂,V₃)=0 is the group velocity umage of Nth-order singularity point, where M=N(N-1)-2n-3c (Quine, 1982), with n and c being respectively the number of nodes and cusps for curve F_N=0. It is shown that the tangential cone does not have cusps (c=0) but can have nodes if N≥6. The inflection points and bitangents for curve F_N=0 respectively result in cusps and nodes for dual curve Φ_M=0. The cusps affect the Gaussian curvature computed in vicinity of singularity point (Stovas et al., 2025). The irregularities in phase and group domain are illustrated in Figure by dots for two-layer model with double singularity points in both layers (N=4, J=2, n=2 and M=8).

Figure. Two-layer model with double singularity points. a) Curve F₄=0 in affine plane (phase domain). Four inflection points on converted wave branch are shown by black dots. Two bitangents are shown by dotted lines limited by gray dots. b) Group velocity image (Φ₈=0) of quartic singularity point (dual curve). Four cusps are shown by black dots, and two nodes are shown by gray dots. Solid and dashed lines stand for pure wave modes (S1S1 and S2S2) and converted (S1S2 and S2S1) waves, respectively.

References

Stovas, A., Roganov, Yu., and V. Roganov, 2024, Singularity points and their degeneracies in anisotropic media, Geophysical Journal International 238 (2), 881- 901.

Stovas, A., Roganov, Yu., and V. Roganov, 2025, Gaussian curvature of the slowness surface in vicinity of singularity point in anisotropic media, Geophysical Journal International 240 (3), 1917-1934.                                                                    

Quine, J.R., 1982, A Plücker equation for curves in real projective space, Proceedings of the American Mathematical Society, 85, no.1, 103-107.

How to cite: Stovas, A.: Singularity points in multi-layered anisotropic medium , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3503, https://doi.org/10.5194/egusphere-egu26-3503, 2026.

Teleseismic shear-wave splitting is a widely-used technique for measuring oriented fabric in the crust and upper mantle; such fabric is an important marker for current or past deformation. The technique yields both the orientation (fast direction) and cumulative intensity (split time) of the net fabric. However, published splitting results, particularly split times, can have puzzling inconsistencies that make mapping splitting over large areas challenging; semivariograms of compiled splitting results show a lack of spatial coherence in split time measurement when studies using different methods are combined. I present modelling work that demonstrates that these inconsistencies result from an inherent bias in splitting measurement, particularly pronounced for split time, that is sensitive to details of the data processing methods and is amplified by averaging single-event measurements. With a correct choice of averaging method (error-surface stacking), this bias can be mitigated sufficiently to allow split time to be mapped over large areas, as demonstrated using compiled data from western Canada. The results show strong spatially-coherent variations along the strike of the Cordillera, which may represent regions of dominant vertical vs. horizontal flow in the upper mantle, driven by complex Cordilleran active tectonics.

How to cite: Frederiksen, A., Phillips, C., and Gu, Y.: Mapping upper-mantle fabric at continental scale: frozen tectonics and active flow patterns in western Canada, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4099, https://doi.org/10.5194/egusphere-egu26-4099, 2026.

Shear wave splitting is widely used to probe seismic anisotropy, but its depth resolution is limited. Building on the formulation of Silver and Savage (1994), we apply a Bayesian inversion that explicitly accounts for uncertainty in shear-wave polarization orientation (θ) to resolve multilayer anisotropy from splitting parameters. We apply this approach to source-side S and SKS data from the Cocos subduction zone, and to SKS data from station NNA above the South America subduction zone and station SNZO at the southern Hikurangi margin. Subduction in all three regions is shallow to flat, minimizing dip-angle effects on SKS. The inversion yields tightly constrained fast directions for both upper and lower anisotropic layers. Three-layer inversions show progressive rotation with depth consistent with two-layer solutions, but are not required by the data. In the Cocos system, the upper-layer fast direction is unambiguously aligned with Cocos plate motion in the NNR reference frame, consistent with subduction-entrained flow. In contrast, beneath NNA and SNZO, the upper and lower layers exhibit trench-subparallel and trench-normal anisotropy, respectively—opposite in layering sense to the poloidal–toroidal flow structure predicted by dynamic models. If both layers reside in the subslab mantle, the trench-parallel upper layer flow would decouple the deeper mantle from the slab, raising questions about how the apparent subduction-driven flow is maintained at depth. Alternatively, the upper layer may reflect deformation within or above the slab. Possible sources of trench-parallel anisotropy include frozen-in oceanic lithospheric fabrics, trench-parallel mantle-wedge flow, or inherited fabrics related to nearby continental shear zones. These results highlight the complexity of subslab dynamics and demonstrate the value of probabilistic multilayer inversion for interpreting shear-wave splitting.

How to cite: Kuo, B.-Y., Peng, C.-C., and Montagner, J.-P.: Subslab flow beneath subduction zones revealed by multiple-layer shear wave splitting inversion, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4243, https://doi.org/10.5194/egusphere-egu26-4243, 2026.

The recent increase in seismic activity in the southern Sichuan Basin has attracted substantial public interest and simultaneously provides an important opportunity to investigate upper crustal anisotropy, which offers key constraints on the regional stress field and crustal deformation. In this study, we obtained 1,845 high-quality local shear-wave splitting measurements from 15 stations, along with 2,027 null measurements from 19 stations. The results reveal a single anisotropic layer characterized by a horizontal symmetry axis at depths of approximately 3–7 km. The fast polarization directions exhibit clear spatial variability, which is primarily controlled by the spatial distribution of earthquakes rather than temporal evolution. Near the Baimazhen Syncline, the fast polarization directions align with the strike of the strata and form a circular pattern around the synclinal core, indicating that the anisotropy in this region is dominantly structure-controlled. In contrast, stations located in the southern Weiyuan Anticline and the western Baimazhen Syncline display fast directions of N171.7°E and N45.9°E, respectively. These orientations are consistent with the P axes derived from earthquake focal mechanisms, suggesting that anisotropy in these areas is primarily governed by the regional stress field. Overall, this study enhances our understanding of the complex geological framework of the southern Sichuan Basin and underscores the need for caution when interpreting potential temporal variations in seismic anisotropy in future investigations. This work was supported by the National Natural Science Foundation of China (Grant 42374124).

How to cite: Qiang, Z., Wu, Q., and Li, Y.: Spatial Heterogeneity of Upper Crustal Anisotropy in the Southern Sichuan Basin (China) Revealed by Local Shear-Wave Splitting, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4419, https://doi.org/10.5194/egusphere-egu26-4419, 2026.

The southwest region of Western Australia is one of the oldest continental regions on Earth, hosting the Archean Yilgarn Craton, bounded by the Proterozoic Albany-Fraser and Pinjarra orogens. Here we calculate shear wave splitting of the PKS and SKS teleseismic phases using stations from Phases 1 and 2 of the WA Array (average station spacing 40 km), as well as other temporary and permanent networks in the study region. We find evidence for coherent seismic anisotropy, with the regional average delay time (1.24 ± 0.62 s) comparable to the global average, δt = 1 s. Although fast polarization orientations show variation, they are not aligned with current plate motion and the expected asthenospheric flow direction. In the South West Terrane, fast polarization orientations match the trend of ancient structural faults. By contrast, structural faults in the Youanmi Terrane and the Eastern Goldfields Superterrane are oriented at an angle compared to the E–W and NE–SW fast polarizations. Instead, the seismic anisotropy pattern shows a striking similarity to E–W trending Precambrian (2.42 Ga) dykes that extend uninterrupted across the Yilgarn Craton. We propose that lithospheric fabrics frozen-in at the time of craton formation (~2.76–2.65 Ga) generated a mechanical weakness which subsequently influenced the orientation and emplacement of the dykes. Further evidence for a similar, ancient (~2.73 Ga) architectural fabric comes from recent isotope geochemistry analysis of primary ENE-trends within the Yilgarn Craton. Overall, these results point toward large-scale, fossilized lithospheric fabric within the Yilgarn Craton, preserved for over two billion years, offering a unique window into the formation and early evolution of the continent. 

How to cite: Gauntlett, M., Eakin, C., Bishoyi, N., Zhang, P., O'Donnell, J.-P., Murdie, R., Miller, M., Pickle, R., and Ebrahimi, R.: Seismic anisotropy analysis across Southwestern Australia reveals ENE‐trending lithospheric architecture linked to Archean Yilgarn Craton formation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4571, https://doi.org/10.5194/egusphere-egu26-4571, 2026.

We constrain the extent and anisotropic properties of the Ivrea Geophysical Body (IGB) beneath the Western Alps using receiver function (RF) analysis of 66 teleseismic datasets. The IGB represents one of the most prominent examples of shallow mantle material within continental crust, yet its geometry, composition, and tectonic significance remain debated beyond its well-known positive gravity anomaly. Using teleseismic waves recorded from temporary seismic deployments and permanent seismic networks across the western Alps, we perform a comprehensive receiver function (RF) analysis that indicates the presence of anisotropic mantle materials at shallow depth, associated with the occurrence of the IGB, in terms of P-to-s converted energy out of the radial plane. We characterise the anisotropic rock volumes solving an inverse problem using a Neighbourhood Algorithm. The results indicate that 35 out of 66 RF data-sets from this study, together with five additional stations from a previous study, display coherent anisotropic characteristics directly above the high-gravity anomaly and can be associated with the IGB. These stations exhibit strong anisotropy (~15%) and a coherent fast-axis pattern that systematically rotates from south to north, following the arcuate geometry of the Alpine trench. The depth distribution of the anisotropic interfaces constrains the IGB as a continuous lithospheric-scale body approximately 170 km long, 30-50 km wide, and 20-45 km thick, with its upper boundary as shallow as 1 km depth. The whole body is slightly dipping towards the East. Seismic velocities and anisotropy magnitudes indicate a dominantly mantle-derived composition, consistent with a peridotitic protolith variably overprinted by serpentinite-rich shear zones. Our results refine the three-dimensional extent of the IGB and demonstrate that its anisotropic fabric records the deformation associated with Alpine subduction, slab rollback, and subsequent exhumation, providing new constraints on the tectonic evolution of the western Alpine lithosphere.

How to cite: Confal, J., Pondrelli, S., Salimbeni, S., and Piana Agostinetti, N.: Mapping the Ivrea Geophysical Body and its anisotropic properties beneath the Western Alps using receiver functions analysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5755, https://doi.org/10.5194/egusphere-egu26-5755, 2026.

The enigmatic radial anisotropy of the upper mantle remains difficult to resolve. Recent global models show strong disagreements and suggest different inferences on mantle dynamics and evolution. Here, we present a new radially and azimuthally anisotropic shear-wave velocity model of the upper mantle, LLCS-2026, and validate its key patterns using independent seismic and thermodynamic phase-velocity inversions for tectonic-type-average 1D profiles. LLCS-2026 is computed using waveform fits of 1,630,432 seismograms (1,252,717 vertical; 377,715 transverse components). Automated multimode waveform inversion is used to extract structural information from surface and S waveforms in very broad period ranges, from 11 to 450 s, with most data sampling in the 20–350 s period range. The vertical and transverse component waveforms are jointly inverted for the isotropic average shear-wave velocities, their pi-periodic and pi/2-periodic azimuthal anisotropy, and radial anisotropy. Statistical and manual outlier analysis yields a final dataset of 1,009,038 seismograms (765,302 vertical, 243,736 transverse components) that constrains the final model, which captures complex patterns of seismic isotropic and anisotropic structure within the Earth. In agreement with previously published models, prominent low-velocity anomalies indicative of thin lithosphere and partial melting are observed at 20-150 km depth beneath mid-ocean ridges. At 300-400 km, however, high isotropic-average velocities are present in the vicinity of some of the ridges in the Indian and Atlantic oceans. They suggest drips of cold, lithospheric mantle material, probably related to rapid lithospheric cooling in the complex 3D context of triple junctions and ridge-hotspot systems. Radial anisotropy is positive (Vsh > Vsv) at 100-150 km depth everywhere in the mantle, with cratons showing smaller anisotropy compared to other units. Below 200-250 km depth, radial anisotropy is negative (Vsv > Vsh) nearly everywhere. The depth at which the anisotropy sign changes varies with tectonic region. The anisotropy sign flips at the shallowest depth (~200 km) beneath young oceans and continents and at the greatest depth (~250 km, on average) beneath cratons. Radial anisotropy is also negative in the top 50 km of the oceanic lithosphere. Together with azimuthal anisotropy observations, this indicates a complex pattern of crystallographic preferred orientations created by mantle flow beneath mid-ocean ridges, with an interplay between the alignment of crystals due to the vertical flow below the ridge and the lateral flow away from it. Independent seismic (Civiero et al. 2024) and thermodynamic (Lebedev et al. 2024; Xu et al. 2025) inversions of phase-velocity data confirm and validate the anisotropy-sign-flip observations and inferences.

References

Civiero, C., Lebedev, S., Xu, Y., Bonadio, R. and Lavoué, F., 2024. Toward tectonic‐type and global 1D seismic models of the upper mantle constrained by broadband surface waves. Bulletin of the Seismological Society of America, 114, 1321-1346.

Lebedev, S., Fullea, J., Xu, Y. and Bonadio, R., 2024. Seismic thermography. Bulletin of the Seismological Society of America, 114, 1227-1242.

Xu, Y., Lebedev, S. and Fullea, J., 2025. Average physical structure of cratonic lithosphere, from thermodynamic inversion of global surface-wave data. Mineralogy and Petrology, 1-12.

How to cite: Lebedev, S., Lavoué, F., Celli, N. L., and Schaeffer, A. J.: Radial anisotropy of the upper mantle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5995, https://doi.org/10.5194/egusphere-egu26-5995, 2026.

