TS1.3 | The crustal stress state and strength: Conceptions, modelling, and uncertainties
EDI

A detailed understanding of the stress state and variable geomechanical properties (frictional strength, Young's modulus, etc.) of the Earth's crust are important parameters for lithosphere dynamics as well as engineering applications related to the extraction, transport, storage and disposal of energy or materials.
In the context of lithosphere mechanics, the strength limits are bounded by two end-members. In one end-member model, the crust is strong and fails at high differential stresses (hundreds of MPa) consistent with the classical Christmas tree envelope and static friction governed by Byerlee's law. In the other end-member model, the crust is weak and fails at low differential stresses (tens of MPa) consistent with stress magnitudes that may result from topographic loading and tectonic forces. How significant are these end-member scenarios and how do they affect our perception of lithosphere dynamics on time scales ranging from a single earthquake to long-term processes such as orogeny? Can the end-members be reconciled or are they mutually exclusive? Do they reflect differences between continental interiors and plate margins or tectonically inactive and active regions?
Geomechanics is focused on providing the most accurate estimate of the present-day stress state, or quantifying criticality in the context of subsurface use. More complex questions can be addressed with numerical models. But how can the uncertainties of model parameters (material properties and structures) and calibration data (stress magnitudes) be quantified?
To address these fundamental questions, we invite contributions from observational, experimental, theoretical, and numerical studies that improve our understanding of the crustal stress state or expand the methodological repertoire. Highly appreciated are presentations about new methods and on strategies to reduce the uncertainties.

Co-organized by ERE5/GD7
Convener: Armin DielforderECSECS | Co-conveners: Moritz ZieglerECSECS, Gian Maria BocchiniECSECS, Mojtaba RajabiECSECS, Marianne Conin, Karsten Reiter, Ismay Vénice AkkerECSECS
Orals
| Mon, 24 Apr, 10:45–12:30 (CEST)
 
Room D1
Posters on site
| Attendance Mon, 24 Apr, 16:15–18:00 (CEST)
 
Hall X2
Posters virtual
| Attendance Mon, 24 Apr, 16:15–18:00 (CEST)
 
vHall TS/EMRP
Orals |
Mon, 10:45
Mon, 16:15
Mon, 16:15

Orals: Mon, 24 Apr | Room D1

Chairpersons: Armin Dielforder, Moritz Ziegler
10:45–10:50
10:50–11:10
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EGU23-13795
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TS1.3
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solicited
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On-site presentation
Carolin Boese, Marco Bohnhoff, Oliver Heidbach, and Georg Dresen

One main goal of the Continental Deep Drilling Program (KTB) of the Federal Republic of Germany was to establish a continuous stress profile from the surface to the final drilling depth of 9.1 km. To characterize stresses with depth, several independent methods were applied: analyses of borehole failure such as borehole breakouts/drilling-induced tensile fractures; hydraulic fracturing mini-tests at several intervals ≤3 km depth as well as two modified hydraulic tests at 6 and 9 km depth; and core disking and strain retardation of core samples. Focal mechanisms of induced seismic events from fluid injection experiments were inverted for stress estimates at different depths. Since then, the KTB is known as a world-class site with regard to crustal stress data. In particular, stress magnitude estimates are still among the deepest and fewest high-quality estimates derived at crustal depth.

The GEOREAL fluid injection experiment aims to characterize the geothermal potential at the KTB site at 4 km depth and to refine the adaptive reservoir stimulation concept employing near-real-time microseismic monitoring with direct feedback on hydraulic parameters. Additionally, a goal of GEOREAL is to investigate spatial and temporal stress variations at this depth. We noticed new borehole breakouts in the open hole section of the pilot well KTB-VB, likely due to the massive fluid production and injection experiments between 2002 and 2005. Together with new logging and seismic data from GEOREAL, these stress estimates will be used to further characterize the stress field from the borehole to the reservoir scale.

The GEOREAL hydraulic stimulation will include a series of hydraulic tests at ≥3.9 km to investigate the effect of pressure build-up and release, the role of continuous and periodically varying flow rates, the effect of relaxation phases and maximum injection pressure on the spatio-temporal propagation of induced seismicity. Induced events will be monitored with high precision using a 12-level geophone chain in the KTB main hole at only ~300 m distance to the stimulation interval. This will be used to determine stress estimates from focal mechanism inversion of induced events on a 100-m source scale.

To better understand the role of the local stress field we use a 3-D geomechanical-numerical model (10 x 10 x 10 km3) of the KTB. This offers a unique opportunity to utilize the detailed knowledge of the subsurface at the KTB site, in particular due to the existing 3-D structural model, high-quality rock property estimates from laboratory work, high-quality stress magnitude data, and new information from GEOREAL. The model provides a continuous description of the 3-D stress field including its changes due to the variability of rock properties to assess the in-situ stability of the intact rock mass and faults. This allows for further detailed studies that require the undisturbed in-situ stress state as one key observable and input parameter to characterize deep geothermal reservoirs and associated processes such as induced seismicity.

How to cite: Boese, C., Bohnhoff, M., Heidbach, O., and Dresen, G.: Integrated stress determination at the KTB deep crustal laboratory, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13795, https://doi.org/10.5194/egusphere-egu23-13795, 2023.

11:10–11:20
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EGU23-11786
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TS1.3
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On-site presentation
Shijie Zhong, Shunjie Han, and Ashley Bellas

