TS1.11 | Rock strength, (paleo)stress, fluids, and seismicity: methods, applications and implications for tectonics and mechanics
Rock strength, (paleo)stress, fluids, and seismicity: methods, applications and implications for tectonics and mechanics
Co-organized by SM4
Convener: Moritz Ziegler | Co-conveners: Olivier Lacombe, Lisa Eberhard, Gian Maria Bocchini, Christophe Pascal, Thomas P. Ferrand, Armin Dielforder
Orals
| Fri, 19 Apr, 08:30–12:25 (CEST)
 
Room K1
Posters on site
| Attendance Fri, 19 Apr, 16:15–18:00 (CEST) | Display Fri, 19 Apr, 14:00–18:00
 
Hall X2
Orals |
Fri, 08:30
Fri, 16:15
The strength of rocks determines how the lithosphere responds to stresses resulting from geodynamic processes, gravitational forces and anthropogenic activities. A thorough understanding of rock strength and stress is therefore crucial for a wide range of topics, from plate tectonics and geohazards to mass transport and engineering applications. However, rock strength and stress remain difficult to measure and our comprehension of both quantities depends much on our ability to constrain them from observations, experiments and models.
One difficulty in constraining strength and stress is their variability in space and time, also because we do not fully understand the factors causing the variability. Fluids are known to reduce rock strength and trigger seismicity by reducing effective stresses and driving mineral reaction, but their exact role in driving mechanical instabilities needs to be better understood, also with respect to other processes like transformation-driven stress transfers.
The current state of stress is mainly assessed on seismic focal mechanisms, fault monitoring and slip inversion, borehole data, and methods such as hydraulic fracturing to determine the magnitude of the applied stress. In addition, the paleostress (ancient state of stress) can be obtained by different methods such as paleopiezometry and fault slip inversion, which mainly yield the direction of paleo-stress axes and the stress ratio. However, full stress tensor remains difficult to determine and investigations typically cover specific spatial and/or temporal scales, with a limited view on possible heterogeneities in space and time. We have to deal with incomplete datasets, part of which are not openly accessible. We must therefore advance and develop mechanical concepts, experiments, measuring methods and data compilations, to refine the models.
This session is intended to bring together researchers from various fields and to facilitate transdisciplinary discussions. We seek contributions that advance the current understanding of the governing mechanics of seismotectonic processes including fluids, the paleo and current in-situ stress state and estimation methods, as well as the strain field of the Earth’s lithosphere.

Orals: Fri, 19 Apr | Room K1

Chairpersons: Lisa Eberhard, Olivier Lacombe, Moritz Ziegler
08:30–08:35
Welcome & Part I: Earthquake mechanisms
08:35–08:45
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EGU24-21427
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On-site presentation
Giulia Mingardi, Julien Gasc, Robert Farla, and Alexandre Schubnel

Numerous studies have illustrated that mineral transformations have the capability to induce faulting at elevated pressure and temperature (PT), circumstances in which ductile flow would typically dominate. This mechanism, commonly known as transformational faulting, emerges as a plausible explanation for the puzzling phenomenon of deep-focus earthquakes occurring at depths up to 700 km. Currently, the debate partly revolves around determining why certain phase transformations lead to faulting while others do not. To better understand this phenomenon, we can compare different transformations taking place in similar experimental conditions and see how they do or do not cause strain localization and faulting. In this regard, we conducted a series of five deformation experiments in the large volume press at the PB61 beamline at DESY synchrotron. Two of these experiments involved deforming germanium-olivine samples as they transformed into ringwoodite (the high-pressure phase). The other three experiments were carried out on quartzite (novaculite) samples while they were transforming to coesite. Throughout the experiments, we collected X-ray diffraction patterns and images concurrently with the collection of Acoustic Emissions (AEs).

The results indicate, in both quartz and olivine experiments, the growth of the high-pressure phase at various rates depending on PT conditions and equilibrium overstep. Specifically, we observed rapid olivine-ringwoodite kinetics at elevated PT, far from equilibrium, while slower kinetics were noted for the quartz-coesite transformation. Thousands of AEs were collected in each experiment, and their locations reconstructed using arrival times on the six transducers used. Interestingly, the spatial distribution of these AEs revealed that for some quartz-coesite experiments, AEs originated from fault planes that formed within the initially intact rock cores. Furthermore, an analysis of the AE catalogues, focusing on the magnitude-frequency distribution, revealed a wide range of b-values influenced by varying PT conditions and transformation kinetics. This observation underscores the different underlying mechanisms since the obtained b-values are high when transformation and strain are distributed and lower when strain is localized (i.e., when a fault plane develops).

Our study supports the major role of mineral transformations in inducing faulting under high PT. These findings will help better quantify the intricate relationships between mineral transformations and faulting and in turn contribute to a better understanding of the fundamental geological processes behind deep and intermediate earthquakes.

How to cite: Mingardi, G., Gasc, J., Farla, R., and Schubnel, A.: Combining synchrotron and acoustic emission techniques to reveal the secrets of high PT faulting, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21427, https://doi.org/10.5194/egusphere-egu24-21427, 2024.

08:45–09:05
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EGU24-16752
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ECS
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solicited
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On-site presentation
Prigent Cécile and Warren Jessica

Seismicity studies on both fast- and slow-spreading ridge systems have found along-strike variations in mantle mechanical behavior on oceanic transform faults (OTFs), at pressure and temperature conditions above the long-term brittle-ductile transition of peridotites at ~700°C. Plate motion on some sections of the fault is accommodated by aseismic slip only (ductile deformation), whereas motion on other sections is by slip and deep swarms of microearthquakes (semi-brittle deformation) of mantle rocks (e.g., McGuire et al., 2012; Yu et al., 2021). To explore the mechanisms responsible for lateral variations in mantle mechanical behavior and the occurrence of this deep mantle microseismicity, we carried out an integrated study on peridotite mylonites dredged from two OTFs on the Southwest Indian Ridge that record deformation at 700-1000°C.

The samples show variable degrees of deformation, ranging from proto- to ultra-mylonitic textures. The most deformed zones of the mylonites are characterized by an increase in the proportion of fine grained (<10 micron) mylonitic shear bands compared to coarse grained (millimeter) porphyroclasts inherited from the protolith. These shear bands contain syn-deformation chlorine-rich amphibole indicating seawater-peridotite interaction during shear band formation.

Olivine and pyroxene porphyroclasts in protomylonites contain evidence for intense brittle deformation. The presence of subgrain walls, high aspect ratios, and internal misorientations crosscut by fractures imply that they deformed by low-temperature plasticity before brittle deformation. Fractures are sealed by the fine-grained shear bands present in the samples. In (ultra)mylonites, porphyroclasts also show evidence of fracturing after flowing through low-T plasticity. Fracturing was coeval with viscous flow of surrounding weak and hydrated mylonitic shear bands and triggered by hardening of larger grains due to dislocation accumulation. From existing flow laws, such brittle deformation of peridotite minerals necessitates high strain rate deformation, from 10-9 to 10-5s-1, similar to strain rates associated with slow slip events.

From these results we propose that swarms of microseismicity on OTFs are triggered by deformation of a heterogeneous mantle. Seismic rupture occurs in lenses of coarse-grained peridotites, possibly driven by aseismic creep of surrounding hydrated mylonitic shear zones. Importantly, observations also suggest plastic flow of brittle (seismic) patches before rupture.

How to cite: Cécile, P. and Jessica, W.: Origin of brittle deformation and microseismicity in the ‘ductile’ mantle on oceanic transform faults, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16752, https://doi.org/10.5194/egusphere-egu24-16752, 2024.

09:05–09:15
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EGU24-19186
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On-site presentation
Lucie Tajčmanová, Sebastian Cionoiu, and Dan Schuppenhauer

Dehydration reactions can influence the occurrence of earthquakes at a range of depths, highlighting the importance of understanding these reactions in the study of seismic activity. We have performed several pilot experiments that included the construction of a controllable fast pressure drop unit attached to the piston-cylinder apparatus. This setup makes it possible to simulate conditions that represent a fast pressure drop during an earthquake event. We focused on serpentinite dehydration because 1/ it plays an important link between the deep geodynamic processes occurring in subduction zones and the seismic and volcanic activity and 2/ the interplay between serpentinite dehydration and deformation during the earthquake cycle is not yet fully understood. To test the experimental setting, we first performed a series of static experiments under the conditions that are already in the olivine stability field. After the static experiments at high pressure (1.1 GPa), we performed the controlled fast pressure drop experiments to 0.3 GPa as well as the ramping experiments, in which a series of pressure build-ups and drops were performed maintaining the high temperatures (570 to 640 °C) to simulate the earthquake cycle. In these experiments, olivine was an order of magnitude more abundant than in the one-hour low-pressure static experiment. The pressure drop occurs in seconds. The ramping experiment lasted only 10 mins before cooling down. The results may challenge conventional wisdom about the timescales of mineral reactions under extreme conditions, such as during earthquakes.