The Alps, together with the Northern Apennines and the Northern Dinarides, represent one of the most complex and best-studied examples of continental collision in the world. Over the years, several active and passive seismic experiments have been deployed in the Alpine region. More recently, the installation of large and dense seismic arrays, such as AlpArray and AdriaArray, has provided unprecedented spatial coverage, enabling the development of increasingly detailed seismic velocity models. However, most existing regional models of the Alps primarily rely on isotropic seismic velocities. Radially anisotropic models, which map the parameter ξ = Vsh²/Vsv², provide complementary information by revealing the preferential orientation of anisotropic minerals and structural fabrics produced by past and ongoing tectonic processes.

In this study, we combine ambient noise and earthquake surface-wave data from more than 3,300 permanent and temporary broadband seismic stations to construct a high-resolution Alpine Radial Anisotropy model (AlpRA25) of the crust and upper mantle. Ambient noise data collected between 2017 and 2019 from approximately 700 seismic stations were used to calculate ~46,000 Rayleigh- and ~40,000 Love-wave dispersion curves. These were merged with ~295,000 Rayleigh- and ~200,000 Love-wave dispersion curves obtained from about 6,000 earthquakes recorded at approximately 3,300 broadband seismic stations between 1990 and 2022, resulting in a total of ~295,000 Rayleigh and ~240,000 Love dispersion curves spanning periods from 3 to 250 s.

These data were inverted for phase-velocity maps using a least-squares algorithm with an average knot spacing of 30 km. An a posteriori outlier analysis discarded 15% of the interstation measurements with the highest residuals, after which the model was recomputed. Local dispersion curves were then extracted at each grid node and evaluated for their frequency-dependent roughness. The Rayleigh and Love local dispersion curves were jointly inverted for 1-D shear-wave velocity structure using a particle swarm optimization algorithm, yielding vertical and horizontal shear-wave velocities (Vsv and Vsh, respectively). The final AlpRA25 model is a high-resolution 3-D model of Vsv, Vsh, and ξ from 5 to 250 km depth, covering the Alps, the Northern Apennines, the Northern Dinarides, and the adjacent foreland and back-arc basins. AlpRA25 provides new constraints on the lithospheric architecture and deformation of the Alpine region, highlighting the role of radial anisotropy in imaging tectonic processes from the crust to the upper mantle.

How to cite: Meier, T., Berger Roisenberg, H., Eckel, F., El-Sharkawy, A., Rosenberg, C., Boschi, L., and Cammarano, F.: AlpRA25: a new radial anisotropy model of the Alps from ambient noise and earthquake surface waves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7464, https://doi.org/10.5194/egusphere-egu26-7464, 2026.

The Haiyuan Fault, a major strike-slip structure in the northeastern Tibetan Plateau margin, is bounded by the first-order Tibetan Plateau and South China blocks, plus the second-order Ordos and Alxa blocks. Seismic anisotropy serves as a robust proxy for probing deep crustal deformation, geodynamic processes, and subsurface seismic structures. We conducted receiver function analyses on teleseismic data from two dense profiles and five broadband stations across the study area; crustal thickness (42–56 km) and Vp/Vs ratios (1.60–1.88) were quantified by the H-κ domain search algorithm, while common conversion point (CCP) imaging delineated the Moho discontinuity across the Haiyuan Arc Fault Zone. Crustal thickening reflects shortening driven by Tibetan-Eurasian collision, with the Haiyuan tectonic evolution linked to high-temperature/pressure regimes induced by Indo-Asian convergence. CCP images reveal a distinct Moho offset and ambiguous continuity beneath the fault zone, confirming it as a Moho-penetrating transcrustal structure associated with intense crustal extrusion from the plateau interior. We characterized multi-scale crustal anisotropy via shear-wave splitting (SWS) analysis of local earthquake data. SWS parameters exhibit clear zoning controlled by the Haiyuan Fault: fast polarization orientations are NNE–NE north of the fault and WNW–EW south of it. Within ~10 km of the fault, fast orientations align with the fault strike (WNW), indicating the fault’s stress influence range spans dozens of kilometers. Enhanced normalized time-delays near the fault signal stronger anisotropy along this strike-slip belt. Upper crustal anisotropy likely arises from crack-induced fabric, whereas middle-lower crustal anisotropy reflects deformation-controlled fabric. Spatial anisotropy patterns imply the combined effects of stress, faulting, and local tectonics. Notably, SWS results suggest the Haiyuan Fault constitutes the actual crustal boundary of the northeastern Tibetan Plateau, ~200 km north of the previously reported plateau block boundary.

How to cite: Shi, Y. and Gao, Y.: Crustal Deformation of the Haiyuan Fault Zone Inferred from Dense Seismic Array Observations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8643, https://doi.org/10.5194/egusphere-egu26-8643, 2026.

    The North China Craton (NCC) exhibits a marked east-west contrast in its present-day tectonic framework. The eastern region  features a thinned lithosphere hosting extensional basins like the Zhangjiakou-Bohai seismic belt and the Shanxi Graben. This area experiences intense crustal deformation and frequent seismicity. Conversely, the western region possesses a thicker lithosphere, greater crustal stability, and weak seismic activity. The central orogenic belt acts as a transitional zone in crust-mantle structure, characterized by dramatic crustal thickness variations, evidence of lithospheric modification, remnants of ancient structures, and is key for studying crust-mantle coupling/decoupling. 
    Crustal anisotropy serves as a crucial indicator for revealing lithospheric deformation. Analysis of seismic wave velocity anisotropy quantitatively constrains crustal stress, assesses fault activity, and infers deep material flow. This provides  essential constraints for seismic hazard assessment and tectonic dynamics research.
    This study utilized local-earthquake data (M>1) from the National Fixed Seismic Network to investigate crustal anisotropy across the NCC using the SAM (Shear-wave splitting Analysis Method) method. The extensive dataset, covering multiple temporal windows of seismic activity, provides strong temporal continuity and spatial coverage for analyzing anisotropy characteristics. Results reveal complex fast-wave polarization directions (FPDs) across the study area. The average FPD (~73°) aligns well with the regional mean maximum horizontal compressive stress (SHmax) direction in the NCC. However, the FPDs also display distinct local features correlating with specific structures, indicating significant local variability in the regional stress field. This manifestation of localized crustal anisotropy characteristics is vital for understanding the region's geodynamic activity. The local stress field variations suggest a heterogeneous crustal stress distribution, likely influenced by multiple geological factors.

How to cite: Wang, Q., Gao, Y., and Liu, G.: Crustal Anisotropy Characteristics in the North China Craton, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8791, https://doi.org/10.5194/egusphere-egu26-8791, 2026.

We investigate the seismic anisotropy of the Rotondo granite (Gotthard Massif, Swiss Alps) by integrating geological and geophysical data from lab to field scale. We compare our modeled anisotropic properties with decameter measurements from boreholes and kilometer-scale regional seismicity data from the Bedretto Underground Laboratory for Geosciences and Geoenergies, demonstrating clear links between deformation fabrics and observed seismic anisotropy across scales. 

Using field- and micro-scale analyses of deformation styles and fabric orientations, we delineate discrete structural domains characterized by varying strain intensities and fabric types, ranging from isotropic granite to fractured zones or proto-mylonitic shear zones. Proto-mylonitic zones exhibit strong phyllosilicate SPO and higher percentage of Vp anisotropy (~8-27%, range is dependent on compositional variations). Fractured zones vary in frequency within the Rotondo massif and also exhibit elevated Vp anisotropy (>7.5%). For each structural domain, we compute effective elastic stiffness tensors (or 'rock recipes') to characterize their intrinsic seismic velocities. We introduce a new approach for combining multiple lithological “rock recipes” that emphasizes collective impact on bulk anisotropy and spatial context, rather than volume-weighted averaging. 

We observe a scale-dependent shift in anisotropic influence, where the control of ductile fabrics (<20 m) is progressively superseded by fractures as the observational scale increases. When these heterogeneous fabrics are aggregated, destructive interference among strongly anisotropic components reduces the bulk anisotropy to ~2.5%, which is below laboratory measured values. We find good agreement between our theoretical results and cross-borehole effective Vp measurements within the Bedretto Lab. We also find qualitative evidence for anisotropy between event relocation models (e.g. Double Difference or NonLinLoc) of background seismicity in the region at the 1-5 kilometer scale. These results demonstrate consistency in seismic anisotropy estimation across methods and scales, and show the utility of geologically-based anisotropy characterization.

How to cite: Hinshaw, E., Ceccato, A., Zappone, A., Behr, W., and Obermann, A.: Multi-scale Seismic Anisotropy of the Rotondo Granite: Linking Deformation Fabrics to Wave Propagation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9651, https://doi.org/10.5194/egusphere-egu26-9651, 2026.

The Western Hellenic Subduction Zone is characterized by a transition from oceanic to continental subduction. The change occurs at the Kephalonian transform fault. However, how this transition takes place at depth remains a topic of discussion. This setting thus provides us with an ideal natural laboratory to investigate how differences in subduction regimes affect the structure and dynamics of the system.
To this end, we compute receiver functions across two seismic arrays from the MEDUSA broadband network, one imaging the oceanic subduction and the other imaging the continental subduction. We computed teleseismic receiver functions and performed harmonic decomposition along both lines. We then inverted these results to image the overriding crust, mantle wedge and slab, in terms of their velocity and anisotropic properties. By comparing the seismic properties of the continental and oceanic slabs, we aim to identify key differences in slab structure, seismic anisotropy, dehydration, and metamorphism between the two subduction regimes.
Preliminary results confirm a dipping low velocity zone in both regimes, corresponding to the slab's crust. Its signal is lost below 60 km in the isotropic component but remains visible to greater depths in the anisotropic component. Furthermore, we identify a low velocity layer within the mantle wedge which could resemble the altered LAB of the overriding plate. What sets the two domains apart is the cutoff depth of the isotropic component of the slab – it can be traced 10 km deeper in the South than in the North – and a generally lower anisotropy in the southern mantle wedge.
Until now the LAB has rarely been observed through conventional receiver function analysis or tomography in subduction zones. We therefore suggest that anisotropic inversion may provide unique insight into the structure of the mantle wedge and the subducting slab.

How to cite: Ziegler, J., Rondenay, S., and Piana Agostinetti, N.: Seismic Anisotropy in the Subducting Slab and Mantle Wedge of the Western Hellenic Subduction Zone from Receiver Functions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10259, https://doi.org/10.5194/egusphere-egu26-10259, 2026.

The Iranian plateau, characterized by the Arabia-Eurasia continental collision in the Zagros and the Makran oceanic subduction system, presents a unique opportunity to investigate the underlying processes of lithospheric deformation and upper-mantle dynamics. Previous studies of upper mantle seismic anisotropy, mostly using SK(K)S splitting and occasionally direct S waves revealed complex patterns for the fast axes. The observed rotations of fast axis obtained from S and SK(K)S waves along different tectonic setting, the difference in fast directions between S and SK(K)S phases in central Iran, evidence for two-layer anisotropy, and ambiguity regarding the depth origin of observed anisotropy emphasize the need for further studies. These challenges, together with the pronounced tectonic heterogeneity of the Iranian plateau, call for tomographic approaches that allows for the localization of anisotropic structure. In this study, we utilize SKS splitting intensity tomography to elucidate the depth distribution of the anisotropic properties of the upper mantle beneath the Iranian plateau. Our dataset includes teleseismic events with magnitude above 5.5 and epicentral distances between 90 and 130 degrees recorded by 151 permanent (2015-2021) and 296 temporary seismic stations (2003-2021). We employ a three-dimensional full-wave anisotropy tomography method using splitting intensity, which provides enhanced depth resolution compared to traditional shear wave splitting methods. This method utilizes perturbation theory to establish the linear relationship between splitting intensity and anisotropic parameters, including the azimuth of fast axis and anisotropy strength, and incorporates Green's function databases to efficiently compute the sensitivity or Fréchet kernels. Splitting intensity measurements are inverted to construct a three-dimensional model of upper-mantle azimuthally anisotropic structure beneath the study area. Results show clear lateral differences in anisotropic strength and fast-axis orientation specifically between the Zagros and Alborz regions, as well as the adjacent domains. Vertical profiles illustrate depth-dependent heterogeneous anisotropic structures across the lithosphere–asthenosphere boundary into the asthenospheric upper mantle. These variations likely reflect the combined influence of regional tectonic processes, continental collision, lithospheric deformation, and present-day mantle flow patterns beneath the Iranian plateau. Our results highlight the potential of SKS splitting intensity tomography to resolve complex mantle anisotropy and shed new light on the three-dimensional deformation structure of the upper mantle. The observed lateral and depth variations in anisotropy provide new insights into how the relationship between surface tectonics and upper-mantle deformation varies spatially across major tectonic domains such as the Zagros, Makran, Central Iran, and the Alborz.

How to cite: Arvin, S., Lan, H., Chen, L., Ke, Z., Lin, Y., Zhao, L., and Talebian, M.: 3-D Anisotropic Structure of the Upper Mantle beneath the Iranian Plateau Using SKS Splitting Intensity Tomography, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10489, https://doi.org/10.5194/egusphere-egu26-10489, 2026.

How to cite: Zailac, K., Pondrelli, S., Salimbeni, S., and Stipčević, J.: Seismic Anisotropy Beneath the Dinarides: Implications for Adria-Eurasia Convergence, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10589, https://doi.org/10.5194/egusphere-egu26-10589, 2026.