An essential feature of plate tectonics is that lithospheric deformation is localized at plate boundaries with substantially larger magnitude than that in plate interiors, suggesting that lithospheric rheology is weaker at plate boundaries than in plate interiors. Numerous mantle convection modeling studies that approximate this empirically derived lithospheric rheology using different formulations or proxies (e.g., pre-existing weak zones, faults, reduced coefficient of friction or yield stress, …) have largely reproduced the observed features of lithospheric deformation. While the rheological formulations in theoretical modeling studies have become increasingly more sophisticated often with an expressed goal to understand the cause of plate tectonics and initiation of subduction, it is important to place constraints on lithospheric rheology using in-situ observations including flexural (i.e., vertical motion) and seismic response to different forcings. Laboratory studies indicate that lithospheric deformation is controlled by frictional sliding, low-temperature plasticity (LTP) and high-temperature creep with increasing temperature. Observations of lithospheric flexure and seismicity at Hawaiian Islands (i.e., plate interior setting) in response to volcanic construction suggest that internal frictional coefficient µf is 0.25, while LTP is significantly weaker than that derived from laboratory studies [e.g., Mei et al., 2010], based on modeling studies of loading response of Hawaiian lithosphere with realistic elasto-frictional-plastic-viscous rheology [Zhong and Watts, 2013]. Further studies [Bellas and Zhong, 2021; Bellas et al., 2020; 2022] showed that µf is 0.3 and activation energy of LTP needs to be reduced from laboratory derived value of 320 KJ/mol to 190 KJ/mol to fit the flexural and seismic deformation at Hawaii, and that the same rheological parameters reproduce the observed elastic thickness at other oceanic islands and seamounts on lithosphere of different ages. The Japan subduction zone shows characteristic features of subducting lithosphere with its outer rise and trench topography and transition from shallow normal/extensional faulting to deep reverse/compressional faulting seismic deformation (i.e., neutral plane) [e.g., Craig et al., 2014]. Dynamic deformation models of subduction have been formulated, using realistic slab buoyancy force and elasto-frictional-plastic-viscous rheology, to interpret the observations of trench-outer rise topography and neutral planes [Han et al., 2022]. The modeling indicated that the observed neutral plane in the Japan subduction zone is consistent with the rheology for subducting lithosphere with LTP activation energy of ~220 KJ/mol and µf~0.3, which are similar to that inferred for the plate interior at Hawaii. The modeling also found that µf<0.1 that is required to generate mobile-lid or plate tectonic convection in mantle convection models [e.g., Moresi and Solomatov, 1998] would not generate the extensional to compressional stress transition (i.e., neutral plane) in the Japan subducting lithosphere, further suggesting the importance of in-situ observational constraint on lithospheric rheology and dynamics of plate tectonics. 

How to cite: Zhong, S., Han, S., and Bellas, A.: Constraints of Flexural and Seismic Observations on Lithospheric Rheology at Plate Interior and Plate Boundary Settings, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11786, https://doi.org/10.5194/egusphere-egu23-11786, 2023.

11:20–11:30
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EGU23-8377
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TS1.3
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ECS
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On-site presentation
Giuseppe Volpe, Maria Eugenia Locchi, Giacomo Pozzi, Elisa Tinti, Marco Scuderi, Chris Marone, and Cristiano Collettini

After many large earthquakes aftershocks activity can reach depths greater than the base of the seismogenic zone that is defined by background seismic activity. This observation is generally explained by strain rate induced embrittlement associated with the increase of post-mainshock strain rate, which favors a transition from ductile to brittle behavior. However, the underlying physical processes are not well understood. Here we integrate geological and geophysical data for the 2016–2017 Central Italy seismic sequence with laboratory experiments to provide a geological and physical interpretation for the post-mainshock transient deepening of the base of the seismogenic zone.

The base of the seismogenic zone in the central-northern Apennines is set typically at 9-10 kilometers and corresponds to the top of the phyllitic basement. Structural studies on exhumed basement rocks highlight a heterogeneous basement fabric consisting of competent, 10 to 200 m wide, quartz-rich lenses surrounded by an interconnected and frictionally weak phyllosilicate-rich matrix. The matrix controls the overall rheology of the basement due to its interconnectivity, and promotes aseismic deformation because its rate-strengthening behavior.

Following each major (Mw > 5.5) event of the 2016–2017 sequence, a dramatic and abrupt increase in seismic rate is observed below 10 km, hence within the basement. Here we document the presence of seismicity clusters made of more than 4 earthquakes and characterized by small magnitude (Mw < 2.5), small dimensions (< 500 meters), small temporal duration (< 14 days) and a swarm-like behavior. Furthermore, these clusters are often represented by multiple or repeating earthquakes with a cross correlation coefficient higher than 0.7 for all the three components. These observations suggest that the increase of shear stressing rate within the basement is responsible for deepening of seismicity. To further explore this idea, we performed laboratory experiments on rocks from exhumed outcrops of basement rocks. We found that shear stressing rate promotes accelerated creep on the phyllosilicate-rich matrix and dynamic instabilities on the quartz-rich gouge belonging to the lenses.     

Our integrated analysis suggests that the mainshocks of the 2016-2017 seismic sequence promoted an increase of shear stressing rate within the basement allowing the phyllosilicate-rich matrix to creep faster hence favoring the loading and the repeated failures of locked seismogenic patches represented by the quartz-rich lenses.

How to cite: Volpe, G., Locchi, M. E., Pozzi, G., Tinti, E., Scuderi, M., Marone, C., and Collettini, C.: Structural and frictional control on the transient deepening of the seismogenic zone following major earthquakes in Central Italy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8377, https://doi.org/10.5194/egusphere-egu23-8377, 2023.

11:30–11:40
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EGU23-8980
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TS1.3
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ECS
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On-site presentation
Ajay Kumar, Mauro Cacace, and Magdalena Scheck-Wenderoth

We study the present-day thermo-mechanical state of the Alpine Himalayan Collision Zone to understand the physics controlling the observed crustal differentiation and the underlying continental-wide geodynamics. We found that the stability of the lithosphere is regulated by a thermodynamically controlled critical crustal thickness (Cr), which is close to the average thickness of the continental crust. Regions with thickness higher than Cr, representing orogenic lithosphere, and higher than average potential energy undergo weakening and dissipate the acquired internal energy, compared to their foreland lithospheres that have crustal thickness close to Cr. A weaker orogenic lithosphere deforming in a dissipative mode to release the acquired potential energy manifests as zones of diffused rather than focused deformation. We additionally find that the energy dissipation path that the orogenic lithosphere takes to either attain Cr (cratonization) or to undergo runaway instability (rifting) is modulated by the feedback between the thermal and mechanical relaxation of the lithosphere. The internal energy stored in the crust from heat-producing elements, fastens the dissipation of the acquired potential energy.

How to cite: Kumar, A., Cacace, M., and Scheck-Wenderoth, M.: Continental lithosphere deformation is a response to its thermodynamically controlled critical crustal thickness, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8980, https://doi.org/10.5194/egusphere-egu23-8980, 2023.

11:40–11:50
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EGU23-7497
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TS1.3
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On-site presentation
Yuval Tal and Lior Wise

Natural faults are rough at all scales and can be described with fractal geometry. This deviation from planarity results in geometric asperities and a heterogeneous stress field. Analytical and numerical studies have shown that roughness introduces additional shear resistance on the fault and promotes failure in the medium surrounding the fault because of the elevated stresses. These studies generally assume a small slip on the fault, i.e., much smaller than the minimum roughness wavelength, λmin. It is important to examine the effects of roughness on shear resistance and near-fault stresses at large sliding, as well as the assumptions incorporated in the derivation of the analytical solutions.