How to cite: Tajčmanová, L., Cionoiu, S., and Schuppenhauer, D.: Could earthquakes cause rapid dehydration of serpentinite?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19186, https://doi.org/10.5194/egusphere-egu24-19186, 2024.

Part II: Stress and paleostress
09:15–09:25
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EGU24-3863
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ECS
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On-site presentation
Jean-baptiste Jacob, Benoît Cordonnier, Jonathan Wright, and François Renard

Understanding the mechanisms controlling brittle rock failure at the grain to sub-grain scale is a fundamental challenge in geosciences. Recent advances in triaxial compression and dynamic shock experiments combined with dynamic X-ray microtomography provide unparalleled insights into the 3D strain field evolution within deforming rocks. However, these methods do not accurately predict the heterogeneous internal stress field prior to failure, which is crucial for predicting microfracture initiation and propagation, leading to macroscopic failure. In the past decade, efforts have focused on developing synchrotron X-ray diffraction techniques leveraging the high penetrative capacity of hard X-rays from the last generations of synchrotron light sources. These techniques offer spatially resolved information on crystal phase orientation and elastic strain within a 3D volume. The local orientation and elastic strain tensor is reconstructed grain-by-grain, with precision down to approximately 10-3 radian for orientation and 10-4 for strain. Stress is then calculated using Hooke's law for anisotropic materials and the elastic constants of the crystal phases. We employed 3D X-ray diffraction to investigate the internal stress field evolution in a rock core sample deformed under triaxial compression in the Hades apparatus. A 5mm-diameter core of Berea sandstone was subjected to axial step loading under constant radial stress of 10 MPa, reaching brittle failure at around 90 MPa differential stress. Elastic strain of individual quartz grains were measured at different load steps, and elastic stresses were calculated, providing maps of the internal strain and stress field in the sample. Results reveal progressive elastic shortening of quartz grains parallel to the compression axis and elongation in orthogonal directions due to the Poisson’s effect. Reorientation of principal stress components is also observed with increasing axial stress, which tend to align with the macroscopic stress field. Internal stresses distribution varies within a range of ca. 300 MPa, suggesting local stress amplifications occurred interpreted as force chains, potentially favoring crack nucleation. This experiment is among the first ones to characterize in-situ the stress distribution in a natural rock under compressive loading, and demonstrates the potential of synchrotron diffraction techniques for investigating strain and stress in geological materials.

How to cite: Jacob, J., Cordonnier, B., Wright, J., and Renard, F.: Mapping internal stress field in deforming rocks using synchrotron high-energy X-ray diffraction, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3863, https://doi.org/10.5194/egusphere-egu24-3863, 2024.

09:25–09:35
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EGU24-16849
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ECS
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On-site presentation
Alix Osinchuk, Brendan Dyck, Dave Wallis, and Alfredo Camacho

Strength-depth profiles for ductile portions of continental crust are derived from either extrapolation of flow laws from deformation experiments or paleopiezometric estimates in deformed and nominally hydrated plate margins. Lower continental crust in intracontinental settings, in contrast, is relatively dry and should be considerably stronger than the lower crust of hydrated plate margins. The relative strengths of dry quartz and feldspar are poorly constrained by experiments and paleopiezometric estimates from such rocks are sparse. As such, the strength of intracratonic lower crust is difficult to ascertain. Here, we use a recently calibrated subgrain-size piezometer to estimate paleostresses from feldspar and quartz deformed in relatively dry (<20 ppm H2O) lower continental crust of the Musgrave Ranges in central Australia. Neocrysts of plagioclase, K-feldspar, and quartz mantle partially recrystallized porphyroclasts, which is indicative of bulging and subgrain-rotation recrystallization. Using crystallographic preferred orientations and plotting misorientation axes of subgrain boundaries of each phase, we infer that dislocation creep involved the slip systems (010)[100] and (010)[001] for plagioclase, (010)[101] for K-feldspar, and (0001)<11-20> and {01-10}<0001> for quartz. Titanium in quartz and gradients in concentration of Ca and K in feldspars within neocrysts and along subgrain boundaries verify that subgrains in all three phases were formed at a temperature of ~650°C under dry, eclogite-facies conditions. Subgrain sizes of 10.6–18.1 µm in quartz, 11.5–16.9 µm in plagioclase, and 12.0–17.5 µm in K-feldspar correspond to differential paleostresses between 22–36 MPa and are consistent with a single mean paleostress of 28 MPa. Our results demonstrate that there is minimal stress partitioning between dry quartz, plagioclase and K-feldspar under typical crustal thermal gradients. Moreover, the differential stress accommodated by felsic rocks in the Davenport shear zone is lower than predicted by previous strength-depth profiles of lower cratonic crust.

How to cite: Osinchuk, A., Dyck, B., Wallis, D., and Camacho, A.: Subgrain-size piezometry of feldspar and quartz records a single paleostress from dry lower continental crust, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16849, https://doi.org/10.5194/egusphere-egu24-16849, 2024.

09:35–09:45
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EGU24-9228
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On-site presentation
Luca Menegon, Giovanni Toffol, Hugo W. van Schrojenstein Lantman, David Wallis, Giorgio Pennacchioni, and Bjørn Jamtveit

Field studies established that seismicity in the lower crust is linked to brittle failure of dry, strong rocks. Failure of these strong rocks implies build-up of differential stresses to gigapascal (GPa) levels, but this requirement contrasts with current models of continental lithosphere deformation, which favour distributed flow of weak viscous lower crust. Although several mechanisms have been proposed to generate transiently high stresses, direct measurements are lacking. Recent advancements in microanalytical techniques (i.e., high-angular resolution electron backscatter diffraction, HR-EBSD) have proven successful at measuring the residual stress resulting from elastic strain retained in mineral grains.

We investigated with HR-EBSD the residual stresses in garnet and diopside from exhumed faults containing pseudotachylytes (quenched frictional melts produced during seismic slip). The samples come from Lofoten (Norway), Holsnøy (Bergen Arcs, Norway), and Musgrave Ranges (Central Australia). Pseudotachylytes from all three localities represent single earthquake events and formed at lower-crustal conditions (T = 500–720 °C, P = 0.5–1.0 GPa). Pseudotachylytes from Holsnøy show an asymmetric damage distribution, where host-rock garnet is pulverized nearby the fault on the side subjected to predominantly tensional stresses during rupture propagation, while garnet is intact on the other side. This asymmetry provides an opportunity to compare the residual stresses on both sides of the fault.

All samples preserve intragrain residual stress heterogeneities reaching 100s of MPa to GPa levels due to local high density of unrecovered lattice defects (dislocations). However, the timing of formation of lattice defects with respect to the seismic event differs. In samples from the Musgrave Ranges, the absence of any later deformation along with the sluggish mobility of dislocations in garnet at the ambient deformation conditions (500 °C, 0.5 GPa) allowed preservation of the high dislocation density produced during the earthquake rupture propagation, recording stress heterogeneities of as much as 6 GPa. In Holsnøy, residual stress heterogeneities of up to 1 GPa are only measured in pulverized grains and are also associated with unrelaxed dislocations generated during the earthquake rupture propagation. Intact garnet grains from the less damaged side of the fault show a limited range of intragrain stress heterogeneities, generally within 100 MPa, and a low density of dislocations. Residual stresses in diopside from Lofoten are only elevated (600 MPa) within 200 µm of the pseudotachylyte. Diopside recorded the progressive build-up of stress during interseismic loading, as suggested by the presence of coseismic cracks crosscutting lattice undulations that preserve the greatest stress heterogeneities. However, the ability of diopside to build up stress is limited, as stress is efficiently dissipated by the development of deformation twins.

In conclusion, great stress heterogeneities can be preserved in mineral grains that experienced the earthquake cycle in the lower crust. Different mineral phases can preserve stress heterogeneities to different extents, depending on the mobility of dislocations after their formation and on other relaxation mechanisms (e.g., twinning). Information on residual stress have important implications for the energy budget of an earthquake, the earthquake cycle deformation, and crustal rheology.

How to cite: Menegon, L., Toffol, G., van Schrojenstein Lantman, H. W., Wallis, D., Pennacchioni, G., and Jamtveit, B.: Earthquake induced residual stresses preserved in fault rocks exhumed from the lower crust, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9228, https://doi.org/10.5194/egusphere-egu24-9228, 2024.