Fractures are ubiquitous throughout the Earth’s upper crust, spanning scales from microscopic cracks to large fault systems. Their hydromechanical behavior plays a critical role in controlling fluid migration in hydrocarbon, geothermal, and groundwater reservoirs, which, in turn, makes fracture detection, characterization, and monitoring an important objective in geoscience and engineering applications. Seismic methods, as indirect and non-invasive tools, have become central to this effort due to their ability to probe fractured media with adequate resolution and depth penetration. This work synthesizes our recent experimental and theoretical advances in the study of seismic characterization of fractured rocks, driven by several key observations. First, most fractured reservoirs exhibit effective seismic anisotropy because fractures often develop preferentially aligned with the local principal stress directions, leading to direction-dependent wave propagation. This anisotropy can be estimated using techniques such as shear-wave splitting, azimuthal velocity variations, and amplitude variations with offset and azimuth. Furthermore, we show that incorporating both fracture-induced and intrinsic background anisotropy, a rather common scenario in fractured environments, into inversion workflows is essential for a robust interpretation. Second, when a seismic wave propagates through a fluid-saturated fractured reservoir, it will be significantly attenuated and dispersed as a result of multiple intrinsic (e.g., inelastic effects due to solid and/or fluid friction effects) and extrinsic (e.g., geometrical spreading) mechanisms. In particular, when a seismic wave propagates through a fluid-saturated porous rock containing fractures, it produces fluid pressure gradients between the more compliant fractures and the stiffer embedding rock as well as between hydraulically connected fractures with different orientations and/or properties. Consequently, fluid flows until the pressure equilibrates, a phenomenon commonly referred to as wave-induced fluid flow (WIFF). This mechanism can alter the effective compliance of the fractures. Such compliance changes can significantly influence velocity and attenuation anisotropy across the seismic frequency range. The dependence of this type of mechanism on the petrophysical properties, fracture-geometry, and distribution makes the analysis of frequency-dependent seismic attributes particularly informative with regard to the hydromechanical properties. [NB2] Third, seismic responses in fractured media are highly sensitive to changes in their stress state, fluid saturation, and geometrical properties, thus, facilitating corresponding monitoring efforts through time-lapse seismic surveys. Finally, highly permeable fractures can often be directly imaged since open fractures with partial surface contacts generally have large mechanical compliance, which, in turn, produces strong scattering of seismic waves. Indeed, there is evidence from full-waveform sonic log data to suggest that the fracture mechanical compliance obtained from P-wave velocity changes and transmission losses correlates with the degree to which fractures are hydraulically open.

How to cite: Barbosa, N., Rubino, G., Caspari, E., and Holliger, K.: A review of the sensitivity of seismic wave velocity and attenuation to fracturing, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11156, https://doi.org/10.5194/egusphere-egu26-11156, 2026.

The 2023 Kahramanmaraş earthquake doublet (Mw 7.8 and Mw 7.7) produced an extensive rupture along several segments of the East Anatolian Faults (EAF), and triggered intense aftershock activity in southeastern Türkiye. This seismic sequence therefore provides an exceptional dataset to investigate the crustal anisotropy in such a complex tectonic area. In this study, we evaluated the crustal anisotropy in the source region of these catastrophic earthquakes by conducting a detailed local S-wave splitting (SWS) analysis on a relocated earthquake catalogue. We have performed shear-wave splitting measurements over several thousand local-S waveforms recordings of 31 permanent broadband seismic stations operated by AFAD (Turkish Earthquake Data Center) that were extracted using precise relocation of aftershock activity. After a visual quality control procedure for each splitting analysis, a total of 486 high-quality measurements were obtained. The results reveal a highly heterogeneous anisotropic pattern, with fast direction oriented from N80°W to N79°E and mean fast direction for the whole dataset of N16°E, reflecting the lateral variations in the regional stress field along the EAF, the Sürgü-Çardak Fault (SÇF), and the Malatya Fault. A transition between stress-induced and structure-related anisotropy is clearly identified across different segments of the EAF. Stations in close proximity to the EAF exhibit a dominant structure-induced anisotropic signature, characterized by the strict alignment of FPDs parallel to the fault geometry. Overall, the obtained results provide a comprehensive perspective on how the upper crust responds to substantial stress release, thus offering critical insights into the mechanical behaviour of complex fault networks where the regional collisional stress regimes and strike-slip faulting systems converge.

How to cite: Erman, C., Baccheschi, P., Yolsal-Çevikbilen, S., Eken, T., and Taymaz, T.: Crustal Anisotropy Variations Revealed by Local S-wave Splitting in the Source Region of 2023 Kahramanmaraş Earthquakes along the East Anatolian Fault Zone, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12795, https://doi.org/10.5194/egusphere-egu26-12795, 2026.

Body- and surface-wave seismic data provide complementary sensitivities to the anisotropic elastic structure of the Earth, and the potential constraints of a simultaneous inversion would extend significantly beyond those of the individual phases. However, joint body- and surface-wave anisotropic imaging remains limited, mainly because of the high nonlinearity of the problem and the different inversion methods traditionally adopted for body- and surface-wave phases. Here, we implement a nonlinear transdimensional stochastic solver based on the reversible-jump Markov chain Monte Carlo (RJMCMC) algorithm to simultaneously invert P-, S- and Rayleigh-wave data. By sampling irregularly meshed anisotropic velocity models for the upper mantle, with different mesh configurations and complexities adaptable to the heterogeneous data constraints, we populate an ensemble of variable solutions describing the data within the uncertainties. The method is validated using independent synthetic seismograms simulated with SPECFEM3D Globe in an anisotropic upper mantle plume model. We show how the different sensitivities of the data translate into different constraints on upper mantle seismic structure, and we analyze metrics to quantitatively assess uncertainties in the inferred solutions.

How to cite: Del Piccolo, G., Byrnes, J., Gaherty, J., VanderBeek, B., Faccenda, M., and Morelli, A.: Joint body- and surface-wave probabilistic transdimensional tomography of upper mantle seismic anisotropy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13229, https://doi.org/10.5194/egusphere-egu26-13229, 2026.

Bridgmanite is the most abundant intrinsically-anisotropic constituent of the lower mantle. Its deformation, thus, potentially translates to large-scale anisotropy that would accumulate in high-stress regions, particularly the interaction between subducted materials and the surrounding mantle. While most of the lower mantle is generally considered well-mixed, recent observations suggest structures at mid-mantle depths (800 – 1500 km). Their origin, however, often remains enigmatic. Recent state-of-the-art deformation experiments in bridgmanite at lower-mantle temperatures and pressures reveal a depth-dependent behavior of anisotropy. Coupled with realistic geodynamic models of mantle convection, the large-scale imprint of the depth-dependent fabric reveals a purely deformation-driven seismic discontinuity between 1000 – 1400 km depth that matches observations. The discontinuity appears sharpest in actively deforming regions, and becomes negligible closer to neutral ones. In this work, we investigate its seismic detectability around a subduction zone using three-dimensional global waveform modeling via AxiSEM3D. Given its likely presence across a broad range of frequencies, we assess the suitability of SS precursors for detecting this feature. Enhanced using array seismological methods, results show the presence of SS precursors in the transverse component generated by the anisotropic discontinuity. In addition, a flattened slab geometry produces a shallower SS precursor due to the interface formed between an overlying isotropic slab and an underlying strongly anisotropic (V_SH>V_SV) layer. We examine the azimuthal dependence of precursor amplitudes and their frequency dependence, with particular emphasis on the microseismic frequency band (~0.05 – 0.25 Hz). In light of these recent findings, we discuss its implications on the nature and origin of mid-mantle discontinuities.

How to cite: Magali, J. K., Yuan, Y., and Thomas, C.: Seismic detectability of a deformation-induced anisotropic discontinuity in the Earth’s lower mantle around a subduction zone using synthetic global modeling of SS precursors, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19103, https://doi.org/10.5194/egusphere-egu26-19103, 2026.

Seismic attenuation is important to understand the thermal and compositional state of the lithosphere, therefore sheds light on its deformation process. However, measuring the attenuation coefficient of seismic waves is still a challenging task because the phase and amplitude can be affected by both elastic velocity structures and anelastic attenuation, let alone these effects are coupled. Here, we developed a 2D joint inversion T-matrix method for the Rayleigh-wave phase velocity and attenuation coefficient simultaneously. Using a matrix inversion calculation to update the background medium Green functions with scattering series, the scattered wavefield can be fully represented in the frequency domain. First, the T-matrix method takes the coupling of elasticity and attenuation on waveform into consideration by joint inversion. Secondly, by calculating the anelastic scattering effects, 2D distribution can be obtained even for weak attenuation, which is a step towards 3D Q structure. Without time domain wave propagation simulations, the method is affordable in regional problems. Therefore, the method can be used to invert 2D Rayleigh wave phase velocity and attenuation coefficients.

How to cite: Yang, P. and Ruan, Y.: A 2D joint inversion method for Rayleigh wave phase velocity and attenuation coefficient, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1247, https://doi.org/10.5194/egusphere-egu26-1247, 2026.

Characterizing high-frequency (~10 Hz) seismic wave propagation is essential for understanding strong ground motions and improving seismic hazard assessment. High-frequency components are strongly influenced not only by source processes but also by small-scale heterogeneity along the propagation path. In the crust, vertical layering coexists with lateral heterogeneity, which plays a key role in controlling the propagation and attenuation of seismic waves. During propagation, seismic energy is reduced by intrinsic attenuation, in which energy is dissipated into heat and acoustic energy, and by scattering due to heterogeneity, which can produce apparent attenuation or amplification. In this study, we analyze S-wave coda from the 2016 Gyeongju earthquake using Multiple Lapse Time Window Analysis (MLTWA) to estimate the intrinsic (Qi), scattering (Qs), and total (Qt) quality factors in discrete frequency bands. Over the central frequency range of 1.5–22 Hz, the inferred Qs values range from approximately 398 to 4399, Qi from 185 to 1390, and Qt from 120 to 1041, revealing a pronounced frequency dependence of attenuation. The observed Qs–frequency relationship is then interpreted using a von Kármán autocorrelation model, yielding crustal heterogeneity parameters ε = 0.048, κ = 0.32, and a = 8.0 km. These parameters reproduce the empirical Qs curve and are used to generate random heterogeneous media for numerical simulations of high-frequency wave propagation. By integrating observation-based heterogenous crustal modeling, this study quantitatively constrains the influence of crustal heterogeneity on high-frequency seismic wave propagation and provides a physical basis for refining strong ground motion prediction models and improving the reliability of seismic hazard assessments.

How to cite: Lee, S., Cho, C. S., and Song, S. G.: High-Frequency Seismic Wave Simulations in Qs-Constrained von Kármán Random Media for the 2016 Gyeongju Earthquake, South Korea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2494, https://doi.org/10.5194/egusphere-egu26-2494, 2026.

Seismic anisotropy and attenuation, often quantified by the inverse of the quality factor (), are powerful, but often underexploited, indicators of fracture architecture, fluid content, and small-scale heterogeneity in the subsurface. At the same time, machine-learning (ML) methods offer flexible, data-driven mappings between seismic attributes and subsurface properties yet are not often designed to exploit seismic anisotropy and attenuation. In this contribution, an integrated workflow that combines laboratory measurements, borehole and VSP data, and surface seismic attributes with ML modelling to achieve advanced subsurface characterization in fractured carbonate systems.

Seismic waveforms collected in Abu Dhabi in the United Arab Emirates (UAE), were recorded at wide frequency range from Hertz to MHz, in the field and laboratory. The lithology of Abu Dhabi subsurface is dominated by carbonates, which are known by their high heterogeneity and multiple fracturing systems. To address the complexity caused by lithology, new methods and processing workflows have been developed and applied on the data. This includes new methods for calculating seismic attenuation from surface seismic, vertical seismic profiling (VSP), and sonic data, allowing an estimate of attenuation magnitude and its anisotropy, in addition to separating between scattering and intrinsic attenuation.

The study includes a suite of field and laboratory studies that quantify azimuthal P-wave attenuation, separate intrinsic and scattering contributions, and relate these to fracture systems and tar-mat occurrence in Abu Dhabi carbonate subsurface. These include multi-offset azimuthal VSP analyses that recover fracture strike and discriminate between open and cemented fractures using attenuation anisotropy, detailed attenuation-mode separation from VSP and sonic data, AVAz-based fracture characterization from 3D surface seismic, and ultrasonic measurements that document the sensitivity of  to petrophysical properties and saturation in carbonate core plugs. Building on this physical understanding, we extend recent work on ML-based prediction of Thomsen’s parameters from synthetic and VSP data to explicitly incorporate multi-scale attenuation attributes. Training data is generated by finite-difference modeling in anisotropic, fractured carbonate media constrained by well logs, FMI, and core information from an offshore Abu Dhabi oilfield. Input features include azimuthally dependent amplitudes of direct and reflected waves, frequency- and traveltime–derived attributes. We benchmark several ML regressors (support vector regression, extreme gradient boosting, multilayer perceptrons, and 1D convolutional neural networks) and use explainable AI tools to rank the relative importance of attenuation- versus kinematics-based features.

This study demonstrates that jointly exploiting anisotropy, attenuation, and ML substantially improves the interpretability and resolution of fracture and fluid systems in complex carbonate media. The proposed workflow is generic and can be transferred to other fractured and heterogeneous settings, offering a practical route to physics-aware, data-driven seismic characterization for reservoir development and monitoring. 

How to cite: Bouchaala, F., Matsushima, J., and Zhao, G.: Integrating Seismic Anisotropy, Attenuation, and Machine Learning for Advanced Subsurface Characterization, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2933, https://doi.org/10.5194/egusphere-egu26-2933, 2026.