In this study, we examine the effects of fault geometry on the shear resistance and near-fault stresses at large sliding numerically, using a method that is based on the mortar finite element formulation, in which non-matching meshes are allowed across the fault, and the contacts are continuously updated. This enables modeling slip larger than λmin and the overriding of asperity contacts. We begin with simulations of an elastic medium and show that, indeed, the roughness results in large and heterogeneous stresses on and off the faults, which increase with the roughness level. However, except for small slip values, the increase of shear resistance with slip is much smaller than the linear increase predicted by the analytical models, which assume small and uniform slip. For self-similar geometry, with Hurst exponent of H = 1, the average shear resistance increases with slip at a decreasing rate. For self-affine geometry, with H < 1, it initially increases with slip, then decreases at a slip larger than λmin /2. Although overriding of asperities is allowed in the simulations, as slip increases, significant stress concentrations are developed on the fault, which may not be realistic for natural rock surfaces. To account for that, we implement wear laws into the method and model the evolution of stresses during a quasistatic slip and cycles of dynamic earthquakes. The wear process redistributes and bounds the stresses on the fault and allows a more realistic characterization of stress distribution near the fault.

How to cite: Tal, Y. and Wise, L.: Shear resistance and near-field stresses on rough faults, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7497, https://doi.org/10.5194/egusphere-egu23-7497, 2023.

11:50–12:00
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EGU23-10150
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TS1.3
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ECS
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On-site presentation
Steffen Ahlers, Karsten Reiter, Tobias Hergert, Andreas Henk, Luisa Röckel, Sophia Morawietz, Oliver Heidbach, Moritz Ziegler, and Birgit Müller

For the safe usage of the subsurface the stress state is of great importance, e.g., for borehole stability, mitigation of induced seismicity or the search and long-term safety of a high-level nuclear waste deposit. However, the state of knowledge concerning the stress state in Germany is limited as only unevenly distributed stress measurements are available which frequently provide only one component of the stress tensor. The SpannEnD (Spannungsmodell Endlagerung Deutschland) project aims to improve this knowledge with the help of a 3D geomechanical-numerical model. The model is calibrated on available stress magnitudes and enables a continuum-mechanics based prediction of the stress state and its local variability for Germany.

The 3D geomechanical-numerical model comprises the upper lithosphere and contains 22 lithological units parametrized with individual mechanical properties (Young’s modulus and Poisson’s ratio) and densities. Linear elasticity is assumed and the finite element method (FEM) is used to solve the equilibrium of forces. Overall, the model contains about 11 million hexahedral elements resulting in a lateral resolution of 2.5 x 2.5 km2 and a vertical resolution of up to 250 m. The model is calibrated by adaptation of displacement boundary conditions with magnitudes of the minimum (Shmin) and maximum horizontal stresses (SHmax). The model results show an overall good fit with these stress magnitudes used for calibration indicated by a mean of the absolute stress differences of 4.6 MPa for Shmin and 6.4 MPa for SHmax. Furthermore, the results agree well with additional data sets excluded from calibration but used for validation, e.g., with a mean of the absolute stress differences of 1.1 MPa for vertical stress magnitudes and an absolute mean deviation of the orientation of SHmax with regard to World Stress Map data of 11.9°.

How to cite: Ahlers, S., Reiter, K., Hergert, T., Henk, A., Röckel, L., Morawietz, S., Heidbach, O., Ziegler, M., and Müller, B.: SpannEnD – Prediction of the recent crustal stress state of Germany using a 3D geomechnical-numerical model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10150, https://doi.org/10.5194/egusphere-egu23-10150, 2023.

12:00–12:10
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EGU23-7038
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TS1.3
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ECS
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On-site presentation
Saeed Mahmoodpour, Michael Drews, and Florian Duschl

The North Alpine Thrust Front (NATF) is an example of the classical onshore fold-and-thrust belt and foreland system [1]. There are ongoing heat production projects in this area. However, complex compaction and stress fields require detailed investigation for safe and economical drilling activities. Previous field investigation of the wedge and foredeep shed light on the possible driving mechanisms for overpressure generation in the wedge, foredeep and footwall in the SE of Germany. To do this, 20 deep wellbores are selected in this area and their geophysical and drilling data are investigated [2]. This study is a complementary work to find possible explanations for observations through numerical modeling. Examining the mechanics behind these complex deformations is beyond the capabilities of the critical taper theory. However, a large strain geomechanical numerical simulator coupled with critical state soil mechanics constitutive model can provide useful insights in this regard. Geomechanical forward modeling requires boundary conditions at far distances. Also, except some basic geometrical features, other deformations are not predefined and they are developing during the simulation. Therefore, it is not only insightful regarding the final shape of the system, but also progressive development of the deformations is trackable [3].  A plane-strain framework is implemented to simulate the interested processes through the Elfen software [4]. A quasistatic criterion is assumed throughout the simulation to decrease the possible boundary effects of the loading. Adaptive-remeshing helps to capture the large-strain behavior of the system in a reasonable computational time. Data from different sources of the drilling, geophysical tools and field observation is used to tune the model and test the capability of the model to estimate the required properties. Numerical simulations result in a similar geometry which is observed in the field works. Obtained stress values and pore pressure are comparable to the field data.  The differences between the simulation results and field observations can be attributed to the assumptions which were made during the simulation. For example, thermal impacts and possible diagenetic processes are neglected during the simulation. Also, a homogeneous material is assumed for the different layers, while in the real case, there are heterogeneities inside the layers.

1. Pfiffner, O, A. (1986) “Evolution of the North alpine Foreland Basinn in the central Alps”, Foreland Basins, 219-228.

2. Drews, M., Duschl, F. (2022) “Overpessure, vertical stress, compaction and horizontal loading along the North Alpine Thrust Front, SE Germany”, Marine and Petroleum Geology, 143, 105806.

3. Albertz, M., Sanz, P, F. (2012) “Critical state finite element models of contractional fault-related folding: Part 2. Mechanical analysis”, Tectonophysics, 150-170

4. Rockfield (2017) “Elfen explicit manual (Version 4.10)”, Swansea, UK, Rockfield Software.

How to cite: Mahmoodpour, S., Drews, M., and Duschl, F.: Geomechanical forward modeling of the stress field, pore pressure and compaction in the North Alpine Thrust Front, SE Germany, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7038, https://doi.org/10.5194/egusphere-egu23-7038, 2023.