09:45–09:55
09:55–10:05
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EGU24-14306
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ECS
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On-site presentation
Stewart Williams, Melodie French, and Claire Rubin

Understanding the stress conditions of active subduction zones has been a longstanding hurdle with critical implications for natural disasters considering stress/strain orientations and magnitudes can control shallow earthquakes and tsunamigenesis. The Sestola-Vidiciatico Unit (SVU) in the Northern Apennines is an exhumed subduction channel with exposures of up to 9 km paleodepth, having reached up to 200°C. This unit experienced a relatively limited deformation history and serves as a rare analog to the shallowest portions of active subduction megathrusts. We use calcite twin data from shear veins along mineralized faults surrounding the exhumed subduction interface to reconstruct paleostress orientations through calcite twin stress inversion. Combining orientation data with calcite twin paleopiezometry and geothermometry, we are able to reconstruct the stress state of the SVU during peak subduction and subsequent exhumation.

During subduction, the maximum principal stress axis was oriented at a low angle to the subduction interface and the minimum principal stress axis oriented at a high angle, indicating N/NE directed compression. As subduction ceased and exhumation initiated, stress orientations inverted with the maximum principal stress axis becoming oriented at a high angle to the subduction interface and the minimum principal stress axis oriented at a low angle, indicating N/NE directed extension driven by primarily the weight of overburden material. These findings are consistent with theoretical orientations for both of these tectonic regimes and agree with previous studies interpreting subduction zone stress orientations. Calcite twin paleopiezometry and geothermometry suggests the rotation of principal stresses coincides with higher differential stresses during early exhumation. Based on the interpreted differential stresses and the reconstructed paleostress orientations, we model different possible explanations including contrasting mechanical strength between the contractional and extensional faults or changes in pore fluid pressure conditions between the two different tectonic regimes.

How to cite: Williams, S., French, M., and Rubin, C.: Determining the deformation temperatures and paleostress conditions of the Sestola-Vidiciatico Unit in the Northern Apennines, an exhumed shallow subduction zone, using calcite deformation twins, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14306, https://doi.org/10.5194/egusphere-egu24-14306, 2024.

10:05–10:15
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EGU24-392
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On-site presentation
Tridib Kumar Mondal and Sirshendu Kumar Biswas

Dykes are essentially magma filled fractures within the earth’s crust often formed by the pressure imparted by the intruding magma. Magnitude of the magma overpressure has been traditionally determined utilizing elastic properties of the host rock and the complete dimension i.e., full length and maximum width of the fractures. Full exposures of dykes (from tip to tip) are rare, however, as most of the dyke bodies encountered in the field are subject to erosion or disruption along its length as a result of geological time, making estimation of aspect ratios challenging.

We propose a new method of estimating total length and maximum width of dykes from their partial outcrops featuring at least one exposed tip. Taking into account the fact that majority of dykes form as dominantly opening mode fractures with an elliptical shape of opening, the method involves solving the equation of this ellipse using every conceivable combination of a pair of ground points recorded on the dyke margin considering the visible tip as the origin. Validity of the method has been checked using published data obtained from incomplete dyke outcrops exposed in the caldera walls of Miyake-jima volcano in Japan. The calculated estimates are in line with the results acquired through a previous published method. The present method has been effectively utilized to calculate the aspect ratios of partially exposed mafic dykes emplaced within the younger granite of the Chitradurga Schist Belt in the western Dharwar craton of peninsular India. We discuss the ranges of their magma overpressure and depths of origin as well as the stress intensity factors associated with the host granite.

 

How to cite: Mondal, T. K. and Biswas, S. K.: Dykes and their magma overpressure, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-392, https://doi.org/10.5194/egusphere-egu24-392, 2024.

Coffee break
Chairpersons: Christophe Pascal, Moritz Ziegler
10:45–10:55
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EGU24-10900
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On-site presentation
Daniel Koehn, Daniel Hafermaas, and Saskia Koehler

Faults are normally thought to present shear fractures that develop at an angle to the main principal stresses, so that they have shear stresses active parallel to the fault plane and thus move. Here we present two “fault” features that deviate from this principle, they develop not due to stress but during kinematic movement, they are both oriented parallel to two of the main principle stresses and as such have no shear stresses in their planes. On the large tectonic plate scale one of these features are the well known transform faults between mid ocean ridges. The ridges themselves are extensional features with the lowest principle stress perpendicular to the ridge. Transform faults are oriented perpendicular to the ridges and show movement only or mainly between the ridges where the plates move in opposite directions. These are faults that do not develop due to shear stress, they develop only because of differential movement and are therefore only or mainly kinematic. On the small scale a very similar feature is the side of a stylolite tooth. Stylolites are dissolution features, they are thus in a way the opposite to mid ocean ridges and have the largest principal stress oriented perpendicular to the stylolite plane. Due to differential movement and growth of the stylolite roughness they develop steep teeth where the sides of teeth become oriented perpendicular to the stylolite plane. These are also movement surfaces or “faults” that have no shear stress.

What does this mean for stress inversion analysis? At least stylolite teeth show a quite pronounced set of striations on their sides and slikolites are also often developed on fault planes, at least in limestone. How do we separate a purely kinematic from a stress-related fault? This discussion and the potential consequences for stress inversion studies is not new, but remains to be very important and should be debated.

How to cite: Koehn, D., Hafermaas, D., and Koehler, S.: The development of kinematic shear-stress free faults , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10900, https://doi.org/10.5194/egusphere-egu24-10900, 2024.

10:55–11:05
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EGU24-5744
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On-site presentation
Simona Pierdominici

Knowledge of the present‐day stress field in the Earth's crust is key for understanding the mechanical behaviour of rocks and structures under tectonic forces. The study of stress fields therefore remains a pivotal area for understanding the mechanical behaviour of rocks, fluid flow at depth and in revealing mechanisms that cause tectonic plates to creep, fail, or rupture. stress patterns in the Earth's crust appear on different scales: first order (plate scale), second order (regional scale), and third order (local scale). The latter is mainly controlled by basin geometry, topography, local inclusions, density contrasts, and active faults and can mask regional and plate stress patterns.

In this contribution, a couple of examples of stress states at the local, regional and large scale are presented using borehole breakouts as main stress indicator for the current stress field orientation.

In order to understand the influence of stress field evolution at local scale, a case study in Hawai´i concerns the effects of the two large overlapping shield volcanoes on the stress field at depth. The analysis reveals that the two-competing gravitational loads primary control the orientation of the present-day stress field, which deviates significantly from the plate and regional tectonic stress field. Therefore, knowledge of local and shallow stress fields can have a significant impact on future borehole planning. From a more regional point of view, an example of current stress orientation in Sweden is presented. The main objectives are to constrain the orientation of horizontal stresses using borehole data, and to discuss implications for geothermal exploration. Thus, obtaining detailed and accurate data on the stress state is of paramount significance to optimise the design of underground installations in order to maximise fluid flow and minimise the risks of wellbore instability. Finally, a large-scale study in Italy investigates the stress field at plate scale to reveal whether the orientation of horizontal stresses may change with depth or laterally indicating stress perturbations and heterogeneities related to areas with complex geo-tectonic setting.

In conclusion, this contribution aims to illustrate and emphasise the relevance of determining the horizontal stress orientation at depth in order to improve the understanding of subsurface stress fields and their applications in different fields of geosciences and different geological settings.

How to cite: Pierdominici, S.: Reconstruction of the state of stress in the upper crust by borehole breakouts stress indicators, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5744, https://doi.org/10.5194/egusphere-egu24-5744, 2024.

11:05–11:15
Part III: Stability of the crust
11:15–11:35
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EGU24-3672
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solicited
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On-site presentation
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Jeanne Hardebeck and Karen Luttrell

Observations of crustal stress orientation from the regional inversion of earthquake focal mechanisms often conflict with those from borehole breakouts. In particular, stress orientations from focal mechanism inversion tend to show little heterogeneity on length scales of kms to 10s of km, while borehole stress measurements often exhibit substantial short-length-scale heterogeneity.  Some of the difference may be because the two methods sample different locations within the crust, possibly indicating local stress heterogeneity, either laterally or with depth. We attempt to reconcile these two types of stress measurements, and investigate the implications for crustal stress heterogeneity. We compiled SHmax estimates from previous studies for 57 near-vertical boreholes with measured breakout azimuths across the Los Angeles region. We identified subsets of earthquake focal mechanisms from established earthquake catalogs centered around each borehole with various criteria for maximum depth and maximum lateral distance from the borehole. Each subset was independently inverted for 3-D stress orientation, and the SHmax direction compared with the corresponding borehole breakout-derived estimate. We find good agreement when both methods sample the basement stress (breakouts are close to the sediment-basement interface), or when both methods sample the mid- basin stress (sufficient earthquakes are present within a sedimentary basin). Along sedimentary basin margins, in contrast, we find acceptable agreement only when focal mechanisms are limited to shallow and close earthquakes, implying short-length-scale heterogeneity of <20 km. While the region as a whole shows evidence of both lateral and vertical stress orientation heterogeneity, we find a more homogeneous stress state within basement rock, over length scales of 1–35 km. These results reconcile the apparently conflicting observations of short-length-scale heterogeneity observed in boreholes, which sample primarily the basins, with the relative homogeneity of stress inferred from focal mechanisms, which sample primarily the basement.