The existence of a typical triple junction in Colombia is crucial for understanding plate convergence and coupling among the South American Plate, the subducting Nazca Plate, and the Caribbean Plate. However, locating this triple junction is challenging due to complex geodynamic evolution and uncertainty in the slab boundaries. Here, we developed a high-resolution Lg-wave attenuation model for Colombia and surrounding areas to constrain crustal magmatic activity, link deep processes with surface volcanism, and identify potential slab boundaries. The area encompassing Central America, western Colombia, and Ecuador exhibits strong Lg attenuation and a concentration of volcanoes, indicating thermal anomalies in the crust. In line with the velocity structure, volcanism, seismicity, and isotopic dating, the thermal anomalies associated with the subducting Nazca and Caribbean slabs suggest the presence of three subducting slabs beneath the South American Plate, with a triple junction located at approximately 7.5°N, 77°W. This research was supported by the National Natural Science Foundation of China (42430306).

How to cite: Zhao, L.-F., Liu, Z., Xie, X.-B., Vargas, C. A., Tian, B., and Yao, Z.-X.: A high-resolution broadband crustal Lg attenuation model beneath Colombia and its implication for triple-junction tectonics, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3654, https://doi.org/10.5194/egusphere-egu26-3654, 2026.

Anelasticity is an intrinsic property of Earth’s interior and it is closely associated with temperature, partial melt, and water content. To date, the development of seismic attenuation models has lagged behind that of velocity models, due to the difficulty in distinguishing attenuation effects from velocity heterogeneities in waveforms, as well as inconsistencies across inversion methods and their resulting attenuation structures. To address these challenges, we recently developed a novel anelastic scattering-integral-based full waveform inversion (FWI) method. Its effectiveness has been verified through numerical experiments using the Northwestern United States region as a realistic case study. Specially, the method can accurately solve 3D anelastic wave equation even in the presence of strong attenuation and computes full anelastic sensitivity kernels incorporating both effects of physical dispersion and dissipation. As an application, we utilize abundant seismic waveform data from the China National Seismic Network to establish, for the first time, a high-resolution 3D anelastic structure model of the lithosphere and asthenosphere in the eastern Tibetan Plateau. Waveform comparisons and checkerboard tests verify the reliability of the inverted model, which achieves a maximum horizontal resolution of 0.6°×0.6°and a maximum vertical resolution of 25 km. This highly accurate anelastic model provides important structural constraints for understanding the deep processes of material extrusion at the eastern margin of the Tibetan Plateau.

This work is supported by the National Natural Science Foundation of China (42204056).

How to cite: Wang, N.: 3D anelastic full waveform inversion and its application, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4131, https://doi.org/10.5194/egusphere-egu26-4131, 2026.

The Southern Granulite Terrain (SGT) in peninsular India is a high-grade metamorphic region formed by intricate Precambrian tectonic processes, serves as a natural laboratory for examining the seismic properties of solid continental lithosphere. Attenuation anisotropy shows how seismic energy loss changes with direction, giving extra information beyond just how fast seismic waves move through rock. It is particularly good at showing processes like grain-boundary relaxation, dislocation creep, and fluid assisted deformation. We have measured the shear-wave splitting parameters (, ) and attenuation anisotropy (, ) for the SKS phases recorded at 13 stations spread over the SGT using the second eigenvalue minimisation method and the instantaneous frequency matching technique, respectively. The attenuation anisotropy parameters for each station, obtained through a weighted-stacking process, vary from 0.1s to 0.85s for differential attenuation () with an average of ~0.36s and -82° to 88° for fast polarisation direction (), with the apparent fast wave () attenuating more, indicating the presence of fluid-filled fractures. Removing the attenuation effects, the station-averaged delay time () lies between 0.73s and 1.27s, with an average of ~0.99s, and fast polarisation direction () lies between -87° and 58°. We further analysed the backazimuthal dependence of the splitting parameters. The melt inclusions and the anisotropic layers beneath each station are characterised using the squirt flow model. The fractures are striking at an angle between ~49° and ~306°, and dipping at an angle between ~36° and 50°. The anisotropic layer thickness varies from 33 km to 115 km beneath the stations. Variations in attenuation anisotropy across major shear zones, like the Palghat–Cauvery and Achankovil sutures, offer important information about reactivated shear deformation, fossil lithospheric fabrics, and potential asthenospheric contributions in the SGT. This information helps to clarify the tectonothermal evolution of this ancient crustal block.

How to cite: Das, R., Hamza, F., and Saikia, U.: Probing Mantle Deformation beneath the Southern Granulite Terrain Using Seismic Attenuation Anisotropy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4344, https://doi.org/10.5194/egusphere-egu26-4344, 2026.

Cratons are traditionally considered to be long-lived and stable owing to their great thickness and rigid lithospheric roots. However, increasing evidence suggests that some cratons have experienced significant lithospheric thinning and destruction. The Sichuan basin, a cratonic basin within the Yangtze Craton, is widely regarded as the cratonic nucleus owing to its long-term tectonic stability and continuous sedimentary subsidence. However, the oldest Archean basement of the Yangtze Craton, represented by the Kongling Complex, is mainly exposed in the eastern Sichuan Basin, raising the question of the spatial location of the ancient nucleus for the Yangtze Craton. Since the Mesozoic, the Yangtze Craton has been affected by the combined influences of Paleo-Pacific subduction and Cenozoic eastward extrusion of the Tibetan Plateau, and the preservation and spatial distribution of its deep lithospheric root remain poorly constrained by geophysical observations. Here, we constructed a high-resolution crustal-upper mantle attenuation model using regional Pn and Lg phases to constrain the coupling/decoupling characteristics between crust and upper mantle beneath the Yangtze Craton. The weak crustal Lg attenuation in the Sichuan Basin does not correspond to the weak Pn attenuation in the upper mantle, indicating that the lithospheric root may mechanically migrate to the eastern Sichuan Basin. The phenomenon is likely associated with the Cenozoic eastward extrusion of the Tibetan Plateau, yet the eastern Yangtze Craton appears to have undergone overall lithospheric thinning and destruction related to Mesozoic Paleo-Pacific subduction. This study was supported by the National Natural Science Foundation of China (42474084) and Deep Earth Probe and Mineral Resources Exploration-National Science and Technology Major Project (2025ZD1005302).

How to cite: Shen, L., Zhao, L.-F., Chang, X., Xie, X.-B., and Yao, Z.-X.: Crustal Lg and upper mantle Pn attenuation structure beneath the Yangtze craton and its implications for the ancient cratonic nucleus, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4670, https://doi.org/10.5194/egusphere-egu26-4670, 2026.

Seismic attenuation offers insights into subsurface material properties, which are independent of the velocity information obtained from seismic tomography. Because seismic‐wave amplitude attenuation is sensitive to several factors such as temperature, mineral grain size, partial melt, and compositional variations, quantitative attenuation analysis provides additional constraints on the thermal and rheological state of the Earth’s interior. However, compared to seismic imaging studies, attenuation characteristics of the subsurface beneath the southern Korean Peninsula remain poorly constrained. In this study, we analyze seismic waveforms recorded at approximately 40 broadband seismic stations deployed across the southern Korean Peninsula between 2009 and 2012, and derive preliminary Rayleigh-wave attenuation estimates over the period range of 20–120 s. The results show generally low attenuation at short periods (20–30 s), which are primarily sensitive to the crust and uppermost mantle, whereas relatively high attenuation is observed at longer periods (80–120 s), corresponding to asthenospheric depths. These patterns likely reflect increasing temperature and rheological heterogeneity in the upper mantle. Future work will expand station coverage and invert the attenuation measurements to construct a detailed depth‐dependent attenuation model beneath the southern Korean Peninsula.

How to cite: Park, S. and Chang, S.-J.: Rayleigh-wave attenuation in the southern Korean Peninsula from Helmholtz tomography, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4722, https://doi.org/10.5194/egusphere-egu26-4722, 2026.

Abstract

Seismic attenuation, controlled by scattering and intrinsic absorption processes, represents a fundamental property for investigating crustal heterogeneity, fracturing, and fluid distribution. Here we present results from 3D attenuation tomography in the Calabrian Arc (Southern Italy), based on a relocated local-earthquake dataset analyzed within the MuRAT3D framework (De Siena et al. 2014). The study relies on a dedicated dataset of ~490 local earthquakes recorded between 2016 and 2024 by integrating local and national seismic networks. Event selection was designed to ensure homogeneous spatial and depth coverage while limiting clustering effects. P- and S-wave arrivals were manually picked, and earthquakes were relocated using a combined deterministic–probabilistic approach, producing a robust dataset optimized for attenuation analysis (Schweitzer, 2001; Chiappetta and La Rocca, 2024).

MuRAT3D enables a multi-parameter characterization of seismic energy loss by exploiting different portions of the seismic waveform. Scattering is investigated through Peak Delay (PD) derived from envelope broadening, while total and intrinsic attenuation are described by the quality factors Q and Qc. Analyses were carried out at discrete frequencies (1.5, 3, 6, 12, and 18 Hz), showing that only specific frequency bands yield stable and physically consistent attenuation parameters, reflecting the validity limits of the underlying assumptions and different seismic wave propagation regimes. The resulting 3D attenuation images display coherent, laterally variable patterns, with strong contrasts between continental and offshore domains and localized anomalies related to pronounced crustal heterogeneities and possible interactions with deep structures.Ongoing analyses aim to further refine attenuation patterns and their geological interpretation.

How to cite: Caporale, G., La Rocca, M., de Nardis, R., and De Siena, L.: Multi-parameter seismic attenuation tomography of the Calabrian crust (Italy) using MuRAT3D, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5110, https://doi.org/10.5194/egusphere-egu26-5110, 2026.

Within the collision zone between the Arabian and Eurasian plates, the Anatolian Plateau represents an early stage in the closure of the Neo-Tethys Ocean (Teknik et al., 2025). The uplift mechanism of the Anatolian Plateau remains debated, as the primary geodynamic drivers likely vary regionally. While upper-crustal shortening dominates the northern margin of Central Anatolia, slab break-off and mantle upwelling are key along the southern and interior margins (Şengör et al., 2008; Yildirim et al., 2011). These processes are not isolated but may be geodynamically linked through subsequent shifts in plate motion and mantle flow following slab break-off. The Pn wave is a seismic phase that propagates primarily within the uppermost mantle. Its attenuation characteristics serve as a proxy for physical properties such as temperature, pressure, and water content in this region. Therefore, high-resolution attenuation tomography of the uppermost mantle using Pn waves can provide key constraints on the tectonic evolution of the Anatolian Plateau. 

In this study, we collected 23,830 seismic waveform data from 853 events recorded by 717 seismic stations between July 1996 and August 2025. Using a joint inversion method (Zhao et al., 2015), we constructed a broadband (0.05 - 20.0 Hz) high-resolution (1.0  1.0) Pn-wave attenuation model for the Anatolian Plateau. A prominent high-Q region observed in the southwestern part of the study area represents the Aegean Slab while a localized high-Q zone surrounded by low-Q anomalies (at approximately 36°E, 39°N) correlates with volcanism in the Central Anatolian Plateau. This work was supported by the National Natural Science Foundation of China (No. 42430306).

How to cite: Cheng, Q.-Y., Zhao, L.-F., Eken, T., Xie, X.-B., Li, H.-Y., and Yao, Z.-X.: Pn-wave attenuation tomography in Anatolia and its implications for slab break-off, mantle upwelling, and plateau uplift, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5341, https://doi.org/10.5194/egusphere-egu26-5341, 2026.

Seismic attenuation provides key constraints on the thermo-mechanical state and small-scale heterogeneity of the crust, but high-frequency observations are strongly affected by the coupling between intrinsic attenuation (Qi) and scattering attenuation (Qsc). This coupling hampers conventional attenuation inversions, particularly in tectonically complex regions such as the eastern margin of the Tibetan Plateau. High-frequency seismic coda wave envelopes provide critical insights into the influence of attenuation structures on energy evolution and serve as an essential data source for scattering studies.  In this study, we combine unsupervised machine learning and physics-based envelope modeling to investigate crustal intrinsic and scattering attenuation across the Longmenshan Fault Zone and adjacent regions. We first apply a Conditional Variational Autoencoder (CVAE) to tens of thousands of high-frequency (2–4 Hz) P- and S-wave envelopes, including their coda, recorded by a regional seismic array. By conditioning on source–receiver distance, the CVAE suppresses geometric effects and extracts latent variables that characterize lateral and vertical variations in envelope shape. Two latent variables are sufficient to describe the dominant envelope features: the first is primarily associated with variations in P-to-S energy ratios and correlates with intrinsic attenuation, while the second reflects changes in envelope width and peak timing, consistent with scattering strength. The spatial distribution of the intrinsic-attenuation-related latent variable reveals a clear contrast between the Tibetan Plateau and the Sichuan Basin, whereas scattering-related variations are mainly controlled by local small-scale heterogeneity and show no systematic dependence on large-scale tectonic units. Guided by these results, we further perform three-dimensional high-frequency envelope modeling using radiative transport theory on ~61,000 three-component seismograms. We constructed two-layer models of intrinsic attenuation and small-scale scattering structures for the crust of Sichuan Basin and Tibetan Plateau regions, respectively. The sedimentary layer of the Sichuan Basin displays strong scattering and intrinsic attenuation, suggesting a porous, potentially fluid-rich structure, which aligns with the presence of abundant oil and gas resources. The relatively weak scattering and intrinsic attenuation in the Sichuan Basin's crust indicate its nature as an ancient, stable geological block. The lower crust of the Tibetan Plateau shows stronger intrinsic attenuation than the upper crust but significantly weaker scattering, suggesting the presence of a high-temperature, viscous flow structure in the region. The upper crust of the Tibetan Plateau exhibits significantly stronger scattering and intrinsic attenuation compared to that of the Sichuan Basin, reflecting the extensively faulted and fractured structure due to ongoing tectonic collisions.

How to cite: Zhang, B., Li, J., Ni, S., and Zhang, H.: Crustal Scattering and Intrinsic Attenuation Across the Eastern Margin of the Tibetan Plateau Revealed by High-Frequency Coda Waves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6112, https://doi.org/10.5194/egusphere-egu26-6112, 2026.