12:10–12:20
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EGU23-1507
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TS1.3
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ECS
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Virtual presentation
Rasha Amer, Ziqiu Xue, and Tsutomu Hashimoto

With the growing need for CO2 storage, risk management is essential to secure the storage sites; these risks include fault reactivation, ground surface deformation/sea-bottom (uplift), well and caprock integrity, and CO2 leakage; managing these risks could be achieved by understanding the hydromechanical behaviors of rock induced by the reservoir pressure build-up caused by CO2 injection. However, this remains a crucial challenge because the rock's mechanical and hydraulic properties are poorly constrained. Moreover, the conventional monitoring methods usually consider CO2 plume migration only, which is not enough to understand the induced pressure front that occurs far beyond the real pressure plume. Although several techniques could image the geomechanical deformation and investigate the surface deformation well, these monitoring methods do not provide a complete image regarding the deformation migration from the subsurface to the surface due to the limited measurement points in addition to the cost issue.

 In this paper, we will introduce Rayleigh scattering-based Distributed Optical Fiber Strain Sensing (DFOSS) as an effective tool for subsurface and surface geomechanical monitoring to track the dynamic responses at each spatial location along the cable due to the deformation caused by injection; this technology could overcome other conventional methods' limitations including continual spatiotemporal measurements, cost-effective installation: vertically along the wellbore and horizontally into the ground surface, covered area and sea bottom. We will review several laboratory and field experiments from our previous studies. First, we will show the laboratory results from the first laboratory test to track the movement of the CO2 plume as it enters the clay-rich critical regions in the reservoir–caprock system using DFOSS and monitoring of the injected water in a sandstone sample using DFOSS in the second test. Both results demonstrated that DFOSS could provide high-resolution information on deformation and fluid activity. Next, we will show our subsurface monitoring field results, where we conducted several water injection tests in a shallow well. We monitored the injection process by installing DFOSS in a monitoring well. Our outcome confirmed that DFOSS could provide critical information for rocks' properties and fluid migrations by geomechanical monitoring, and it could be a real-time and permanent monitoring tool for wellbore, caprock integrity, and CO2 leakage. Finally, we will show the surface deformation monitoring results, where we installed the fiber cable into the surface horizontally in a shallow trench; the airbag inflation and deflation tests were conducted under the fiber cable to simulate uplift and subsidence caused by the fluid injection and production in the subsurface. The results suggested that DFOSS could locate any anomaly along the cable.

 Our results demonstrate that installation of DFOSS in fiber cables horizontally into the surface around the injection site and vertically in a well to incorporate well-based strain sensing with surface monitoring, allowing geomechanical monitoring (horizontally into the surface and vertically in the subsurface) in three dimensions via a cost-effective, real-time and permanent monitoring system.

How to cite: Amer, R., Xue, Z., and Hashimoto, T.: Geomechanical Monitoring at CO2 storage sites with Distributed Fiber Optic Strain Sensing: Insights from laboratory and Field experiments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1507, https://doi.org/10.5194/egusphere-egu23-1507, 2023.

12:20–12:30
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EGU23-15990
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TS1.3
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ECS
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On-site presentation
Thomas Niederhuber, Martina Rische, Thomas Röckel, Birgit Müller, and Frank Schilling

The Ruhr region is characterized by centuries of coal mining at depths reaching more than 1000 meters. After the closure of the last mines, their controlled flooding started. The Floodrisk project investigates ground uplift, stress changes due to pore pressure changes and the reactivation potential of faults to explain induced seismicity. We focused on monitoring the eastern Ruhr area and are investigating in detail the relationship between mine water rise, tectonic stress and induced seismicity in the Haus Aden drainage area.

In the region of the former "Bergwerk Ost", which had the highest seismicity in the Ruhr area during active mining, the RUB has installed a network of up to 30 short-period seismic stations. Continuous monitoring of seismicity and mine water levels is available for this region from the active mining phase through the post-mining phase to flooding. The temporal evolution of the mine water level after the pumps were shut down in mid-2019 shows a strong correlation with the temporal evolution of the observed microseismicity. Over 2200 induced events have been located since the beginning of flooding, showing spatial clustering. A comparison of the mine galleries, which today serve as the main underground waterways, with the localizations of the events shows that most of the events occur about 300 m below the main pillars located between the longwall panels.

This study provides a compilation of the regional stress state in the eastern Ruhr area based on the mine measurements, which were re-evaluated to derive the regional stress component and compared with stress orientations from independent sources (information on stresses in deep boreholes and earthquake focal mechanisms). The spatial distribution of stress orientations in the Ruhr region shows a rather homogeneous stress pattern with only very few locations where stress orientations differ significantly from the average.

Based on the geometry of the pillars, shafts and longwall panels, a generic numerical FE-model was developed using the compiled stress data for model calibration. The results indicate increased vertical stresses within and below the pillars as a result of stress arching. The horizontal stress changes are minor, thus differential stress increases in the vicinity of the event localizations.

How to cite: Niederhuber, T., Rische, M., Röckel, T., Müller, B., and Schilling, F.: Flooding Induced Seismicity in the Ruhr Area – a geomechanics numerical modelling approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15990, https://doi.org/10.5194/egusphere-egu23-15990, 2023.

Posters on site: Mon, 24 Apr, 16:15–18:00 | Hall X2

Chairpersons: Gian Maria Bocchini, Karsten Reiter
X2.193
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EGU23-204
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TS1.3
Keisuke Ariyoshi

Natural faults host various types of migrating slow earthquake phenomena, with migration speeds much lower than seismic wave speeds and different moment-duration scaling from regular earthquakes. To advance the obtained quantitative understanding of the migration process and long duration of slow earthquakes,  I study a chain reaction model in a population of brittle asperities based on a rate- and state-dependent friction on a 3-D subduction plate boundary. Simulation results show that the migration speed is quantitatively related to frictional properties by an analytical relation derived here. By assuming that local pore water in front of the migration drives rapid tremor reversal and is so local as to hold a constant stress drop, the application of the analytical solution to observational results suggests that (i) the temporal changes of observed migration speeds for the rapid tremor reversal could be explained by about 70% reduction of the effective normal stress; (ii) effective normal stress for the deeper extension of seismogenic segment in the western part of Shikoku is about 1.5 times greater than that in the central part. Applying rupture time delay between slow earthquake asperities for a duration longer than the regular earthquake, I also conclude that (iii) the characteristic slip distance of rate-and-state friction for low-frequency earthquakes is roughly between 30 μm and 30 mm; (iv) the stress and strength drops of very low-frequency earthquakes is much smaller than 1 MPa.