How to cite: Hardebeck, J. and Luttrell, K.: A Unified Model of Crustal Stress Heterogeneity from Borehole Breakouts and Earthquake Focal Mechanisms , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3672, https://doi.org/10.5194/egusphere-egu24-3672, 2024.

11:35–11:45
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EGU24-6650
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On-site presentation
Rebecca M. Harrington, Deborah Kilb, Alessandro Verdecchia, and Pia Victor

Transient stress perturbations caused by passing waves of distant earthquakes have been observed to activate fault slip. Observations of remotely triggered earthquakes at distances greater than ~2-3 mainshock fault lengths suggest that certain conditions promote fault activation, including large-amplitude shaking at periods below ~ 10 seconds within a geothermal setting and extensional and/or transtensional tectonics. Yet, it is still unclear if remote dynamic triggering is ubiquitous. An additional complication to determining the prevalence of triggering is that there are likely many small-magnitude earthquakes within sparsely instrumented regions that are uncataloged. Additionally, large mainshock signals can mask smaller local events that also are missing from the local catalogs. As a result, possible triggering mechanism(s) remain enigmatic. Bounding the necessary physical conditions for remote triggering, such as determining the upper or lower bounds of stress or strain amplitudes, the orientation of the seismic wave’s traversal with respect to the local stress field or fault geometry, or the geologic properties conducive to triggering can help provide clues about the physics of remote triggering. 

            The northern Chilean subduction margin provides an ideal setting to study remote dynamic triggering. Its dense instrumentation provides a long history of both seismic and aseismic deformation in both the subduction system and forearc faults, including the Atacama fault system. Our investigation combines a new, detailed regional earthquake catalog (2007-2021) from Sippl et al., (2023) and documented cases of triggered aseismic slip in the Atacama fault system (Victor et al., 2018). We use a twofold approach to determine the prevalence of earthquake triggering by candidate mainshocks that produce strains at our target location ranging from 1 to ~140 microstrain. The approach uses 1) a difference-of-means test of cataloged seismicity outside of the mainshock cluster (including foreshocks and aftershocks), and 2) a waveform-based approach to look for earthquake triggering at seismic stations located close to creepmeters that recorded triggered aseismic slip events.  We find a lack of evidence of persistent, statistically significant seismicity increases outside of the mainshock cluster associated with any of the candidate mainshocks.  Notably, there is an absence of significant seismicity changes outside of clustered foreshock or aftershock seismicity associated with the series of 11 M6.2-8.2 earthquakes that produced high-strain-rate events during the 2014 Iquique sequence. Seismic recordings of the 2011 M9.1 Japan earthquake at stations CX.PB01-CX.PB14 located near creep meter stations CAR3 and CH01 on the Mejillones peninsula near the Chomache fault reveal evidence of remote triggering. We observe local, uncataloged earthquakes that are only visible after applying a high pass filter that removes the mainshock signal that otherwise overprinted and swamped the local signals.  The uniformity of particle motions (circular or oblong) generated by local earthquakes on multiple stations (N=9) is lacking in most non-triggering mainshocks. This uniformity suggests that the orientation of transient stress perturbations imparted by the mainshock waveforms, in relation to the local fault orientations, may play a role in the triggering process. 

How to cite: Harrington, R. M., Kilb, D., Verdecchia, A., and Victor, P.: Putting faults into motion by remote dynamic triggering: local ground-motion orientations seem to eclipse strain in the northern Chilean subduction forearc crust, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6650, https://doi.org/10.5194/egusphere-egu24-6650, 2024.

11:45–11:55
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EGU24-7664
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On-site presentation
Rostislav Melichar, Ivo Baroň, Matt Rowberry, Jan Jelének, Ľuboš Sokol, Maria del Puy Papí Isaba, Christiane Freudenthaler, Helmut Hausmann, Lukas Plan, Bernhard Grasemann, Josef Stemberk, Richard A. Schultz, and Roland Bürgmann

Short-term earthquake prediction remains one of the primary goals of seismotectonics. Here detailed observations of unusual fault kinematic behaviour and near-surface crustal stress variations are presented from before, during, and shortly after an earthquake series which culminated with two Mw 4.6 and 4.4 events near Breitenau, Vienna Basin, Austria, on 30 March 2021 and 19 April 2021, respectively. The oblique normal NNE-SSW trending Pitten Fault is exposed in Altaquelle Cave close to the southern margin of the Vienna Basin in the eastern Alps, which is known to have hosted several historical earthquakes of Mw = > 5. This cave has developed in Triassic marbles of the Central Alpine Permomesozoic. The observed branch of this active steeply dipping fault is associated with the seismogenic sinistral Vienna Basin Fault and the NE-SW trending Mur-Mürz Fault. To investigate the fault activity, TM71 moiré extensometers have been used to obtain precise three-dimensional records of fault kinematic behaviour at the micron scale while the recently developed SMB2018 protocol has been used to define the stress state associated with each fault reactivation event. The observations were then compared to the Copernicus European Ground Motion Service InSAR time series derived from Sentinel-1 data. From late 2018 to early 2021, the three-dimensional kinematic behaviour of the fault comprised a variety of different on-plane as well as out-of-plane hanging block displacements ranging in magnitude from 3 to 19 μm. Then, around the time of the earthquake series in 2021, four significant displacement events were recorded: (i) 0.186 mm along a vector of 186/-12° (i.e. upward) on 16 March; (ii) 0.615 mm along a vector of 177/-88° (upward) on 26 March; (iii) 0.066 mm along a vector of 013/26° (downward) on 30 March; and (iv) 0.022 mm along a vector of 308/54° (downward) on 11 May. The third of these events occurred on the same day as the largest earthquake. These events are all much larger than any other record of fault displacement recorded in the Eastern Alps since 2013. This contribution details this unusual fault displacement behaviour and compares the calculated stress states with both the focal solutions for each earthquake and InSAR maps of E-W and vertical ground motion. A comprehensive understanding of this important seismotectonic event helps to shed further light on potential earthquake precursory phenomena.

How to cite: Melichar, R., Baroň, I., Rowberry, M., Jelének, J., Sokol, Ľ., del Puy Papí Isaba, M., Freudenthaler, C., Hausmann, H., Plan, L., Grasemann, B., Stemberk, J., Schultz, R. A., and Bürgmann, R.: Unusual fault kinematic behaviour and near-surface crustal stress variations before and during an earthquake series in the Vienna Basin (Austria) in spring 2021, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7664, https://doi.org/10.5194/egusphere-egu24-7664, 2024.

11:55–12:05
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EGU24-20951
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On-site presentation
Marek Jarosinski, Kinga Bobek, and Radomir Pachytel

The 1D Geomechanical Models (1DGM) were done for four vertical boreholes in the Early Paleozoic shale sequences of the Baltic Basin in Poland. The models assumed an elastic rock behaviour with anisotropy in VTI symmetry. The far-field horizontal stresses were calculated as the sum of two components: the vertical stress derivative acting on the horizontally constrained rock column and the effect of elastic tectonic strains. Local stresses in the borehole wall were assumed to induce breakouts (BBs) and drilling-induced tensile fractures (DITFs). Hydraulic fracturing tests additionally validated the stress modelling results.

Micro-resistivity images from XRMI logging revealed irregular BBs, usually confined to individual layers, often non-symmetrical, and with a tendency to encompass the entire borehole wall. Despite their irregularity, statistical analysis of their orientation provides a good quality and stable stress orientation.

The initial modelling results, balancing the cumulative length of the modelled (BBM) and observed (BBO), revealed a systematic misfit between BBm and BBo locations. A detailed comparison between the BBm and BBo intervals concluded that the artificial degradation of Young's modulus and Poisson's ratio is caused by the perturbation of the velocity of the acoustic wave from the dipole acoustic tool passing through the intervals with irregular BBs. This makes it impossible to model the BBs in the places where they are present. To deal with this, the initial stress models were recalculated to the final models in which the BBm were avoided in intervals where there were no BBO.