Seismic attenuation provides a highly sensitive constraint on fluid-driven processes in the shallow subsurface. These attenuation-derived spatiotemporal insights complement conventional seismic velocity monitoring and can be used for environmental monitoring and engineered subsurface infrastructure management. In this study, we implemented time-lapse Rayleigh-wave attenuation measurements during controlled shallow water injections to quantify the coupled evolution of seismic attenuation and pore-fluid infiltration. The monitoring experiment was conducted over a 14-day period at a localized test site where two vertical wells were hydraulically connected by a permeable pipeline. The frequency-dependent Rayleigh-wave attenuation coefficients are estimated from spectral-ratio slope fitting of multichannel active-source surface-wave records. These measurements are subsequently combined with phase velocities and S- and P-wave velocities to invert for depth-dependent energy dissipation factors  and  within a layered medium. The resulting attenuation variations are interpreted as proxies for changes in fluid saturation and hydrological properties in the shallow subsurface. The attenuation images clearly delineate the boundary between the pipeline and the surrounding medium and exhibit pronounced temporal variations driven by injection-induced fluid migration. Daily time-lapse variations of attenuation over the 14-day experiment reveal frequency-dependent responses to the intermittent injection schedule, with peak values near the period of maximum injection. These patterns reflect the migration and redistribution of pore fluids within the near-surface formation. The inverted Q images further identify localized low-Q zones around the pipeline and the two wells, indicating enhanced energy dissipation associated with fluid accumulation and increasing saturation. This study establishes a powerful framework for monitoring fluid migration and its physical impacts from time-lapse seismic attenuation. Our results highlight the importance of attenuation-based imaging for advancing high-resolution characterization of near-surface hydrological and engineered subsurface environments.

How to cite: Liu, X., Mi, B., Xia, J., Guan, J., Zhou, J., and Sun, H.: Spatiotemporal monitoring of soil moisture dynamics from Rayleigh-wave attenuation: A controlled field experiment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6591, https://doi.org/10.5194/egusphere-egu26-6591, 2026.

We estimated shear-wave splitting parameters and splitting intensity using core-refracted phases (SKS and SKKS) recorded at 90 digital broadband seismic stations across the South Indian Shield, encompassing the Western Dharwar Craton (WDC), Eastern Dharwar Craton (EDC), and Southern Granulite Terrain (SGT). Observed delay times range from 0.4 to 1.5 s, with a mean of ~0.9 s, while fast polarization directions vary from NW to NE–NNE. Although delay times show no significant variation among the three tectonic domains, fast polarization directions exhibit pronounced spatial differences. The EDC is characterized predominantly by NE–NNE orientations, the WDC by N–S to NW directions, and the SGT by a mixed pattern ranging from NW to NE. The splitting intensity varies smoothly across the region, with values ranging from 0.8 to 1.0. These observations suggest that seismic anisotropy beneath the South Indian Shield reflects a complex interplay between the Archean lithospheric architecture and subsequent domain-specific deformation driven by deep Earth processes.

How to cite: Saikia, U., Shameer, S., and Das, R.: Seismic Anisotropy and Splitting Intensity Beneath the South Indian Shield: Evidence for Archean Lithospheric Fabric and Post-Archean Deformation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6793, https://doi.org/10.5194/egusphere-egu26-6793, 2026.

Strong trade-offs between earthquake source and attenuation term remain a major challenge in source parameters inversion and attenuation structure. Spectral ratio methods alleviate this problem by using nearby small earthquakes with highly correlated waveforms as empirical Green’s functions (EGF), thereby reducing path and site effects and enabling robust relative estimation of source parameters, particularly corner frequency. However, limited signal-to-noise ratios and spikes at high frequencies significantly affect the estimation of corner frequency. In addition, different choices of EGF may further increase the uncertainty in corner frequency estimations. To reduce the effects of high-frequency spectral instability and EGF selection on spectral ratios, we first perform single-spectrum fitting to obtain physically constrained and smoothed amplitude spectra. These fitted spectra are then used to construct spectral ratios, from which corner frequencies can be robustly estimated. The source parameters constrained by the spectral ratio analysis are then incorporated as prior information, with the introduction of controlled perturbations, a joint inversion of the source parameters (M₀ and f_c) and the attenuation factor t* is carried out using single spectra fitting. We applied this method to earthquakes that occurred in the southern Sichuan Basin. We applied this method to 257 earthquakes with magnitudes ≥1.5 recorded in the Weiyuan area of the southern Sichuan Basin, China, between November 2015 and November 2016. Seismic moments and corner frequencies are obtained through the combined use of spectral ratio analysis and single spectral fitting, from which stress drops are estimated assuming a circular crack model. The resulting t* measurements are subsequently used to invert for the regional attenuation structure, providing an independent evaluation for the robustness of the inferred source parameters. This study was supported by the National Natural Science Foundation of China (42474084).

How to cite: Chang, X., Shen, L., and Zhao, L.-F.: A spectral-ratio-constrained joint inversion of source parameters and attenuation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6910, https://doi.org/10.5194/egusphere-egu26-6910, 2026.

In both geophysics and medical physics, the propagation of seismic waves through highly complex, heterogeneous viscoacoustic-viscoelastic media follows the same physical principles. The attenuation of seismic waves in the Earth's heterogeneous interior is identical to the way ultrasound waves behave when passing through the human skull or bones (transcranial ultrasound).
In this work, we utilize the core concepts of wave physics and spectral element method (SEM), a well-known numerical simulation technique within geophysics that is used to study the scattering and attenuation caused by the skull during transcranial ultrasound. In the Earth, P-waves can convert to S-waves at interfaces; similarly, at the interface of the skull, ultrasound undergo mode conversions, and also generates Lamb waves, which further complicates the energy transmission. Despite the massive difference in physical scale, both medical ultrasound and geophysics involve a similar number of wavelengths between the source and receiver.

The interface between the skull and soft brain tissue creates a high impedance contrast causing most of the energy to reflect and only a small amount of energy is transmitted through skull.
3D numerical phantoms replicating skull-like properties with varying thicknesses were constructed. SEM, a high-order numerical modeling technique, is used for full waveform modeling of both elastic-acoustic and viscoacoustic-viscoelastic waves through heterogeneous media. A conformal hexahedral mesh is implemented to precisely resolve the irregular geometry of the bone. This ensures that the simulated reflections and refractions are physically accurate and thereby avoid numerical staircasing artifacts. 

The difference in the amplitude and waveform propagation is studied between the acoustic-elastic and viscoacoustic-viscoelastic mediums. Elastic modeling assumes energy is conserved, while viscoelastic modeling incorporates the quality factor (Q) to simulate intrinsic attenuation. 
Amplitude decay measures the difference between the peak pressure value of the transmitted waves. Amplitude decay and difference between wavefields are analyzed to quantify how the heterogeneous internal structure affects the wavefront, and also demonstrating that SEM, a proven geophysical method, effectively simulates and quantifies medical ultrasound wave propagation.

How to cite: Lohan, I., Marty, P., and Fichtner, A.: Quantifying attenuation and scattering in skull-like phantoms using the spectral element method, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8977, https://doi.org/10.5194/egusphere-egu26-8977, 2026.

Hong Kong, one of the most densely populated financial centers in the world, has received limited attention in subsurface structure imaging due to its tectonic quiescence. However, it sits atop the core of the Lianhuashan Fault Zone and was a center of multiple super volcanic eruptions during Yanshanian movement. The complex fault systems and widespread geothermal resources in adjacent region are legacies of these intense tectonic events. We deployed a temporary array of 13 portable seismic nodal sensors covering Hong Kong core area and recorded 21-day seismic data. Using ambient noise adjoint tomography, we imaged the upper 8 km of the crust at high resolution. Significant fault-controlled heterogeneity revealed indicates both geothermal potential and seismic hazard. A deep-seated fault beneath Lantau Island experienced intense fault dilation and volcanic activity as it served as a main magma conduit during Mesozoic. It left behind fractured felsic rocks (low velocity) and rigid mafic intrusions (high velocity), forming a potential seismogenic structure. Pronounced low-velocity anomaly beneath Tai Mo Shan may reflect geothermal activity. Combined with pervasive fracturing and abundant precipitation in Hong Kong, this suggests the presence of an uplift-driven convective geothermal system in the region.

How to cite: Li, Z., Wang, X., Liu, X., Yang, H., and Zhao, G.: Ambient Noise Tomography Reveals Heterogeneous Structure of the Igneous Rocks in Hong Kong’s Upper Crust, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9760, https://doi.org/10.5194/egusphere-egu26-9760, 2026.

The collision between the Indian and Eurasian plates has resulted in one of the most tectonically active zones in the world. To characterize the crustal structure and thermal properties of the region, we present a high resolution Lg wave attenuation model along with Lg wave propagation efficiency map for the Indian Shield, the Himalayas, and the Tibetan Plateau and neighbouring areas. Using a dataset comprising more than 1,800 regional earthquakes recorded by 795 broadband seismic stations, we inverted spectral amplitudes using the least squares orthogonal factorization (LSQR) method to map the lateral variation of the Lg wave quality factor (Q_Lg ) and its frequency dependence (η). The resulting tomographic images reveal a sharp contrast in crustal attenuation across the collision zone. The Indian Shield exhibits significant tectonic stability and low attenuation (high Q_Lg ) along with high Lg wave propagation efficiency, consistent with the transmission of seismic energy through a rigid cratonic lithosphere. Conversely, the Tibetan Plateau is dominated by widespread high attenuation (low Q_Lg ) and significantly reduced Lg wave propagation efficiency, with the lowest values observed beneath the Qiangtang and Songpan-Ganzi terranes. The variation in the η parameter highlights the distinction between intrinsic and scattering attenuation, correlating strongly with regional heat flow variations. We observe a clear spatial correlation between low Q_Lg anomalies and the presence of partial melt or aqueous fluids within the Tibetan crust. These results provide new insights into the geophysical understanding of the collision zone and the geometry of the crustal structure.

How to cite: Bose, S., Singh, C., and Singh, A.: Lg Wave Attenuation across the Indo-Eurasian Collision Zone, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10547, https://doi.org/10.5194/egusphere-egu26-10547, 2026.

Myanmar is located at the southeastern margin of the collision zone between the Indian and Eurasian plates, occupying a key position in the Eastern Himalayan Syntaxis. It serves as a natural laboratory for studying oblique subduction, accretionary orogeny, and crust-mantle dynamics. However, the complex crust-mantle kinematic decoupling mechanism in this region, as well as the control of deep slab geometry on magmatic thermal evolution, remain subjects of debate. Since seismic attenuation is highly sensitive to temperature, partial melting, and fluid content, conducting high-resolution attenuation tomography is crucial for revealing the deep physical state of materials and geodynamic processes in this area. In this study, we performed high-resolution 3-D P-wave attenuation tomography of the Myanmar Orogen using seismic data recorded by 70 stations from the China-Myanmar Geophysical Survey in the Myanmar Orogen (CMGSMO I) between June 2016 and February 2018. We utilized 2,313 seismic events obtained from a deep-learning-based catalog and extracted 14,273 high-quality P-wave t* measurements. By employing the trans-dimensional Bayesian Markov Chain Monte Carlo (MCMC) method, we constructed a high-precision 3-D attenuation model of the study region. The inversion results reveal two significant high-attenuation anomalies: a shallow high-attenuation zone beneath the Indo-Burma Ranges (IBR) at depths of 0–40 km, and a deep high-attenuation anomaly beneath the Central Basin at depths of 80–120 km. The shallow high-attenuation zone coincides well with low-velocity structures; we attribute this to high porosity and fluid saturation within the accretionary wedge sediments, as well as fluid overpressure and rheological weakening caused by deep metamorphic dehydration. This rheologically weak layer likely acts as a lower crustal detachment, facilitating kinematic decoupling between the upper crust and the underlying lithosphere. The deep high-attenuation anomaly reflects asthenospheric upwelling triggered by a "slab window" resulting from the tearing of the Indian Plate. The injection of high-temperature material into the mantle wedge induces partial melting and significantly enhances seismic wave attenuation.

How to cite: Feng, Y., Ai, Y., Zhang, Z., He, Y., Jiang, M., Wei, S. S., Mon, C. T., Thant, M., and Sein, K.: New Insight into the Indo-Burma Subduction Zone: Implications from Seismic Attenuation Tomography in Central Myanmar, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12286, https://doi.org/10.5194/egusphere-egu26-12286, 2026.

The Val d’Agri basin in southern Italy is largest onshore hydrocarbon systems in Europe and, at the same time, one of the most seismically active sedimentary basins in the Apennines. This area appears as an ideal natural laboratory for investigating how fluids, rock damage and stress interact in the shallow crust thanks to the production and fluid injection that take place in this oilfield.
We analyze a dense local earthquake dataset recorded in the Val d’Agri area using seismic attenuation tomography. Attenuation is imaged with the MuRAT workflow, a Matlab algorithm that exploits multi-frequency measurements of direct and coda-wave amplitudes to recover three-dimensional distributions of scattering and absorption. These parameters are highly sensitive to fracture density, lithology, and fluid saturation, and therefore provide a physically meaningful view of the reservoir and fault system.
The resulting attenuation volumes allow us to identify zones of strong energy loss and high heterogeneity that may correspond to highly fractured, fluid-rich areas within the sedimentary cover and along major fault systems. Such features are particularly relevant in a georesource context, as they can act both as preferential fluid pathways and as mechanically weak volumes prone to seismic activation. Results of these analyses provide new light on the internal structure of the reservoir and its surrounding fault network, while also highlighting their interaction with industrial operations.
Overall, this work demonstrates how seismic energy attenuation tomography can provide a powerful framework for imaging fluid–fault interactions in active hydrocarbon systems. The results offer new insights into the processes controlling induced and triggered seismicity in the Val d’Agri basin and contribute to the development of geophysically informed strategies for sustainable resource exploitation and seismic risk management.