References:

Ariyoshi, K. (2022). Extension of aseismic slip propagation theory to slow earthquake migrationJournal of Geophysical Research: Solid Earth127, e2021JB023800. https://doi.org/10.1029/2021JB023800

How to cite: Ariyoshi, K.: Physical interpretation of slow earthquake migration process based on a friction law, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-204, https://doi.org/10.5194/egusphere-egu23-204, 2023.

X2.194
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EGU23-3401
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TS1.3
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ECS
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Gian Maria Bocchini, Armin Dielforder, Kilian B. Kemna, Rebecca M. Harrington, Andrea Hampel, and Onno Oncken

Stress in active subduction forearcs is controlled primarily by friction along the megathrust and the gravitational force. The competing deviatoric compressive and tensional stresses generated by megathrust friction and gravity, respectively, are of the same order of magnitude and result in very low deviatoric stress in the forearc. The near neutral stress state in subduction forearcs is supported by observations of stress reversal, that is a change from deviatoric compression to deviatoric tension, caused by very small megathrust shear stress drops (<10MPa) after recent large megathrust earthquakes. However, studies that quantify and compare the stress state in subduction forearcs at various stages of the seismic cycle are still limited. Here, we use two-dimensional finite-element force-balance models to quantify and constrain forearc stresses at different locations along the Chilean and Japanese subduction margins that are at different stages of the seismic cycle. The models consider forearc topography, slab geometry, crustal thickness, and water load to quantify the elastic stress in the forearc due to gravity and friction along the megathrust. The models indicate low deviatoric stress values (10s of MPa) in the subduction forearcs, which imply a weak forearc crust in areas of active seismic deformation. We validate the model results by estimating seismic stress drop values of forearc earthquakes from high-quality seismic waveform recordings. We estimate spectral corner frequency using both single-spectrum and spectral-ratio estimates and depth-dependent shear-wave velocity models. Spectral-ratio estimates provide more robust corner-frequency estimates that we employ to validate and interpret the increasing stress drop trend down to depths of ~50-60 km. The slight depth dependency of seismic stress drop values is consistent with depth dependency of deviatoric stress obtained from the finite-element models. Moreover, we find that average seismic stress drop values are systematically lower or similar to maximum deviatoric stress obtained from our models, which is consistent with a partial stress release during earthquakes in the forearc. Our results suggest a relation between seismic stress drop values and ambient stress in subduction forearcs.

How to cite: Bocchini, G. M., Dielforder, A., Kemna, K. B., Harrington, R. M., Hampel, A., and Oncken, O.: Ambient Stress in Subduction Forearcs Constrained by Numerical Models and Earthquake Static Stress Drop Values, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3401, https://doi.org/10.5194/egusphere-egu23-3401, 2023.

X2.195
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EGU23-6297
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TS1.3
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ECS
Renato Gutierrez Escobar, Candela Garcia Sancho, and Rob Govers

We aim to quantify the likely ranges of magnitudes and directions of forces that may explain present-day natural stresses within the Eurasian plate. We first focus on one of the driving forces, horizontal gravitational stresses (HGSs) resulting from lateral variations in gravitational potential energy, which 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 show that different published models of lithospheric density including lateral variations in the lithosphere-asthenosphere boundary result in significantly different HGSs. Other driving forces are mantle convective tractions including dynamic topography, and plate interaction tractions with bounding plates. Second, we include observed major faults into a 2D spherical cap elastic model of the Eurasian plate. We show results of forward FEM calculations based on the best model parameters of Warners et al. (2013) and compare them with observed stress directions. We propose different objective functions that quantify the (relative contributions to 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 study represents a stepping stone towards a Bayesian inference workflow to constrain the dynamics of the Eurasian plate of which we show preliminary results.

Warners-Ruckstuhl, K. N., R. Govers, and R. Wortel, 2013, Tethyan collision forces and the stress field of the Eurasian Plate: Geophys. J. Int., v. 195, no. 1, p. 1–15, doi:10.1093/gji/ggt219.

How to cite: Gutierrez Escobar, R., Garcia Sancho, C., and Govers, R.: New steps toward estimating the driving and resistive forces on the Eurasian plate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6297, https://doi.org/10.5194/egusphere-egu23-6297, 2023.

X2.196
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EGU23-3062
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TS1.3
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ECS
Dyuti Prakash Sarkar and Takehiro Hirose

The present mean convergence rate of Himalaya is ~ 15 mm/year. In comparison to the convergence and stress accumulation, only few stress release events represented by greater than M5+magnitude earthquakes in the Himalayan region have been observed. Understanding the constraints leading to the disparity in stress accumulation and stress release, is crucial to understand the stress accommodation mechanism and seismicity in the Himalayas. The current active subduction boundary is marked by Main Frontal Thrust separating the sub-Himalayas and the Gangetic alluvial plains. The rock types within the Main Frontal Thrust sheet show two primary types of sandstone protoliths, and gouges exhibiting cataclastic to foliated microstructural features. In this study, we have performed rotary-shear velocity step experiments on the powdered samples of the sandstone within the Main Frontal Thrust to determine their frictional properties at slow (creep) to fast (seismic) velocity under 10 MPa effective normal stress condition.  We discuss these results and their implications on seismic nucleation in Himalayan Main Frontal Thrust.

How to cite: Sarkar, D. P. and Hirose, T.: Frictional properties of sandstone gouges within Himalayan Main Frontal thrust: constraints on seismicity of shallow crustal deformation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3062, https://doi.org/10.5194/egusphere-egu23-3062, 2023.

X2.197
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EGU23-14361
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TS1.3
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ECS
Olivia Lozano Blanco, Björn Lund, Puy Ayarza, Joaquina Álvarez-Marrón, Dennis Brown, and Yih-Min Wu

The active Taiwan mountain belt is located in a complex geodynamic setting that involves two subduction processes. To the northeast, the Philippine Sea Plate subducts northward beneath the Eurasian Plate at the Ryukyu Trench, while in the southwestern part, the Eurasian Plate subducts eastward under the Philippine Sea Plate, where it obliquely collides with the Luzon Volcanic Arc. The Taiwan thrust-and-fold belt is created as a result of this ongoing arc-continent collision. Regardless of the predominance of compression in the overall structure of the island, several studies have also reported normal faulting. This study aims to estimate the local and regional stress field using earthquake focal mechanism data to contribute to a better understanding of crustal deformation in the complex tectonic setting of Taiwan.