The initial and final stress models differ significantly in terms of the tectonic strain values and the combined length of the BBm. Still, their stress profiles are similar due to the small contribution of tectonic strain to the far-field stresses. We concluded that irregular BBs developed due to small differential horizontal stresses, causing abrupt BB failure with rapidly growing angular width. The stress layering between lithostratigraphic units was obtained with a dominance of the normal faulting stress regime in the lower borehole sections and the reverse faulting present in the upper sections. The minor regional elastic tectonic strain value for the shale sequence was determined to be an order of magnitude lower than the strain in the crystalline basement, as determined from the satellite geodetic strain rate. We expect that this discrepancy could be explained by a higher rate of viscous relaxation in the shale sequence with > 60% of the clay mineral content. This suggests the need to implement the viscous relaxation into the 1DGM of sedimentary sequences.

How to cite: Jarosinski, M., Bobek, K., and Pachytel, R.: Geomechanical models of the shale sequence of the Baltic Basin (Poland): possible case of elastic properties degradation and viscous stress relaxation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20951, https://doi.org/10.5194/egusphere-egu24-20951, 2024.

12:05–12:15
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EGU24-16807
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ECS
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On-site presentation
Jorge Nicolas Hayek Valencia, Ingo Leonardo Stotz, Hans-Peter Bunge, Sara Carena, and Sia Ghelichkhan

Understanding the intricate dynamics of mantle flow and their influence on lithospheric stress patterns is critical for assessing reservoir responses to potential CO2 or nuclear waste storage, as well as for hazard and risk assessment. Stress patterns play a first-order control in the mechanical response within inherited tectonic structures, with varying stress sources governing different spatiotemporal scales. Our understanding of the present-day mantle flow state has much improved over the past decades, reflected in models that are consistent with first-order features. The World Stress Map (WSM) project serves as a primary observational dataset to validate our understanding of Earth's dynamics through a global compilation of crustal stress indicators.

Here we study mantle flow models as a simplified superposition of Couette and Poiseuille flow types, which have been useful in explaining sub-continental scale deformation in the lithosphere. We aim to understand the role of mantle flow as a stress driver by generating stress fields from an analytical representation of upper mantle flow, derived from the superposition of steady-state flow models. Our approach allows us to build first-order expectations and conduct fast hypothesis testing for upper mantle flow states.

How to cite: Hayek Valencia, J. N., Stotz, I. L., Bunge, H.-P., Carena, S., and Ghelichkhan, S.: First-order global stress patterns inferred from hierarchies of upper mantle flow models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16807, https://doi.org/10.5194/egusphere-egu24-16807, 2024.

12:15–12:25
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EGU24-17191
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ECS
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On-site presentation
Natalia Nevskaya, Alfons Berger, Holger Stünitz, Weijia Zhan, Oliver Plümper, Marcus Ohl, and Marco Herwegh

To comprehend the rheology of the Earth's crust and the relevant rock properties, one key approach is to deform rocks and minerals at elevated pressures and temperatures and then extrapolate the measured stress and strain rate values to natural conditions using constitutive equations. Laboratory experiments are mostly conducted on monomineralic rocks, with quartz being considered as the weakest constituent of the middle continental crust. However, field observations suggest that this is an oversimplification, and polymineralic fault rocks may be weaker than monomineralic quartz rocks. This study presents the first experiments on fine-grained, solid, natural rock samples, containing their natural homogeneities and inhomogeneities, demonstrating that granitoid rocks may be weaker than quartz at mid-crustal conditions. It also highlights the importance of pre-existing faults and polymineralic fine-grained zones for strain localisation and proposes values for extrapolation to natural conditions and their use in numerical models of the deformation of the granitoid crust.

Cylindrical granitoid ultramylonite samples, composed of qtz + ab + K-fsp + bt + ep, with grain sizes of 125-15 μm are deformed in a Grigg’s type apparatus at T=650°C, confining P=1.2 GPa, strain rates=10-3 to 10-5s-1, and 0.2 wt% H2O added. Mechanical data are combined with light microscope, SEM, TEM, and quantitative image analysis to connect microstructures with stress and strain evolution. We show that polymineralic granitoid rocks deform through other mechanisms than monomineralic quartz aggregates at pressure and temperature conditions characteristic for the middle crust: Ultra-fine grain size reduction down to <50nm is developed by nucleation and growth of new grains in a polymineralic mixture. Grain size remains small because of pinning processes. We therefore refer to the deformation mechanism as pinning-controlled dissolution-precipitation creep (P-DPC).

Furthermore, we establish a new constitutive equation for this P-DPC, based on an exponential diffusion creep flow law, to model our experiments and tackle the extrapolation to various natural conditions. This flow law is supported by the microstructural evidence for the deformation mechanisms. Extrapolations show that the shear zones of the granitoid middle crust may be magnitudes weaker than extrapolated so far, and deformation may occur at magnitudes faster rates. The brittle to viscous transition may be shifted to shallower levels. This may have implications for the seismogenic zone and/or stress fields below geothermal reservoirs. Most importantly, we show the necessity to take polymineralic rocks into consideration for various numerical model applications.

How to cite: Nevskaya, N., Berger, A., Stünitz, H., Zhan, W., Plümper, O., Ohl, M., and Herwegh, M.: Implications for the strength of the Earth’s middle crust from novel experiments on natural fine-grained granitoid rocks , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17191, https://doi.org/10.5194/egusphere-egu24-17191, 2024.

Posters on site: Fri, 19 Apr, 16:15–18:00 | Hall X2

Display time: Fri, 19 Apr, 14:00–Fri, 19 Apr, 18:00
Chairperson: Moritz Ziegler
X2.34
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EGU24-19980
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ECS
Renato Gutierrez Escobar and Rob Govers

Our goal is to constrain magnitudes and directions of forces that may explain present-day natural stresses within the Eurasian plate. Driving forces such as horizontal gravitational stresses (HGSs), mantle convective tractions including dynamic topography, and plate interaction tractions with bounding plates are considered. HGS resulting from lateral variations in gravitational potential energy are particularly relevant in the context of the Eurasian plate because there are no major slabs attached to it (i.e., no slab pull force). We illustrate that recently published models of lithospheric density including lateral variations in the lithosphere-asthenosphere boundary result in significantly different HGSs. Furthermore, we include observed major faults into a 2D spherical cap elastic model of the Eurasian plate. We present results of forward FEM calculations and compare them with observed stress directions from the world stress map. We propose different objective functions that determine the misfit of the modelled and observed stresses, fault slip directions, and magnitudes, the deviation of the net torque on the plate from zero, and the model representation error. Our analysis represents a first step towards a Bayesian inference workflow to constrain the dynamics of the Eurasian plate.

How to cite: Gutierrez Escobar, R. and Govers, R.: Eurasian plate-scale stress model considering driving and resistive forces, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19980, https://doi.org/10.5194/egusphere-egu24-19980, 2024.

X2.35
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EGU24-16586
Steffen Ahlers, Karsten Reiter, Andreas Henk, Tobias Hergert, Luisa Röckel, Sophia Morawietz, Oliver Heidbach, Moritz Ziegler, Birgit Müller, and Victoria Kuznetsova

Knowledge of the recent crustal stress state is crucial for a better understanding of crust stability. However, the amount of available stress data in Germany is low. Therefore, a reliable and comprehensive prediction of the complete stress tensor is not possible with these only. However, 3D geomechanical-numerical models, which represent the geometry of the subsurface and its mechanical properties and are calibrated to stress data, allow a continuum-mechanics based prediction of the complete stress tensor and its lateral and vertical variability. A new geomechanical-numerical model of Germany provides new insights into the recent crustal stress field. In contrast to previous models, an improved geological model with a significantly higher stratigraphic resolution is used, a high vertical resolution of ~40 m allows a better mechanical representation of individual units and mechanical inhomogeneities and new data records are used for calibration.

The results provide a comprehensive prediction of the complete stress tensor for Germany and can be used for a wide range of scientific questions and applications. Examples are the prediction of the fracture potential, the slip tendency of faults or as boundary conditions for small-scale models usable for example for engineering applications.

How to cite: Ahlers, S., Reiter, K., Henk, A., Hergert, T., Röckel, L., Morawietz, S., Heidbach, O., Ziegler, M., Müller, B., and Kuznetsova, V.: The recent crustal stress state of Germany - results of a new geomechanical–numerical model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16586, https://doi.org/10.5194/egusphere-egu24-16586, 2024.