How to cite: Avella, M., De Siena, L., Garcia, A., and Zaccarelli, L.: Attenuation Tomography Analysis in the Val d’Agri Oilfield, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13919, https://doi.org/10.5194/egusphere-egu26-13919, 2026.

Attenuation is a fundamental process of seismic wave propagation, yet its role in site–city
interaction remains poorly constrained and rarely quantified. In particular, un- derstanding
how buildings collectively dissipate seismic energy through scattering and absorption is essential
for assessing earthquake impact in urban areas. Numerical studies have recently introduced the
concepts of urban attenuation and urban mean free path to describe these processes. However,
observational evidence based on real data is still lack- ing, leaving open questions about how
such mechanisms manifest in practice.
In this study, we address this gap using the META-FORET experiment, in which a dense
pine forest is considered as a natural analogue of an urban environment. Trees act as distributed
reso- nant scatterers, allowing us to investigate urban-like scattering and attenuation processes
under well-characterized and repeatable conditions. We analyze both ambient noise and active
shot data to extract key ground motion parameters that are directly relevant to seismic hazard
assessment, including horizontal-to-vertical spectral ratios (H/V), spatial variability of ground
motion, wave attenuation and intensity indices. Passive data reveal frequency-dependent
scattering signatures around tree resonances (20 and 50 Hz), includ- ing perturbations of H/V
curves, reduced coherence and absorption.
Active shot analyses further show a systematic reduction of Arias intensity and a strong
increase in Trifunac duration within the forest compared to the open field, especially near
resonance frequencies. These observations indicate that resonant scatterers redistribute seismic
energy, reducing direct-wave amplitudes while enhancing coda wave durations.
This study provides the first experimental quantification of urban-like scattering and
attenuation from real seismic data. By bridging fundamental wave physics and ground motion
indicators, we propose a noise-based technique to characterize seismic wave atten- uation in
urban environments.

Keywords: urban-like scattering, wavefield coherence, absorption, seismic attenuation,
spectral ratios.

How to cite: Celorio Murillo, M. H., Guéguen, P., Touma, R., and Roux, P.: Experimental characterization of urban-like scattering and attenuation from a dense nodal array: implications for seismic ground motion, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14980, https://doi.org/10.5194/egusphere-egu26-14980, 2026.

Impoundment of the Baihetan Reservoir has triggered intense micro-seismicity, raising questions about the underlying hydro-mechanical drivers. While stress drop variations suggest fluid lubrication reduces effective normal stress, distinguishing fluid-saturated conduits from dry fracture networks remains challenging with traditional tomography. Standard attenuation imaging (Q_t^-1) inherently conflates scattering (structural heterogeneity) and intrinsic absorption (anelastic loss), obscuring the true physical state of the subsurface.

To resolve this, we apply Multi-Resolution Attenuation Tomography (MuRAT) to a dense local seismic array dataset. By utilizing Radiative Transfer Theory, we independently invert for scattering (Q_sc) and absorption (Q_i) attenuation coefficients. Our results reveal a distinct spatial decoupling of these mechanisms. Scattering anomalies (low-Q_sc) correlate strongly with the surface traces of the Zemuhe and Xiaojiang fault zones, effectively imaging the pre-existing fracture network. In contrast, intrinsic absorption anomalies (low-Q_i) are concentrated at depths of 5–10 km. These high-absorption features are spatially consistent with theoretical zones of fluid infiltration. By separating structural damage from fluid presence, we provide independent geophysical constraints that support fluid-diffusion hypotheses derived from source parameter analysis.

How to cite: Hu, Y., De Siena, L., Liu, R., He, X., and Xu, L.: Attenuation Tomography of the Baihetan Reservoir: Separating Fluids from Fractures in Induced Seismicity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15304, https://doi.org/10.5194/egusphere-egu26-15304, 2026.

The conspicuous eastward expansion of the Tibetan Plateau is evident and uncontroversial from geological surface expressions and remote sensing, but the mechanisms that cause it have remained enigmatic. The extrusion has been attributed to ductile deformation of a weak crust. This is consistent with the discovery of mid-lower crustal low (seismic) velocity zones (LVZs), but the cause of crustal weakness and the origin and nature of the LVZs are debated, with competing hypotheses including channel flow from central Tibet, local fluid content, and mantle-derived processes. We present a high-resolution 3D seismic attenuation (Qp) model of the crust and uppermost mantle in southeastern Tibetan Plateau. Our results reveal high-attenuation anomalies in the middle-lower crust that overlap with previously imaged LVZs but extend across the Moho into the uppermost mantle. These anomalies correlate spatially with Cenozoic magmatism, mantle-derived helium isotope signatures, Zircon Hf-isotopes, and major strike-slip faults. This suggests that the crust in southeastern Tibetan plateau is weakened from below, possibly by upwelling induced by tearing of the subducted Indian slab.

How to cite: Zhang, H., Wang, J., Hou, Z., Xu, B., Guo, H., Thurber, C., and van der Hilst, R.: Crustal Weakening by Mantle Upwelling in Southeastern Tibetan Plateau, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17001, https://doi.org/10.5194/egusphere-egu26-17001, 2026.

Accurate magnitude estimates and reliable propagation models are essential for seismic hazard assessment. Unfortunately, the magnitudes of small earthquakes remain subject to significant uncertainties, primarily due to complex high-frequency propagation effects. Similarly, spatial variations in attenuation properties are crucial for refining ground motion models and reducing epistemic uncertainties in seismic hazard assessment. This study proposes (1) to map attenuation properties (scattering and absorption) in Metropolitan France using the radiative transfer theory of elastic waves, and (2) to simultaneously estimate source and site spectra through a generalized inversion. The recovered source spectra provide access to the moment magnitude Mw​, corner frequency fc​, and apparent stress σ_app​.

We apply the entire inversion procedure to approximately 21,000 recordings from the EPOS-FR and CEA databases, including events with local magnitudes ML​ ranging from 2.0 to 5.9, and stations with hypocentral distances of less than 250 km. The estimated attenuation maps reveal strong spatial and frequency-dependent variations. Scattering dominates absorption at low frequencies (< 1 Hz), while absorption prevails at high frequencies. Strong scattering anomalies are concentrated in recent sedimentary basins at low frequencies and in deformed regions or deep sedimentary basins at medium and high frequencies. Conversely, Variscan units exhibit low scattering attenuation, especially at low frequencies. Absorption is highest in the French Alps and the western Pyrenees and lowest in the Armorican Massif. Concurrently, a catalog of 1,279 Mw​ magnitudes and 577 site terms is established for Metropolitan France. The obtained magnitudes are consistent with those in the unified Euro-Mediterranean catalog. Its comparison with the SI-Hex catalog highlights the importance for correcting the attenuation variations before extracting source parameters and especially the magnitude. The analysis of the apparent stress σ_app​ reveals a moderate increase with the seismic moment M0​ (scaling exponent of 0.24±0.08), without any marked regional trend. Finally, we emphasize the importance of rigorously correcting for site effects, using reference stations on bedrock and of ensuring inter-event connectivity during the generalized inversion process through the existence of common stations across event records. The next step is to integrate this approach and related results in CEA seismic alert operational framework.

How to cite: Heller, G., Sèbe, O., Margerin, L., Traversa, P., Calvet, M., and Mayor, J.: Magnitude and source spectra estimation using an elastic radiative transfer modelling of seismic wave-field attenuations: application to a French dataset. , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19330, https://doi.org/10.5194/egusphere-egu26-19330, 2026.

Strain partitioning in quartzo-feldspathic rock is closely related to the degree of phase mixing. Both quartz and feldspar tend to form the load-bearing framework (LBF) in naturally deformed rocks, provided there are softer phases such as mica, which behaves as the interconnected weak layer (IWL). In cases where mica is absent, the scenario becomes complicated. Quartz in augen gneisses often behaves as the IWL and feldspar takes the role of the LBF. However, the relative degree of weakening in deformed quartz and feldspar depends on their respective deformation mechanisms. As the mechanisms are different, there is a possibility of dissimilar weaking, followed by a strength reversal.

We have studied a deformed quartzo-feldspathic vein from the Bundelkhand Craton in central India. Despite being Archean, this craton experienced long hiatuses between deformation events, which makes the delineation between different events simpler. The sample we collected from this craton is the result of the latest stage of deformation. A high-temperature fluid entered through fractures and softened the granitic country rock. The fluid, being syn-tectonic, allowed the granitic vein to facilitate different deformation mechanisms in quartz and feldspars.

We investigated the crystal-plastic behaviour of quartz and two feldspars in the deformed vein via electron backscatter diffraction. The quartz crystallographic preferred orientation (CPO) and misorientation index (M) is strongest when quartz grains are adjacent to each other. There is no significant difference in CPO strength in feldspars when the proportion of similar neighbouring phases changes. Additionally, a monomineralic quartz layer exhibits a class 3 buckling fold, implying a higher competency than the adjacent matrix, which contains recrystallised feldspar grains. However, the microstructural evidence suggests that the parent feldspar porphyroclasts are stronger than the recrystallised monomineralic quartz bands. From the inverse pole figure of low-angle (2–10°) misorientation axes in quartz, prism <a> activity is observed which is dominant in the temperature range of 500–650°C. Hence, we infer a deformation temperature of at most 650°C, although it can be lower depending upon the water weakening as such weakening activates prism <a> at lower temperatures. Randomised CPO in feldspar suggests strain accommodation via diffusion creep, followed by grain boundary sliding mechanism might have operated in feldspars. These processes could result in greater softening than that in quartz, which deformed by dislocation creep. Isolated quartz grains existing in the triple junctions of feldspars are not part of such pure dislocation creep; rather, it is more likely that they are byproducts of albitic transformation reactions. Hence, higher strength in quartz is limited to the monomineralic bands, which are purely affected by dislocation creep in the deformed quartzo-feldspathic vein of the Bundelkhand Craton.

How to cite: Sikdar, A., Wallis, D., and Misra, S.: Strength Reversal in Recrystallisation: an EBSD-based Study in a Naturally Deformed Granitic Vein, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1143, https://doi.org/10.5194/egusphere-egu26-1143, 2026.

Earthquake records preserved in rocks provide key insights into the processes that govern crustal deformation and seismic energy dissipation. This manuscript presents new approaches for identifying mineralogical signatures of paleoearthquakes using advanced microstructural analyses, including electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). These techniques enable observations from the millimetre to nanometre scale of features associated with plastic deformation, including crystal reorientation and deformation twinning. Here, we investigate deformation microstructures in monazite, a key geochronometer, with the aim of assessing the impact of deformation on geochronological interpretations, as deformation-induced crystallographic defects can act as high-diffusivity pathways leading to Pb loss. Understanding the deformational behaviour of monazite is therefore critical for interpreting geochronological data. We examine monazite from an eclogite facies mylonite in the Musgrave Province (central Australia) to elucidate mechanisms of seismic deformation under dry (<0.002 wt% H₂O), lower-crustal conditions. The studied monazite grain is directly cross-cut by a pseudotachylite vein, indicating that the observed microstructures formed during the associated seismic event. EBSD and ECCI analyses reveal crystal-plastic deformation in the form of twinning with three distinct orientations: 180° <100>, 180° <001>, and 95° <201>. The latter is associated with dynamic recrystallization via subgrain boundary rotation. ECCI further reveals nanometre-scale (<15 nm) porosity within both parent grains and twins. These microstructures are consistent with those reported in monazite deformed during impact events. Recent studies of shocked monazite have shown that deformation by twinning can liberate Pb during rupture of rare-earth-element–oxygen (REE–O) bonds, enabling rapid diffusion along crystallographic defects and complete expulsion from the crystal, effectively resetting the geochronometer. The new insight provided by these microstructural focussed observations likely accounts for the disparity of electron probe microanalysis (EPMA)-based geochronology on the same monazite grain, which yielded ages of 1309 to 691 Ma. Seismicity in the Musgrave Province is primarily associated with the Petermann Orogeny (~550 Ma), suggesting that the younger EPMA ages were partially reset as a result of the twinning. Our results demonstrate the potential for monazite to record and date seismicity, opening new avenues for reconstructing paleoearthquake histories from deep crustal rocks.

How to cite: Dubosq, R., Camacho, A., and Britton, B.: Seismic twinning in monazite: Microstructural records of deep crustal earthquakes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4322, https://doi.org/10.5194/egusphere-egu26-4322, 2026.

Prograde metamorphic reactions that reduce solid volume are common in subduction zones and orogenic settings [1]. These reductions are often linked to irreversible deformation such as viscous compaction and deep mantle earthquakes [2]. Viscous compaction involves permanent closure of reaction-generated porosity and fluid release, making porosity transient property of metamorphic reactions [3]. The pore closure observation is often linked to the intuitive loss of the elastic strength of the rock leading to permanent strains [4, 5], but these assumptions are very rarely demonstrated experimentally. In most cases, the nature of field relationships and the design of experiments do not allow for such an assessment to be made. Time-resolved in situ experiments enable the observation of a sample volume undergoing metamorphic transformation to check such assumptions at every stage of the reaction (loading, heating, cooling and unloading).

In this contribution, we provide preliminary visual and quantitative strain mapping during the metamorphic reaction cycle of a rock sample at various stress states and reaction extent.

Using Mjolnir, an X-ray transparent miniature triaxial deformation rig, we performed a series of gypsum (Ca₂SO₄·2H₂O) dehydration experiments into bassanite (Ca₂SO₄·1/2H₂O) at constant confining pressure of 20 MPa, pore fluid pressure (5 MPa) and subjected to similar temperature paths (up to 125ºC).  We used a series of differential stresses (radial, hydrostatic and axial principal stresses) to explore how the rock volume responds mechanically while also displaying different reaction fabrics (see [6]). This dehydration reaction produces a solid volume reduction of ~30% [4] enabling the investigation of the evolution of elasticity by unloading fully and partially transformed samples.