Manually clustered earthquake focal mechanisms are inverted to obtain an estimate of the principal stress (σ1, σ2, σ3) orientations and the stress ratio (σ12)/(σ13), from which the direction of the maximum horizontal stress (SH) is calculated. The initial data set contains 11,587 earthquake focal mechanisms compiled from several sources dating between 1990 and 2020. All deep earthquakes in the Ryukyu subduction zone were removed from the data set. The Chi-Chi 1999 and other major earthquakes and aftershocks were also removed as they may reflect a distorted stress field. After preprocessing, a database consisting of 8,510 events with focal depths between 1-144 km and magnitudes ML=0.7-5.9 was used in the inversion. Depth division was performed in a regular 7 km grid up to 28 km depth, all events deeper than 28 km being considered in the same layer.

Preliminary results show that, to the southwest, the notable clockwise rotation of SH from SW-NE to a W-E direction and a change in the fault type from strike-slip to reverse to the east coincides with the interaction between the ENE-striking reactivated inherited structures of the Eurasian continental margin and the NNE-striking thrust faults of the foreland thrust-and-fold belt. To the centre-east, results show normal faulting in the upper crust, which changes to reverse faulting with depth, suggesting that there is a stress transition at approximately 14 km. Beneath that depth, there is a general state of compression. Ongoing research aims at integrating these results with those of numerical modelling and with field data in an effort to understand the locus of deformation and the occurrence of extensional tectonics in compressional settings, here and in other mountains belts worldwide.

This research is part of project PGC2018-094227-B-I00 funded by the Spanish Research Agency of the Ministry of Science and Innovation of Spain. Olivia Lozano acknowledges funding from the same agency through contract PRE2019-091431. Funding from SERA European Union H2020 INFRAIA-2016-2017 Agreement, 170522 is also acknowledged.

 

How to cite: Lozano Blanco, O., Lund, B., Ayarza, P., Álvarez-Marrón, J., Brown, D., and Wu, Y.-M.: Evidence of Synorogenic Extension in the Upper-Middle Crust in Central Taiwan , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14361, https://doi.org/10.5194/egusphere-egu23-14361, 2023.

X2.198
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EGU23-5060
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TS1.3
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ECS
|
Fernando Linsalata and Giorgio Spada

We explore the strength of the lithosphere beneath the Graham Land region (Antarctic Peninsula) using numerical modeling which simulate lithospheric deformation as a function of geological and geophysical parameters. First, we process 21 GNSS time series data spanning 1997–2022 provided by the Nevada Geodetic Laboratory, to produce a robust tectonic velocity solution and calculate a new geodetic strain rate model using an optimal mesh grid definition of 0.5 x 0.5 degrees that best fits our study area. Second, we combine our new geodetic strain rate model with the Moho depth and rheological parameters such as geothermal heat flow (GHF), heat productions and thermal conductivity previously published in the literatures to determine yield strength envelope (YSE) along three profiles (A, B and C respectively) beneath Graham Land. The lithospheric strength values are in a range from 0 to 500 MPa and depend more on strain-rates at the surface and thermal regime (GHF) than on crustal thickness. The highest values for the crust (500 MPa) are mostly concentrated in the profile A, near Cape Alexander, where the second invariant of the strain rate present the smaller value (5-15 μstrain/yr) and the principal strain rates are compressive approximately in the N-S directions. In contrast, the highest values for the mantle mainly depend on the thermal structure of the lithosphere and Moho depth and the highest values are concentrated in the profiles B (297 MPa) and C (279 MPa), in the Trinity Peninsula. Here, the second invariant of the strain rates, present the larger value (50-80 μstrain/yr) and the principal strain rates are extensive in the W-E directions, with a maximum value of 30 μstrain/yr. The results of our study demonstrate that both “jelly sandwich” and “crème brûlée” models are valid for the Graham Land lithosphere, depending on specific thermal and rheological conditions considered.

How to cite: Linsalata, F. and Spada, G.: Strength of the lithosphere derived by geological and geophysics data: the Graham Land (Antarctic Peninsula) case study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5060, https://doi.org/10.5194/egusphere-egu23-5060, 2023.

X2.199
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EGU23-13955
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TS1.3
Sofie Gradmann, Marie Keiding, Odleiv Olesen, and Yuriy Maystrenko

The Nordland area in NW Norway is one of the tectonically most active areas in Fennoscandia. It exhibits patterns of extension, which are in contradiction to the first-order regional stress pattern that reflects compression from ridge-push. The regional stress field stems from the interaction of ridge push and GIA (glacial isostatic adjustment); the local stress field mainly results from gravitational stresses, as well as the flexural effects of sediment erosion and re-deposition.

We develop 3D finite element numerical models of crustal scale, using existing geometric constraints from previous geophysical studies. Internal body forces, induced by variations in density, topography or Moho depth, already yield significant deviatoric stresses, which are often omitted in stress models. We show that these can strongly influence the near-surface stress regime, in particular for the continental-margin setting we are considering. Similarly, existing weakness zones (such as faults) control the local stress field.

We apply the far-field stress fields (GIA, ridge-push, sediment redistribution) as effective force boundary conditions to the sides or base of the model. This way, we can account for all stress sources at once, but can also vary them separately in order to examine their relative contributions to the observed stress and strain rate fields.

We compare our models to the stress and strain observations derived from different recent seismological and geodetic data sets. These point to a correlation of seismicity with major changes in the crustal geometry.

How to cite: Gradmann, S., Keiding, M., Olesen, O., and Maystrenko, Y.: The 3D stress field of Nordland, northern Norway - insights from numerical modelling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13955, https://doi.org/10.5194/egusphere-egu23-13955, 2023.

X2.200
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EGU23-2823
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TS1.3
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ECS
Natalia Nevskaya, Weijia Zhan, Holger Stünitz, Alfons Berger, and Marco Herwegh

According to well-established hypotheses based on field observations of natural faults, viscous deformation may localize following pre-existing brittle fractures. The weak behaviour can be explained by brittle grain size reduction and phase mixing, which may activate grain size sensitive processes in the viscous field. To prove this hypothesis, it is necessary to perform experiments to observe the strain and stress evolution in faulted and non-faulted rocks. Pec et al. (2012) performed experiments on granitic rocks by shearing manually crushed granitic powder between coarse solid granitic forcing blocks. However, in their study, there are unavoidable boundary conditions between the forcing blocks and the gouge, and a comparison to an intact rock without fracture is difficult.