X2.36
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EGU24-6837
Mojtaba Rajabi, Moritz Ziegler, Oliver Heidbach, and Malte Ziebarth

The complex interplay between the Pacific and Australian plates in New Zealand offers a unique opportunity to investigate the present-day stress field in a tectonically active area. This study examines the present-day stress pattern of New Zealand through the analysis and compilation of data from 289 boreholes, 4291 earthquake focal mechanism solutions, and 72 neotectonic geological structures. Utilizing the Moho depth of New Zealand, we developed both crustal and mantle stress maps. Stress data above the Moho depth is categorized as the crustal stress map, while data below the Moho is classified as the mantle stress map.

The crustal stress map reveals a consistent ESE-WNW orientation of the maximum horizontal stress (SHmax) across much of the South Island, presenting a high angle to the strike of major active strike-slip faults. In the North Island, the crustal SHmax pattern is variable, highlighting the predominant role of the Pacific Plate subduction beneath the Australian Plate along the Hikurangi Margin. Within the Hikurangi Subduction Zone, both the crustal and mantle SHmax orientations are variable. However, the Taupo Rift Zone exhibits completely different stress pattern in mantle and crust, highlighting the Moho as a strong decoupling horizon in this region.

An examination of neotectonic stress regimes, derived from the neotectonic fault database, in comparison with the present-day stress regime from our stress database across 28 tectonic domains in New Zealand indicates a correlation between observed faults, faulting styles, and the stress field. Nevertheless, discrepancies emerge in certain domains, where the acting stress field diverges from the one expected according to the observed faults, suggesting non-optimal fault orientations.

How to cite: Rajabi, M., Ziegler, M., Heidbach, O., and Ziebarth, M.: Neotectonic stress characterization of New Zealand along the Australia–Pacific plate boundary , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6837, https://doi.org/10.5194/egusphere-egu24-6837, 2024.

X2.37
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EGU24-7176
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ECS
JianHui Tian, Yuan Gao, and Ying Li

The Sichuan-Yunnan region of the SE and E margin of the Tibetan Plateau, situated at the transitional nexus between the seismically-active intensely-deformed Tibetan Plateau and the tectonically stable Yangzi block with comparatively low seismicity, has experienced substantial geological transformations during the Quaternary. Given the pronounced seismicity, there is an escalating imperative for an accurate and refined distribution of the stress field in the region. To unravel the contemporary stress state within major active blocks and along active faults in the study area, an elaborate computation of their tectonic stress field is undertaken by comprehensive updated earthquake focal mechanisms catalog. The tectonic stress field in Sichuan-Yunnan region exhibits obvious lateral variations, with the principal compressive stress direction demonstrating a notable correlation with the azimuth of the P axis. The directions of the stress field show a variation from north to south at ~ 28°N. The directions of the maximum and minimum principal compressive stress in the north show nearly E-W compression and N-S tension, respectively. Conversely, in the south, there is a discernible clockwise rotation trend from east to west. Localized normal faulting stress regimes are observed in the middle section of Xianshuihe fault and southwest side of Litang fault. The extensional environment of the former may be attributed to the tectonic activities such as block translation, clockwise rotation and vertical uplift, as well as the clockwise rotation of the Xianshuihe fault from NW-SW to NNW-SSE. The latter may be related to the extensional structures, rift basins and the normal fault movements in the crust formed by the detachment of the plates and delamination of the mountain roots at the end of Triassic. We also found that the tectonic stress field under the large faults, such as Longmenshan fault, Red River fault, Xiaojiang fault and Lijiang-Xiaojinhe fault, show segmented variation. The findings yield invaluable insights into the intricate dynamics of tectonic deformation along the SE margin of the Tibetan Plateau [supported by NSFC Projects 42330311, 42074065 & 41730212].

How to cite: Tian, J., Gao, Y., and Li, Y.: Present-day stress field in Sichuan-Yunnan region based on comprehensive updated earthquake focal mechanisms catalog, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7176, https://doi.org/10.5194/egusphere-egu24-7176, 2024.

X2.38
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EGU24-4458
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ECS
Peng Chen, Huaning Qiu, and Xinyu Chen

Abstract

The shale reservoir in the Lower Permian Fengcheng Formation of Mahu Sag, the Junggar Basin is prospective in hydrocarbon exploration and development. Due to the complex structures of the study area and the strong heterogeneity of shale reservoirs, the distribution of in-situ stress in the research area has always been poorly understood. Previous studies on in-situ stress are mostly limited to mechanical experiments, logging calculation and simple 2D numerical simulations. Nevertheless, this study combines multiple technical means to simulate the complex 3D in-situ stress in a more accurate and precise way. In this study, a detailed geological model was established by utilizing the method of ant tracking. Post-stack acoustic impedance, logging data and acoustic emission tests were used to jointly invert the accurate 3D geomechanical model. The orientation of the in-situ stress in the study area was determined by digesting the information from FMI (Formation MicroScanner Image) while the boundary condition was fixed by acoustic emission experiments. Finally, the in-situ stress distribution of the study area was clarified through finite element numerical simulation. As is shown by the simulation results, the in-situ stress modeling revealed that the complicated stress state, stress differences, and stress difference coefficients, all of which can provide valuable guidance for well deployment optimization and hydraulic fracturing in the study area, are closely related to burial depth, faults, and rock mechanics parameters. The stress regime in the research area is mainly reverse faulting type. However, as the burial depth increases, the stress regime will change accordingly, transforming from reverse faulting stress regime to strike-slip faulting stress regime. In the same time, the existence of faults will also affect the stress regime to a certain degree. In addition, most faults in the research area are stable and show little tendency of slippage, but there may be a higher risk of slippage in the deep strata. Therefore, it is advisable to avoid these areas as much as possible during geological exploration.

Keywords: 3D in-situ stress field, numerical simulation, shale reservoir, Mahu Sag

How to cite: Chen, P., Qiu, H., and Chen, X.: 3D numerical simulation of complex in-situ stress fields in shale reservoirs: A case study in the northwestern Junggar Basin of China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4458, https://doi.org/10.5194/egusphere-egu24-4458, 2024.

X2.39
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EGU24-20499
Martin Schöpfer, Mario Habermüller, Nicola Levi, and Kurt Decker

Drilling-induced tensile fractures (DITFs) form due to stress concentrations around a wellbore and are in vertical wells typically parallel to the largest horizontal far-field stress and normal to the least horizontal far-field stress. The peak pressure in the wellbore exerted by the drilling mud that the wall rock can sustain is given by the so-called Hubbert-Willis (H-W) criterion, which predicts that wall rock failure takes place when the circumferential effective stress at the borehole wall reaches the tensile strength of the wall rock. However, even though the H-W criterion is a valuable fracture-initiation criterion, it cannot predict if and how an initiated fracture propagates. Linear elastic fracture mechanics (LEFM) can provide a solution to these questions under simple loading conditions, e.g. a vertical borehole in a rock mass that is under an Andersonian stress state. Predicting the initiation and propagation of DITFs in more complex settings, such as inclined boreholes or wellbores in mechanically layered sequences, however, necessitates three-dimensional numerical modelling.

Here we present results of three-dimensional numerical Rigid Body Spring Network (RBSN) lattice modelling. The model comprises so-called rigid blocks (tetrahedra in the present study), that interact with each other and can be bonded at their contacts; these bonds fail when the effective normal stress exceeds the bonds tensile strength, which corresponds to micro-fracture of the wall rock. Coalescence of these micro-cracks leads to the formation of macroscopic fractures. Wellbore failure is modelled by means of a hollow cylinder discretised by these bonded rigid blocks. The remote (tectonic) stress is applied to the hollow cylinder’s outer surface whilst the pressure exerted by the drilling mud is applied to its inner surface. Fractures connected to the wellbore receive the same internal pressure as the wellbore and quasi-static fracture propagation is achieved by gradually increasing the pressure on the borehole wall.

Validation of the numerical model under simple loading conditions illustrates that fracture lengths and associated aperture profiles as a function of wellbore pressure correspond well with LEFM predictions. Numerical models of moderately inclined boreholes exhibit stepping DITFs, where fracture stepping is most pronounced when the wellbore is inclined towards the least horizontal stress direction and no fracture stepping occurs when the wellbore is inclined towards the greatest horizontal stress direction. In mechanically layered sequences comprised of layers with different Young’s modulus, DITFs first nucleate in the stiff beds. Complex fracture geometries emerge in mechanically layered sequences, such as fracture stepping within individual beds or at layer boundaries. The detailed evolution of these more complex DITFs depends on several factors, such as the orientation of the remote principal stresses and layering relative to the wellbore axis. Our numerical modelling approach permits to systematically investigate the effects of these different factors on the geometry of DITFs and therefore offers a new tool that can assist in construing the mechanical genesis of fractures imaged in borehole logs and the current stress in the Earth’s crust.