Using synchrotron microtomography at the I13-2 beamline of the Diamond Light Source (MG34156), we performed high resolution imaging (1.625 microns/ voxel edge) during gradual unloading to observe and quantify the elastic behaviour both using the mechanical data from the Mjolnir rig [7], sample dimensions (using stitched images; [8]) and digital volume correlation (DVC) techniques (Avizo). 

Our results show the complexity of strain distribution and partial preservation in metamorphic rocks with:

• Significant elastic strain preservation during metamorphic reactions and its apparent minimisation during the ultimate stages of the reaction (textural “maturation” via pressure/solution).
• Complex strain distribution influenced by bassanite anisotropy, sample fabric, geometry, and stress state.

These experiments enable to visualise in 4D the grain-scale development of a complex porous network during the reaction. It opens pathways to document the emergence of poro-elasticity (initial solid has very low porosity) and then the release of the elastic strains. This dataset further demonstrates the importance and the complexity of elasticity in metamorphic systems, with complex displacement vector fields under relatively simple boundary conditions.

References:

[1] Brown & Johnson (2019). https://doi.org/ https://doi.org/10.2138/am-2019-6956

[2] van Keken & Wilson (2023). https://doi.org/10.1186/s40645-023-00573-z

[3] Putnis (2015). https://doi.org/10.2138/rmg.2015.80.01

[4] Leclère et al. (2018). https://doi.org/10.1016/j.epsl.2018.05.005

[5] Llana-Fúnez et al. (2012). https://doi.org/10.1007/s00410-012-0726-8

[6] Gilgannon et al. (2024). https://doi.org/10.1130/G51612.1

[7] Butler et al. (2020). https://doi.org/ 10.1107/S160057752001173X

[8] Turpin et al. in prep

How to cite: Freitas, D., Gilgannon, J., Duggins, D., Butler, I., Rizzo, R., Turpin, L., Chen, B., Reinhard, C., and Bourne, N.: The complex evolution of elasticity during metamorphic transformation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4517, https://doi.org/10.5194/egusphere-egu26-4517, 2026.

Deformation and metamorphism are fundamental processes that act synchronously throughout the Earth; however, their interaction remains unclear. Theoretical models predict that an applied tectonic stress has both a dramatic effect or no effect at all. While the small set of deformation experiments that document the interaction between reaction and deformation are either hard to compare or cannot provide the necessary time resolution to test the various theories. These disagreements of predictions and the gap in data invites new time-resolved experiments to be run that can probe details of model predictions and connect existing datasets from different materials deforming at a range of metamorphic conditions. To this end, we use state-of-the-art time-resolved synchrotron-based x-ray microtomography (4DSµCT) deformation experiments to map out the effect of a differential stress on the kinetics of the dehydration of polycrystalline gypsum samples. Our experiments are highly resolved in space (µm) and time (s), which allows us to track and contrast the emergence of the first small crystals (~100 µm3) and their growth through time in hydrostatic and differentially stressed conditions. We find that the kinetics of a metamorphic reaction are profoundly affected by the addition of deformational energy. Differentially stressed samples transform up to ~90% sooner than in the hydrostatic case, and reaction rates increase by a factor of ~5 with increasing differential stress. Importantly, our findings can be expanded to other published data for reactions occurring in the lower crust and the mantle to show that it is changes in the elastic strain energy that drive accelerated metamorphic kinetics. We find that, when we compare kinetic data from these different reactions and normalise the differential stress to each material’s yield strength, a trend emerges that shows stresses larger than the yield do not contribute to accelerating a reaction. Our results showcase the material-independent effect of a differential stress on metamorphic reactions and support theoretical models which place emphasis on the role of changes in stored energy. Current geodynamic models largely ignore the role of stored energy because it is assumed that it is not relevant at long time scales, our results show that its effect is important and should be accounted for when coupling deformation and metamorphism.

How to cite: Gilgannon, J., Vass Payne, E., Butler, I., Freitas, D., and Fusseis, F.: The material-independent effect of a differential stress on metamorphic kinetics, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6735, https://doi.org/10.5194/egusphere-egu26-6735, 2026.

The study of the evolution of petrophysical properties and alteration of host rock in an active fault system is essential for understanding the mechanisms of deformation localization. The distribution of alterations is closely linked to fluid flow paths, while the formation of new deformation structures depends on the mechanical contrasts induced by these alterations. Our study focuses on granodiorite samples from a borehole drilled in 1996 at Hirabayashi by the Geological Survey of Japan, one year after the Nanbu-Kobe earthquake. Crossing the active Nojima fault, this borehole intersects the fault core at 625 m. The analyses include imaging of samples using neutron and X-ray microtomography (ILL – NeXT) and thin sections, as well as mineralogical quantification by X-ray diffraction (XRD). X-ray diffractograms on oriented slides and Rietveld analyses of XRD data acquired on disoriented powders reveal the presence of secondary mineral phases (e.g., montmorillonite, kaolinite, laumontite, siderite, ankerite), representative of different fluid-rock interaction conditions during the exhumation of the massif. Their proportions, which increase as they approach the fault, reach more than 30% of the volume of a sample at 625 m. Whereas X-ray µ-tomography imaging allows us to observe density contrasts within the samples (e.g., mineral phases and fracture network). On the other hand, neutron imaging allows us to observe the distribution of hydrated mineral phases due to the high neutron absorption coefficient of hydrogen (e.g. for 25 meV neutrons, hydrogen attenuation is 3.44 vs 0.17 and 0.11 for oxygen and silicon, respectively). Neutron and X-ray image registration in the same reference frame allows us to perform joint image segmentation, using gaussian-mixture-model to quantify uncertainties, based on the neutron and X-ray coefficients of absorption of the pre-identified mineral phases. The volumes segmented in this way enable us to (i) quantify in a non-destructive way the volume of secondary mineral phases present in the samples along the fault damage profile and (ii) obtain their spatial distribution and assess the anisotropies of distribution in relation to the deformation structures. This work will subsequently enable us to understand the impact of both the distribution of secondary mineral phases and the network of microfractures on the evolution of the petrophysical and mechanical properties of a seismogenic fault.

How to cite: Jamet, M., Baron, F., Beaufort, D., Dazas, B., Patrier, P., Tengattini, A., Iaquinta, R., Doan, M.-L., Pezard, P., Gibert, B., and Luquot, L.: Neutron and X-ray µ-tomography-based 3D imaging of alteration phases in faulted granodiorite at Nojima (Japan), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7324, https://doi.org/10.5194/egusphere-egu26-7324, 2026.

During the International Ocean Discovery Program (IODP) Expedition 402 in the Tyrrhenian Sea, two of the six drilled sites, the U1613 and U1617, were located on the thinned continental crust of the Cornaglia and Campania terraces, where the deposition of evaporites during the Messinian Salinity Crisis (MSC) had been imaged with seismic data. Expedition 402 recovered Messinian evaporites beneath a relatively thin sedimentary cover at both drill sites. At Site U1613, the Messinian section is extremely thin (a few meters only). In contrast, at Site U1617, a complete 102 m-thick evaporitic sequence ranging from gypsum-enriched terrigenous sediments through anhydrite to halite layers was sampled. This scientific drilling site is the only one in the Mediterranean that penetrated the complete Messinian evaporitic sequence, providing a unique opportunity to study the properties of the so-called Upper, Mobile and Lower units. A series of physical property measurements was performed on these cores on board of the JOIDES Resolution drillship, including P-wave velocity, density, magnetic susceptibility, natural gamma ray and thermal conductivity. In addition, we collected representative discrete samples to measure P-wave velocity (Vp), bulk density, grain density and porosity. These data allowed us to analyze the sealing properties of the halite unit and its interaction with salt-induced tectonics. Furthermore, from Vp and density used as input to calculate reflection coefficients, we generated a 1D synthetic seismogram at Site U1617. We compared this synthetic seismogram with the multi-channel seismic data acquired across the drill site, namely the Medoc 6 line. These new data allowed us to compare the Messinian units recovered in situ with multichannel seismic data and thereby revise seismic interpretation of these units. Thanks to the unique opportunity offered by the IODP Expedition 402, we now have reliable data on the physical properties of Messinian evaporites and we are able to provide new constraints on the interpretation of Messinian facies.

How to cite: Loreto, M. F., Ligi, M., Filina, I. Y., Abe, N., Shuck, B. D., Pezard, P. A., Estes, E. R., Malinverno, A., Ranero, C. R., Yang, L., and Zitellini, N.: A new concept of Messinian Salinity Crisis based on physical properties from the IODP Exp.402 in the Tyrrhenian Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7733, https://doi.org/10.5194/egusphere-egu26-7733, 2026.

The mechanisms of intraslab earthquakes at depths of > 40 km are fundamentally different from those of shallow earthquakes because the frictional strength of silicate rocks is proportional to the confining pressure. To understand the process triggering intraslab earthquakes, many experimental studies on faulting of slab-forming rocks have been conducted at upper mantle pressures. Previous studies have revealed that shear localization induced by dehydration of hydrous minerals (e.g., Okazaki & Hirth, 2016) or adiabatic shear heating (e.g., Kelemen & Hirth, 2007) is essential for the occurrence of faulting at high pressures. Although acoustic emission (AE) monitoring technique for D-DIA apparatuses enabled us to discuss the process of microcracking at high pressures, mechanical behavior at the onset of faulting is still unclear due to low time-resolution stress/strain measurements using synchrotron X-rays. The cause of bottleneck in stress/strain measurements is a long exposure time required for the acquisition of a two-dimensional X-ray diffraction pattern of minerals. Considering that the timescale of stress drop associating faulting is on the order of 0.01 sec (e.g., Okazaki & Katayama, 2015), a significant improvement for time resolution of stress/strain measurements is required. To improve the time resolution of stress/strain measurements, we installed a series of new devices at BL15XU, SPring-8.

We conducted in situ triaxial deformation experiments on olivine aggregates at pressures of 1-3 GPa and temperatures of 700-1250 K under nominally dry conditions using a D-DIA apparatus, installed at BL15XU, SPring-8. Two-dimensional radial X-ray diffraction patterns and radiographic images were alternately acquired by adjusting sizes of the incident slit and operating a flatpanel detector and a CCD camera using a high-flux pink beam (energy 100 keV) from an undulator source (0.2 s of exposure time for both ones). Pressure and differential stress were determined from the d-spacing of olivine. Strains of deforming samples were evaluated from the distance between platinum strain markers. AEs were recorded continuously on six sensors glued on the rear side of the 2^nd-stage anvils, and three-dimensional AE source location were determined.

Stress increased with strain at the beginning of sample deformation, and it reached the yielding point at strains of ~0.1 or less. AEs from the deforming sample were detected when stress exceeded ~1 GPa and the amplitude of AE is positively correlated with the magnitude of stress. At strains higher than 0.1 (i.e., beyond the yielding point), both softening (i.e., decrease in stress and/or increase in strain rate) and a decrease in AE rate were observed prior to the occurrence of faulting. Faulting was observed at 880-1150 K. Most of unstable slips proceeded within 1 s and associated a sudden stress drop (~0.5 GPa) and temporal radiation of large AEs. In contrast, neither stress drops nor AEs were associated with a few “aseismic” unstable slips. Differential stress continuously increased when stable slip proceeded and the stable slip was terminated by the occurrence of another unstable slip. Our observations suggest that unstable slips can be divided into two types (i.e., seismic and aseismic ones) under the P-T conditions of shallow subducting slabs.

How to cite: Ohuchi, T., Higo, Y., Tsujino, N., Kakizawa, S., Ohsumi, H., and Yabashi, M.: In situ observation of faulting in olivine at high pressures and high temperatures using high-flux synchrotron X-rays, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7751, https://doi.org/10.5194/egusphere-egu26-7751, 2026.

Evidence for the nature of fault slip across the seismic cycle is hard to decipher. Fault-related deformation near the fault surface develops over the seismic cycle, characterized by rapid coseismic slip and intense deformation, followed by slower interseismic slip and stress accumulation. While considerable focus has been placed on characterizing deformation through fracturing and mesoscale structures, the analysis of grain-scale plastic processes has been largely neglected. However, transient temperature increases due to frictional heating, combined with the ability of calcite-bearing rocks to deform plastically at relatively low temperatures, suggest that microstructural damage and subsequent recovery processes could leave diagnostic evidence in carbonate fault rocks. Indeed, Pozzi et al. (2019) demonstrated that shearing gouge at seismic rates (~1 m/s) develops a crystallographic preferred orientation (CPO), accompanied by grain growth and sintering. These observations, however, were confined to nanometer-scale grains, localized at the fault surface.

Here, we present a detailed microstructural analysis of carbonate samples from the North Anatolian Fault Zone. We used Electron Backscatter Diffraction to map the calcite grains' orientations and characterize intragrain deformation and grain-boundary morphologies. We identify three distinct layers extending from the fault surface to a distance of ~4 mm. Layer I, with a thickness of tens of µm to 0.5 mm, exhibits predominantly angular grains with grain sizes ranging from unresolved (<1 µm) to tens of µm. Layer II, with a thickness of 0-200 µm, is comprised of small equant grains (1-5 µm) and some larger grains (10-30 µm), characterized by wavy grain boundaries, suggesting active grain boundary migration. No CPO was observed in layers I and II. Layer III, with a thickness of ~2-3 mm, contains large grains (hundreds of µm) that can be divided into two populations of grains. Rounded grains with wavy grain boundaries indicate the progressive consumption of smaller grains. At the core of the layer, grains contain faceted boundaries and are elongated parallel to the fault surface. This layer is the only one to exhibit a distinct CPO with the c-axis oriented normal to oblique to the slip surface. Importantly, the large grains in layer III also comprise small, isolated ‘islands’ of finer grains.