In our study, we reduce the boundary conditions to a minimum and can directly compare the stresses and microstructural evolution during deformation of intact and fractured granitic ultramylonites at 650°C, confining pressure of 1.2GPa, and a constant displacement rate of 10-8m/s. We perform these experiments on initially solid cylindrical samples in two experimental sets: In set A, we slowly apply the load and confining pressure, to ensure an intact rock sample is deformed. In set B, we create fractures before the experiment starts but already in the closed system of the experimental setup. Once experimental P/T conditions are reached, both experimental sets are deformed to different finite strains to investigate the associated microstructural evolution. The deformation is disseminated in the set A experiments, but localizes strongly along the fracture in experimental set B. The strain is accommodated by viscous granular flow incorporating an impressive grain size reduction of up to 1000x and dissolution/precipitation processes. In addition, the stress records show that in experiments A, initially a 30% higher yield stress has to be overcome before steady state flow, while in set B steady state flow is reached directly without a strain softening increment. In both sets, steady state stresses range around 300MPa, i.e. far below the confining pressure.

Applying microstructural observations and mechanical data of our experiments to deformation of granitoid crust in nature reveals that fractures serve to reach mechanical steady state earlier compared to non-fractured crust. As a matter of strain, however, both settings may yield at the same mechanical strengths of resulting shear zones. It is important to note that polymineralic fine-grained ultramylonites are up to four times weaker than monomineralic quartz, presenting an important behaviour of efficient strain localization and rheological properties substantially below those of the end member minerals.

 

Pec, M., Stünitz, H. and Heilbronner, R., 2012. Semi-brittle deformation of granitoid gouges in shear experiments at elevated pressures and temperatures. Journal of Structural Geology, vol. 38, pp. 200-221. https://doi.org/10.1016/j.jsg.2011.09.001

How to cite: Nevskaya, N., Zhan, W., Stünitz, H., Berger, A., and Herwegh, M.: Experimental evidence for viscous deformation and strain localization in fractured granitoid rocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2823, https://doi.org/10.5194/egusphere-egu23-2823, 2023.

X2.201
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EGU23-6615
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TS1.3
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ECS
|
Weijia Zhan, Natalia Nevskaya, André Niemeijer, Alfons Berger, Chris Spiers, and Marco Herwegh

Fault gouges of granitoid composition represent the principal non-cohesive tectonites within fault zones in the continental crust. The spatial distribution and strength of granitoid fault gouges is therefore crucial for understanding how weak the upper continental crust could be due to the formation of fault zones. Although several laboratory investigations reported the mechanical weakening of granitoid gouges in shear experiments, the deformation mechanism responsible for such behavior remains not well understood.

To address this issue, we conducted two series of shear experiments on granitoid gouges by using a ring shear apparatus. The starting gouge powders were derived from crushed granitoid mylonite with a median grain size of 45 μm. In a first set of experiments, gouges were sheared at a sliding velocity of 100 μm/s for a displacement of 15 mm. Temperatures explored ranged from 20°C to 650°C in order to determine the temperature dependence of gouge strength. The second set of experiments is identical to the first ones, except that the applied sliding velocity was set at 1 μm/s to study how fault slip rate influences the strength of gouges.

We observe that differences in gouges strengths as a function of sliding velocity and temperature: At a sliding velocity of 100 μm/s, the steady-state shear stress (τ) remains relatively constant at τ=76-82 MPa over the entire temperature range. Contrastingly, at a sliding velocity of 1 μm/s the steady-state shear stress remains temperature-insensitive with τ≈75 MPa up to tempertures of 450°C, but decreases then to τ≈50 MPa at 650°C (Fig.1 a). Furthermore, the amount of decrease of shear stress is strain dependent (Fig.1 b). At even slower sliding velocity of 0.1 µm/s, the shear stresses decrease further to τ≈38 MPa.

Microstructurally, all gouges deformed at T≦450°C show typical cataclastic features, where angular clasts with grain size of ~10 μm are surrounded by a fine-grained matrix. Intergranular fracture arrays in Riedel- and Y-shears are well developed over the entire cross section, indicating homogeneous bulk deformation. In contrast, gouges sheared at 650°C with τ≈50 MPa show strain localization in a principal slip zone. It is shear plane parallel with widths up to ~50 µm. Inside the principal slip zone, all grains are dramaticly reduced to nm-size and tightly packed. No intergranular fracture arrays are observed. Outside the principal slip zone, rounded grains with size of ~5 μm are loosely packed, with meniscus cement growing in between. The aforementioned strain localization is enhanced at temperature above 450°C and slip rate below 1μm/s, suggesting that viscous creep mechanisms (e.g. pressure solution) control the deformation process at slow sliding velocities, which is not the case in fast rate experiments. Our results show that the activation of viscous creep mechanisms leads to significant fault zone weakening, while contrasts in grain size keep deformation localized.

Figure 1 Shear stress plotted as a function of temperature. Shear stress data collected at (a) 15mm displacement in steady-state, and at (b) 5mm displacement.

How to cite: Zhan, W., Nevskaya, N., Niemeijer, A., Berger, A., Spiers, C., and Herwegh, M.: Weakening of granitoid gouge in hydrothermal ring shear experiments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6615, https://doi.org/10.5194/egusphere-egu23-6615, 2023.

X2.202
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EGU23-1637
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TS1.3
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ECS
Jose Bastias, Gabriel Rau, and Philipp Blum

Earth tides exert small gravitational variations in the subsurface which lead to pore pressure changes and water level fluctuations in groundwater monitoring wells. This groundwater response to Earth tides has been used to estimate subsurface hydraulic and geomechanical properties. However, existing approaches are based on simplifying assumptions and their reliability has not been tested for realistic conditions. To simulate how Earth tides affect the subsurface, we developed and verified a numerical model that couples hydraulic and geomechanical theories. We modelled the response of a semi-confined aquifer which exchange water with an observation well for the dominant M2 Earth tide component. We reveal that undrained (i.e., groundwater does not flow in response to stress) and confined (i.e., groundwater is under pressure) conditions are necessary for the analytical solution to be valid. For the M2 frequency we assess that this occurs at depths ≤ 50 m and requires specific storage at constant strain sε ≥ 10-6 m-1, hydraulic conductivity of the aquitard kl ≤ 5 • 10-5 ms-1 and aquifer kl ≥ 1 • 10-4 ms-1, respectively. Further, we illustrate that established analytical solutions are valid in unconsolidated systems, whereas consolidated systems require additional consideration of the compressibility ratio between the porous medium and the porous skeleton (i.e., inclusion of the Biot coefficient). Overall, we find that a priori knowledge of the subsurface system increases the reliability of the groundwater response interpretation. Our results improve understanding of the effect of Earth tides on groundwater systems and provide a framework for evaluating subsurface properties.