How to cite: Schöpfer, M., Habermüller, M., Levi, N., and Decker, K.: Three-dimensional numerical modelling of drilling-induced tensile wall fractures, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20499, https://doi.org/10.5194/egusphere-egu24-20499, 2024.

X2.40
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EGU24-16995
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ECS
Subha Kundu, Uddipta Mohanta, and Sudheer Kumar Tiwari

Paleostress analysis is commonly used to understand the brittle exhumation process of deeper crustal rocks to the surface. In this research, we have assessed different stress fields and associated tectonic events in the southern part of Chotanagpur Gneissic Complex (CGC) using fault slip data. The southern CGC comprises two significant crustal-scale shear zones: South Purulia Shear Zone (SPSZ) and the North Purulia Shear Zone (NPSZ), along and across which our fault slip data has been collected. These shear zones exhibit high-grade (amphibolite to granulite) facies Proterozoic rocks consisting mainly of felsic gneisses and migmatites in which low-grade metapelite of North Singhbhum Mobile Belt (NSMB), calc-silicate and mafic granulites of CGC occur as enclaves.

 There has not been any previous study to determine the major stress orientations during brittle exhumation acting upon the Proterozoic rocks in the study area. Thus, our main aim in this study is to understand the variation in stress regime along and across these shear zones and also try to reconstruct the paleostress orientation to determine the history of brittle exhumation of the lower crustal rocks during major orogenic stages of the Proterozoic period. Almost 1000 homogeneous fault slip data have been analyzed using Win-Tensor software. The primary fault data within both shear zones exhibits an approximate E-W orientation, whereas other sets range from NW-SE to NE-SW. Reconstructing stress fields using the age, overprinting relationship and sense of fault movements show that during the Neoproterozoic period (1.0-0.95 Ga), the direction of the compressional stress regime was in N-S orientation. This indicates an oblique-slip movement (thrusting and sinistral strike-slip fault) of the northern CGC block with respect to the NSMB resulting in crustal thickening. The evidence of E-W striking, orogen-parallel normal faults was produced from an N-S directed extensional stress and is primarily responsible for brittle exhumation of these widely distributed granulite facies rocks especially the CGC gneisses in the Purulia region through crustal extension and thinning at 0.95–0.85 Ga.

How to cite: Kundu, S., Mohanta, U., and Tiwari, S. K.: Paleostress reconstruction from fault slip data along the Purulia Shear Zone, Chotanagpur Gneissic Complex, India  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16995, https://doi.org/10.5194/egusphere-egu24-16995, 2024.

X2.41
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EGU24-15970
Christophe Pascal

Paleostress inversion methods based on fault slip data (i.e. “fault slip inversion methods” or FSIMs) were formalised fifty years ago to become, shortly after, classical tools in structural geology and tectonics. The great popularity quickly gained by the methods was as remarkable as the enduring scepticism they prompted in the geological community. FSIMs belong to the rather narrow collection of methods, which allow for bridging traditional field observations and measurements (of fault planes and their respective slickenlines in the present case) to the stress tensor, a complex mathematical object. The latter statement highlights the originality of the approach and, perhaps, the roots of FSIM scepticism: stress are “observed” (or derived from observation of the nature) and not “measured” with the help of physical instrumentation, as it is traditionally done.

FSIMs are thus methods that involve mathematical processing of field data after adequate encoding of these. They rely primarily on the so-called “Wallace-Bott hypothesis”, which assumes parallelism between the measured fault stria and the computed maximum resolved shear stress, and on additional background conditions. The purpose of the present contribution is to discuss the limits of FSIMs in the light of their theoretical background and of realistic geological situations. The discussion will mostly focus on key-issues (e.g. is the stress restored by FSIMs in agreement with the formal definition of mechanical stress?) and will try to propose some future research tracks.

How to cite: Pascal, C.: Paleostress inversion of fault slip data: what is the problem?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15970, https://doi.org/10.5194/egusphere-egu24-15970, 2024.

X2.42
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EGU24-15071
Silvia Mittempergher, Francesca Remitti, Telemaco Tesei, and Giancarlo Molli

During their activity, faults experience multiple seismic cycles, implying that faults recover strength between subsequent failures. Fault strengthening after failure (“fault healing”) occurs through processes having different space and time scales, including fault rock compaction, contact strengthening, contact area increase, fracture self-healing and precipitation of minerals in fractures (sealing). The efficiency of different processes varies depending on the geological setting, fault mechanics and availability and geochemistry of fluids. Here, we present preliminary data from a field-based study of the healing mechanisms of thrust faults inside the Sestola Vidiciatico Unit (SVU) in the Northern Apennines, a tectonic unit interpreted as the plate boundary shear zone between the Ligurian complex and the underthrusting Adria microplate during early-to-middle Miocene, active at temperatures up to 150°C.

The thrusts are sharp surfaces lined by calcite shear veins juxtaposing hectometer to kilometer-sized tectonic slices consisting of marls, shales, sandstones and mud-rich mass transport deposits. The marls and shales of the SVU bear a penetrative deformation pattern of fractures and incipient cleavage planes bounding oblate lithons, whose flattening planes define a foliation approximately parallel to the tectonic contacts. In the marls and shales adjacent to the main thrusts decimetric to metric-thick sheared domains may be observed showing an oblique foliation compatible with the sense of transport of the thrusts. Subvertical extensional calcite veins are common in the competent rock lithons. Multiple generations of normal faults lined by calcite shear veins crosscut the thrust faults, the oldest being rotated and deflected within the thrust-related shear zones. Calcite shear veins, in both thrusts and normal faults, display crack and seal domains and implosion breccias.

The lack of cataclastic rocks along faults indicates that the thrusts and normal faults were active at low differential stresses and high fluid pressures. Normal faults and subvertical extensional veins mutually crosscutting with thrusts are compatible with episodes of post-failure switching from reverse to normal fault stress regimes. Fault healing is dominated by calcite precipitation, which occurs both during fault slip (in crack and seal veins and implosion breccias), and after failure into subvertical extensional veins. Thrust compaction due to stress rotation is likely to be a factor promoting fault compaction and healing. Furter investigations will be conducted to constrain the origin of fluids involved in the vein cementation and the role of stress rotation in promoting different healing mechanisms.

How to cite: Mittempergher, S., Remitti, F., Tesei, T., and Molli, G.: Healing and strength cycling in a regional scale overthrust: insights from the Sestola Vidiciatico Unit in the Northern Apennines, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15071, https://doi.org/10.5194/egusphere-egu24-15071, 2024.

X2.43
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EGU24-782
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ECS
Saskia Köhler and Daniel Koehn

Over the last two decades application of stylolite roughness inversion has become a common tool to reconstruct paleostress-fields, stress magnitudes and burial depth. While the orientation of the highest principal stress is free of any doubt, there are uncertainties coming along with tectonic stylolite inversion that require a differentiated debate. This includes data quality, rock physical parameters, timing of stylolite growth and burial depth.

We present results of statistical data analysis, showing that data quality depends on the number of samples as well as on the sample size. Thus, the dataset can be of very high quality and stable in outcrop scale. This is faced by field and microscopic observations and results of the stress inversion itself, that demonstrate that some common assumptions, i.e. that modelled paleodepth can be used for tectonic stylolite inversion, are not generally valid. We want to open the discussion towards the question of how we can refine such assumptions and parameters for our paleo stress models and better prediction of recent deformations.

 

How to cite: Köhler, S. and Koehn, D.: (Un)certainties in tectonic stylolite stress inversion, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-782, https://doi.org/10.5194/egusphere-egu24-782, 2024.

X2.44
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EGU24-17092
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ECS
Giovanni Toffol, Giorgio Pennacchioni, Marco Scambelluri, and Luiz Fernando Grafulha Morales

Exhumed pseudotachylytes (quenched coseismic frictional melts) and their wall-rocks represent a source of information to investigate earthquake mechanics at hypocentre depth. Pseudotachylytes produced at eclogite-facies conditions in subducted oceanic rocks are of particular interest as they open a window into the elusive mechanics of intermediate-depth earthquakes1.

Here we present observations from pseudotachylytes hosted in oceanic gabbros and peridotites from Moncuni (Lanzo Massif, W Alps). These pseudotachylytes record seismic faulting occurred at ca. 70 km of depth during subduction of oceanic lithosphere and have been explained as the result of brittle failure under high differential stress in dry rocks2,3.