We infer that deformation mechanisms vary systematically with distance from the fault surface. Layer I records cataclastic flow at the fault surface, whereas layer II, characterized by very small grain sizes, exhibits shearing by grain boundary sliding that resulted in grains with low intragrain misorientation and the absence of CPO. The most striking microstructural record is preserved in layer III, which initially shows strong evidence for recovery processes by abnormal grain growth. We propose that this latter process occurred during or immediately after coseismic frictional heating, resulting in the consumption of previously deformed grains, which maintains the CPO record of deformation and provides a microstructural record of the seismic cycle at millimeter-scale distances from the fault surface.

Pozzi, et al., 2019. Coseismic ultramylonites: An investigation of nanoscale viscous flow and fault weakening during seismic slip. Earth and Planetary Science Letters.

How to cite: Boneh, Y., Levi, T., Nuriel, P., and weinberger, R.: Abnormal grain growth in carbonate samples from the North Anatolian Fault: Microstructural evidence of the seismic cycle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7796, https://doi.org/10.5194/egusphere-egu26-7796, 2026.

X-ray ptychography is a lensless, coherent-diffraction imaging technique developed over the last 20 years that affords 10-nm resolution for optically thick specimens¹. It reconstructs the optical transmission function (OTF) of a sample from raster-scanned overlapping 2D transmission diffraction patterns through iterative phase retrieval algorithms^1,2. The OTF projects the refractive index of the sample along the incident beam and thus quantifies the phase shift and amplitude attenuation of the transmitted beam², which in turn relate to the projected electron density of the specimen. X-ray ptychography is therefore an ultramicroscopy technique that is very well suited to mapping nano- and micron-sized objects with significant density differences relative to the bulk such as pores and dense accessory minerals.   

In this contribution, we present a primer for the application of X-ray ptychography to nano- and micro-scale studies of rocks. First, we illustrate the underlying physical principles that guide the data processing and interpretation of ptychographs. Then, we show exemplary applications to a wide range of rock samples (e.g., seismogenic brittle fault rocks, mylonites, veins, shale, and micrite) imaged at the XFM beamline of the Australian Synchrotron³ over the last 5 years^4,5. Application examples include the measurement of sample surface roughness, imaging of cracks and pores, 3D porosity measurements, and the detection of buried accessory phases.

References

1          Pfeiffer, F. X-ray ptychography. Nature Photonics 12, 9-17, doi:10.1038/s41566-017-0072-5 (2018).

2          Wittwer, F., Hagemann, J., Brückner, D., Flenner, S. & Schroer, C. G. Phase retrieval framework for direct reconstruction of the projected refractive index applied to ptychography and holography. Optica 9, 295-302, doi:10.1364/OPTICA.447021 (2022).

3          Howard, D. L. et al. The XFM beamline at the Australian Synchrotron. Journal of Synchrotron Radiation 27, 1447-1458, doi:doi:10.1107/S1600577520010152 (2020).

4          Jones, M. W. M. et al. High-speed free-run ptychography at the Australian Synchrotron. Journal of Synchrotron Radiation 29, 480-487, doi:https://doi.org/10.1107/S1600577521012856 (2022).

5          Schrank, C. E. et al. Micro-scale dissolution seams mobilise carbon in deep-sea limestones. Communications Earth & Environment 2, 174, doi:10.1038/s43247-021-00257-w (2021).

How to cite: Schrank, C. E., Jones, M. W. M., Kewish, C. M., van Riessen, G. A., Hinsley, G., Berger, A., Herwegh, M., Schwichtenberg, B., Bishop, N. D., Howard, D., Langendam, A. D., and Paterson, D. J.: Nano- and micro-scale imaging of rocks with X-ray ptychography, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8362, https://doi.org/10.5194/egusphere-egu26-8362, 2026.

Dehydration embrittlement is the dominant mechanism proposed to explain deep-focus earthquakes between 100–350 km in depth. Antigorite dehydration was extensively investigated in previous experimental studies, which demonstrated contrasting results regarding the seismic potential of antigorite dehydration. Additionally, microstructural aspects of antigorite dehydration and their implications for deep seismicity are scarce. Localized dehydration, on the other hand, might generate strain weakening, potentially leading to failure at depths relevant to deep earthquakes. Localized antigorite dehydration is demonstrated to occur in nature and the laboratory; however, it is not clear if this is a passive or dynamic process.

To better understand the micro-mechanisms of localized antigorite dehydration, we conducted high-pressure, high-temperature experiments under isostatic and non-isostatic conditions. Experiments were run at 3 GPa and temperatures within and above the antigorite stability field (530 °C–800 °C). Antigorite cores with 2 mm diameter were mounted in cubic assemblies and deformed in a 6-ram multi-anvil press at the Bayerisches Geoinstitute. Pure shear deformation was applied by inserting one pair of anvils while simultaneously removing the remaining two pairs orthogonal to it.

Results show that isostatic dehydration of antigorite at 3 GPa starts at ~530 °C and completes at ~800 °C. Localized dehydration occurs in isostatic and non-isostatic conditions within the antigorite stability field. It is enhanced during deformation experiments, resulting in the formation of nanocrystalline veins and networks containing olivine and pyroxene. These results demonstrate that localized dehydration might occur through passive and dynamic processes with the development of different microstructures.

How to cite: Silva Souza, D., Thielmann, M., Heidelbach, F., and Frost, D.: Localized dehydration of antigorite during experimental deformation at subduction zone conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9434, https://doi.org/10.5194/egusphere-egu26-9434, 2026.

U-Pb geochronology via Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) is a fast and reliable method for in-situ dating of calcite that is used across disciplines in earth science. However, the heterogeneous distribution of U (and Pb) in individual calcite crystals represents a yet unmitigated challenge and identifying zones of sufficiently high U concentrations that can provide precise constraints on timing of calcite precipitation is an inefficient “hit or miss” process. Moreover, it is challenging to confirm that targeted domains of a calcite crystal retain their pristine geochemical signature, given the range of post-crystallization dissolution-reprecipitation and solid diffusion processes that can affect this mineral. There is thus an urgent need to understand the spatial and temporal mechanisms of U incorporation and mobilization in calcite to ultimately improve this key geochronological tool. To determine where specific trace elements are located within calcite crystals, investigate how they are incorporated during crystal growth and how they are affected by post-crystallization fluid-assisted deformation processes, we applied Synchrotron X-Ray Fluorescence Microscopy (XFM) with emphasis on U mapping, Electron Backscatter Diffraction (EBSD), and LA-ICP-MS to a set of calcite veins. Samples were collected from drillcores through the Middle and Upper Jurassic carbonates and marls (max. 85°C) in the Neogene Molasse Basin in central northern Switzerland. By combining high-resolution trace element maps with information on the crystal lattice structure of calcite we show two main textural types of trace element distributions within syntaxial calcite veins: 1) oscillatory crystal growth zonations that reflect preferential incorporation of trace elements into structurally different growth steps and faces of growing calcite crystals during growth and, 2) complete overprint of the initial growth zonation upon potential secondary fluid infiltration and trace element replacement. The anti-correlation between Fe, Mn and Sr, U demonstrates the role of kinetic factors during trace element partitioning between fluid and calcite, pointing to the inhibition of Fe incorporation at higher growth rates. Where the Sr uptake during calcite growth is generally enhanced with growth rate. The results of this project give valuable insights in the complexity of fluid overprint during multi-staged deformation cycles in the modification of trace elements in calcite, with clear implications for the applicability and reliability of U-Pb geochronometer in calcite.

How to cite: Akker, I. V., Schrank, C. E., Jones, M. W. M., Howard, D., Tavazzani, L., and Morales, L.: Trace element mapping in vein calcite with synchrotron XFM: implications for U-Pb geochronology, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9614, https://doi.org/10.5194/egusphere-egu26-9614, 2026.

Salt caverns are formed in the subsurface during solution mining of salt. After the end of salt production, caverns need to be safely abandoned or may be repurposed for storage of energy carriers such as hydrogen. Salt caverns locally disturb subsurface stresses, leading to creep of the surrounding rock salt. Creep can cause cavern convergence at depth and may result in surface subsidence, with consequences for infrastructure and public safety. Accurate forecasting of cavern stability during abandonment or assessment of suitability for storage requires a deep understanding of the grain scale deformation mechanisms and processes controlling rock salt strength and creep rate. For rock salt, important deformation mechanisms are dislocation creep and pressure solution creep. Laboratory experiments have shown that dynamic recrystallization (DRX) associated with dislocation creep can be activated and contribute to mechanical weakening. However, the weakening effect of DRX is not included in engineering constitutive laws used in salt cavern numerical models. These laws are commonly based on low-strain laboratory experiments, where the influence of DRX is limited, and microstructural data are relatively rarely reported. The aim of this study is to experimentally investigate the dominant DRX process in deforming natural rock salt and its effect on the mechanical behaviour.

Lab experiments have been carried out on natural wet salt samples from the Zechstein formation. We conducted constant strain rate experiments using a triaxial compression apparatus. Experiments were performed at a confining pressure of 20 MPa and a temperature of 125 °C, using constant displacement rates corresponding to strain rates of approximately 5 × 10⁻⁵ s⁻¹ and 5 × 10⁻⁷ s⁻¹, up to 30–40% axial strain. After the experiment, all samples were studied using optical microscopy. Electron backscatter diffraction (EBSD) analysis was performed on the starting material and on two deformed samples, one from each strain-rate condition.

For all samples, we observed an initial transient creep stage followed by a quasi-steady state stage. The transition to quasi-steady occurred at a strain of about 10% for samples deformed at a strain rate of ~5 × 10^-7 s^-1. For samples deformed at the faster strain rate of ~5 × 10^-5 s^-1, continuous hardening occurred up to axial strains of 30%, with a gradually decreasing hardening rate approaching steady state. Light optical and EBSD microstructural analysis revealed grains with a dense substructure including subgrain walls, euhedral shape grains with low to no substructure, and grains with irregular shaped grain boundaries including bulges. We infer that the dominant deformation mechanism in the tested natural samples was dislocation creep, providing sufficient local differences in dislocation density to activate DRX dominated by grain boundary migration processes. DRX led to rheological weakening and quasi-steady deformation. We are working on robust understanding of the parameters controlling DRX as this is essential to evaluate the zones prone to weakening by DRX around salt caverns.

How to cite: Dialeismas, E., de Bresser, H., Hangx, S., and ter Heege, J.: An experimental investigation of dynamic recrystallisation processes and their influence on the mechanical properties of natural rock salt samples, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9678, https://doi.org/10.5194/egusphere-egu26-9678, 2026.

Bio-cementation is a new, environmentally-friendly soil-reinforcement process. It is used for civil engineering purposes, such as the fabrication of construction materials, as well as the preservation of monuments. This process uses bacterial activity, mainly that of Sporosarcina Pasteurii, that is capable of  hydrolysing the urea present in the medium, leading to the precipitation of CaCO₃ (calcite) crystals between sand grains, therefore binding them together, and reinforcing the soil. The macro and micro (contact scale) mechanical properties of bio-cememted sand have been extensively studied. However, the microstructure of the precipitated calcite crystals remains undiscovered, which induces mechanical differences under different conditions of cementation. The goal of this study is to investigate the microstructure of biogenic calcite, issued from bio-cementation of sand, and how it varies under different cementation conditions. For this, high resolution synchrotron diffraction imaging at the ESRF was performed, utilizing scanning 3DXRD (s-3DXRD) on ID11 and Dark-Field X-ray Microscopy (DFXM) on ID03. For this, the main experiment was performed on three samples that consist of 3D printed resin cells in which cementation was performed under different conditions, by varying the substrate on which the calcite was grown (between sand grains and glass beads), as well as varying the salinity of the medium. After each cementation cycle, and for each sample, layer measurements were acquired using s-3DXRD. A significant difference was observed between the sand and glass bead cases: the precipitated crystals on the glass beads were much smaller than those precipitated on the sand grains. DFXM measurements showed defects that are only present in the case of high concentration of NaCl in the medium, which could potentially alter the mechanical properties of the material. These two complementary techniques allowed for an in-depth study of the microstructure of the precipitated calcite crystals.

How to cite: Sarkis, M., Detlefs, C., La Bella, M., Naillon, A., Geindreau, C., Emeriault, F., Watier, Y., Ball, J. A. D., and Yildirim, C.: High-resolution microstructural study of calcite crystals precipitated through bio-cementation under different conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11242, https://doi.org/10.5194/egusphere-egu26-11242, 2026.

When deformed by dislocation creep the dominant slip (Burgers) vectors of olivine dislocations are parallel to [100] or [001]. Dislocations with an [010] Burgers vector component (b dislocations) have been recorded rarely. Here we show an experimentally deformed olivine sample has a substantial population (17%) of b dislocations. Electron Backscatter Diffraction maps of crystal orientations provided information on dislocations from the orientation gradients. Maps show the b dislocations form subgrain walls like those formed by other dislocation types and are interpreted to form similarly by glide and climb, so b dislocations are mobile. To confirm our approach, we used EBSD maps to select an area for Transmission Electron Microscopy imaging, down to an atomic scale image of a b dislocation. Our sample was deformed within range of subduction zone conditions; our approach can be used to investigate the scale and conditions of b slip in the mantle more widely.

How to cite: Wheeler, J., Hunt, S., Eggeman, A., Donoghue, J., Gholinia, A., Li, Y., Tillotson, E., and Haigh, S.: Olivine Deformation: to B Slip or not to B Slip, that is the Question, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11671, https://doi.org/10.5194/egusphere-egu26-11671, 2026.