We focus on the pervasive damage surrounding pseudotachylytes within olivine-bearing gabbros. Brittle deformation comprises aseismic (cataclasite bands and foliated cataclasites) and coseismic (pulverized domains with shattering in-situ) features associated with the pseudotachylyte veins. Fluid-absent conditions promoted preservation of the pristine brittle features, including pseudotachylyte glass, throughout the exhumation path.

Pseudotachylyte veins and the associated sharp micro-faults are commonly bound by cataclastic domains. Locally, these domains develop an S-C fabric with ultracataclasites along the shear planes. This fabric shows a progressive localization of strain toward the core pseudotachylyte that cut through the S-C fabric, with the cataclastic aggregates proximal to the pseudotachylyte frequently impregnated by melt. Wall-rock olivine grains show evidence of low-temperature plasticity (deformation lamellae and undulatory extinction) and microfracturing. Both deformation lamellae and microfractures are oriented perpendicular to olivine c-axis. These deformation microstructures are also shown by olivine clasts within the cataclasites bounding the pseudotachylytes suggesting a temporal sequence of (i) crystal plastic deformation and (ii) shattering and pulverization. The small olivine clasts in contact with the sharp margin of the pseudotachylyte show substructures a few hundred nanometres in size and are characterized by absence of Kikuchi diffraction patterns. The lack of diffraction bands is interpreted as evidence of extremely high density of dislocations leading to amorphization of the material.

We interpret the low temperature plasticity of olivine and the progressive evolution of the S-C fabric to represent the precursory stage of stress localization predating the abrupt propagation of the seismic rupture, whose instantaneous high stress pulse is recorded by the shattered olivine clasts.

 

[1] Toffol et al., Earth and Planetary Science Letters, 2020, 578: 117289

[2] Scambelluri et al., Nature Geoscience, 2017, 10.12: 960-966

[3] Pennacchioni et al., Earth and Planetary Science Letters, 2020, 548: 116490

How to cite: Toffol, G., Pennacchioni, G., Scambelluri, M., and Grafulha Morales, L. F.: Record of high-stress deformation before and during an earthquake at intermediate-depth conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17092, https://doi.org/10.5194/egusphere-egu24-17092, 2024.

X2.45
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EGU24-8186
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ECS
Nucleation Mechanism of the 2022 Ms6.8 Luding Earthquake in Sichuan, China: Insights from Lithospheric Effective Elastic Thickness and Numerical Simulation
(withdrawn after no-show)
Fan Wang
X2.46
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EGU24-9630
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ECS
Gian Maria Bocchini, Patricia Martínez-Garzón, Armin Dielforder, Luca Smeraglia, Rebecca M. Harrington, and Marco Bohnhoff

The Armutlu Peninsula in north western Türkiye, a horst zone in an active transtensional pull-apart basin, is bounded by two major sub-branches of the North Anatolian Fault zone and host high rates of seismicity in the northern part. The ~25-station SMARTnet surface seismic network was installed in 2019-2020 with the purpose of augmenting permanent seismic stations in the northern part of the Armutlu Peninsula and to help increase the detection of small-magnitude earthquakes. Here, we employ a waveform-based clustering method that integrates detailed information from the seismicity and focal mechanism distribution enabled by the added station coverage to investigate the geometry and kinematics of the seismically active structures. We start by using an enhanced earthquake catalog of >4,000 double-difference-relocated events and >150 focal mechanisms obtained using P-wave polarities and amplitudes in the time period between January 2019 and February 2020. We perform a formal inversion of the stress field orientation from focal mechanisms to investigate the regional deviatoric stress field and its relation with activated fault structures. The stress-field inversion uses input data that combines the enhanced focal-mechanism catalog from background seismic events together with published focal mechanisms of M≥2.5 events that occurred between 1999 and 2019. Stress inversion results show an extensional stress regime for the broader northern Armutlu Peninsula and a transtensional stress regime for a narrow region of ~80 km2, referred to as the Esenköy Seismic Zone (ESZ). Within the ESZ, the minimum principal stress (σ3) is approximately horizontal and NE-trending, while the maximum (σ1) and intermediate (σ2) principal stresses are close in magnitude and vary between near vertical and near horizontal. We observe clusters of normal and strike-slip faulting events identified in the ESZ through waveform-based clustering analysis that are optimally oriented with respect to the stress field we derive for the area. The minimum principal stress in the ESZ is rotated clockwise by ~10-15° with respect to the minimum principal stress inferred for the broader Armutlu Peninsula and eastern Sea of Marmara. Based on the Mohr-Coulomb failure criterion, we quantify the relative and absolute magnitudes of the principal stresses, determine the local crustal stress and strength conditions, and will present a discussion of the implications for regional tectonic forces.

How to cite: Bocchini, G. M., Martínez-Garzón, P., Dielforder, A., Smeraglia, L., Harrington, R. M., and Bohnhoff, M.: Quantitative constraints on crustal stress and strength from seismological observations in the Armutlu Peninsula (northwestern Türkiye), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9630, https://doi.org/10.5194/egusphere-egu24-9630, 2024.

X2.47
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EGU24-10591
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ECS
Thomas P. Ferrand, Jacques Précigout, David Sifré, Frédéric Savoie, Rémi Champallier, and Fabrice Gaillard

Electrical conductivity measurements on well-characterized materials in the laboratory allow accurate interpretations of electrical anomalies within the lithosphere and asthenosphere. But so far, most measurements have been performed statically, and hence, the effect of deformation and/or differential stress on electrical conductivity remains largely unknown. Here we report the first successful deformation experiments performed in a new-generation Griggs-type apparatus adapted for electrical conductivity measurements. The experiments were conducted on samples of Åheim dunites at a confining pressure of 1 GPa and temperatures of 500, 650 and 800°C. In silicate polycrystals, electrical charges are known to preferentially travel through grain boundaries, which act as high-diffusivity pathways. Our results show that stress and strain can significantly impact the electrical conductivity of peridotites by changing the thickness and the number of grain boundaries, respectively. At fixed P-T conditions, the electrical conductivity varies within an order of magnitude during deformation. This motivates to reappraise interpretations of electrical anomalies in mantle rocks, at least in tectonically active regions. The design presented in this study is fully stable at 1 GPa (≈ 30 km depth) and should be stable up to 2 GPa at least, and to average temperatures as high as 1000°C. Further developments should soon enable similar measurements at pressures up to 4 GPa (120 km depth). These experimental achievements open a new research field, which will help to understand electrical anomalies and strain localization processes in rocks under stress at depth, notably within the lower crust, the upper mantle, and subducting slabs.

How to cite: Ferrand, T. P., Précigout, J., Sifré, D., Savoie, F., Champallier, R., and Gaillard, F.: Electrical conductivity in a Griggs apparatus: a new experimental geophysical tool to investigate geological processes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10591, https://doi.org/10.5194/egusphere-egu24-10591, 2024.

X2.48
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EGU24-249
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ECS
Xiongjie Zhou and Regina Katsman

Methane (CH4) bubbles developed in shallow aquatic muds present a significant environmental risk. Macroscopic CH4 gas content in the muds is accommodated in discrete bubbles that grow from below the pore scale size to the maximum size defined by muddy sediment mechanical properties. The bubbles force out the water within the pores and distort the structure of the muddy sediment by moving the grains apart at their growth above the pore scale. However, the interaction between growing bubbles was not understood. This study uses a mechanical/reaction-transport numerical model to simulate the interaction of competitive CH4 bubbles paired with fracture-driven growth of varying initial sizes in aquatic muds. It reveals that mechanical and solute transport dynamics play a crucial role at different stages of bubble growth, particularly hindering the development of smaller bubble growth in competition. The stress from the larger bubble impacts the inner pressure and diffusive CH4 flux to the smaller bubble, slowing its initial growth (at t < 40 s). Additionally, the larger bubble later diverts CH4 from the smaller one, further inhibiting its growth expansion. This interaction may cause more horizontally oriented smaller bubbles and significant deformations in the larger bubble, especially as the distance between the bubble pair decreases. Such competitive bubble growth may explain the bubble size distributions observed in lab experiments and in situ, promoting CH4 retention in muddy sediments and the formation of gas domes, which are precursors to pockmarks that can cause abrupt gas releases to the water and potentially the atmosphere. The study provides a foundation for upscaling to different models of gassy muddy sediment acoustic characteristics and models of gas retention evolution, while maintaining single bubble growth metrics. It contributes to better evaluating and potentially reducing long-persisting uncertainties around CH4 emissions from shallow aquatic sediments.

How to cite: Zhou, X. and Katsman, R.: Competitive methane bubble growth in aquatic muds, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-249, https://doi.org/10.5194/egusphere-egu24-249, 2024.