How to cite: Bastias, J., Rau, G., and Blum, P.: Numerical simulation provides conditions for interpreting the groundwater response to Earth tides, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1637, https://doi.org/10.5194/egusphere-egu23-1637, 2023.

X2.203
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EGU23-4865
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TS1.3
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ECS
Minzy Kang and Chandong Chang

Borehole breakouts, rock compressive failure at the wellbore wall, are one of the most widely utilized stress indicators, providing useful site-scale in situ stress states for a variety of geo-engineering projects. A 1 km deep vertical borehole drilled to study earthquakes in southeast Korea showed borehole breakouts rotated in azimuth at several depths by as much as 35° from the average azimuth, enlarging uncertainty in representative stress orientation. These breakouts developed in a highly fractured tuffaceous rock at a depth range from 840 m to 1000 m and breakout rotation always occurred adjacent to fractures and faults. While breakout rotation adjacent to fractures/faults has often been observed previously, there are several issues that have to be addressed regarding such a rotation, that is, would it be a local perturbation associated with drilling that can be ignored when assessing representative in situ stress states?; what aspect of fracture perturbs the stress indicator? To address these questions, we carried out a series of 3D finite element modeling, in which the rock mass consists of a single competent rock type (metamorphosed tuff) with a thin and soft planar fracture crossing the model. A borehole penetrates the center of the model vertically. The fracture orientation was varied from model to model for a given far-field boundary stress condition. The model results show that the rotation of breakouts increases generally (but with wide scattering) with an increase in slip tendency of the fracture. A more detailed analysis shows that the azimuthal rotation of breakouts tends to increase in a clearer manner with an increase in the horizontal shear displacement (or shear strain) component along fracture having relatively high slip tendency. For the reasonable values of mechanical properties assumed in the model, the breakout rotation can be as high as ~34° from the boundary stress orientation imposed in the model. Such stress rotation occurs throughout the extent of the fracture and is reflected in breakout rotation. The model results are quite comparable to the breakout rotations observed in the borehole. Our study suggests that breakout rotation is not just a local feature around the borehole but reflects a site-scale stress rotation associated with the presence of fractures having specific orientations and slip direction.

How to cite: Kang, M. and Chang, C.: Rotation of borehole breakouts by the effect of fractures/faults: observation and numerical model study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4865, https://doi.org/10.5194/egusphere-egu23-4865, 2023.

X2.204
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EGU23-5935
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TS1.3
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ECS
|
Sophia Morawietz, Moritz Ziegler, Karsten Reiter, Oliver Heidbach, Inga Moeck, Ingmar Budach, Hartwig von Hartmann, and Jennifer Ziesch

The stress field of Earth's upper crust is crucial for geodynamic processes and of key importance in planning and managing the utilization of the subsurface, such as geothermal energy extraction, stimulation of enhanced geothermal systems, or safety assessment of deep geological repositories. The contemporary 3-D stress state also provides the basis to assess the impact of induced stress changes which can lead to the reactivation of pre-existing faults, the generation of new fractures, or subsidence due to long-term depletion.

However, information on the stress state of Earth's crust is sparse and often not available for the areas of interest. So far, the stress tensor orientations and stress regimes have been systematically compiled and provided by the World Stress Map (WSM) project in a public-domain database. Yet, the acquisition of stress tensor orientations is not necessarily accompanied by the determination of the stress magnitudes, which, however, are required when investigating questions related to stability and hazard mitigation strategies of georeservoirs. To estimate the 3-D stress state, geomechanical-numerical modelling is applied. For the calibration of such models, stress magnitude data are essential. A major challenge is to bridge the scale gap between the widely scattered data that is required for model calibration and the high-resolution small-scale geological model in the target area. Ziegler et al. (2016) presented a multistage approach to resolve this challenge. For this, two successively calibrated models are created – one large-scale model with coarse resolution but available stress magnitude data for calibration, and one local model with fine resolution, e.g., based on a 3-D seismic survey of the target area, but without any stress data. Synthetic data obtained through the large-scale model is used to calibrate the small-scale local model.

First, we validated the multistage approach by means of generic models to rigorously quantify the associated introduced uncertainties. For this purpose, we implemented a highly simplified model lithology with only vertical stratification and no lateral changes. In a second step, we applied the multistage approach in a real-world setting and demonstrated the applicability on a local model of Unterhaching, south of Munich/Germany, where a geothermal district heating plant is located. Here, a local high-resolution model based on a 3-D seismic survey (Budach et al., 2018) has been successfully calibrated on a regional-scale stress model of the Bavarian Molasse Basin. The results of the local-scale model agree with the large-scale model. At the same time, stress change due to rock property variability, only resolved in the local-scale model, is shown.

How to cite: Morawietz, S., Ziegler, M., Reiter, K., Heidbach, O., Moeck, I., Budach, I., von Hartmann, H., and Ziesch, J.: A stress field model for the Unterhaching geothermal plant: Challenges and solutions in local model calibration, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5935, https://doi.org/10.5194/egusphere-egu23-5935, 2023.

X2.205
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EGU23-12847
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TS1.3
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ECS
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Luisa Röckel, Steffen Ahlers, Sophia Morawietz, Birgit Müller, Karsten Reiter, Oliver Heidbach, Tobias Hergert, Moritz Ziegler, Andreas Henk, and Frank Schilling

For many underground operations such as geothermal energy exploitation, mining, oil and gas production or the storage of high-level radioactive waste, active tectonic or induced seismicity is of concern. Seismicity usually occurs on pre-existing faults that are reactivated under adequate stress conditions. Thus, an assessment of the reactivation potential of faults can aid in the identification of areas particularly prone to the occurrence of seismic events or such areas where adequate geotechnical measures have to be taken to avoid anthropogenic fault reactivation. A tool for the assessment of the fault reactivation potential is the so called slip tendency, which is the ratio between the maximum resolved shear stress on the fault plane and the normal stress. Such an analysis requires information about the stress field acting on the fault plane and information about the fault geometry, fault orientation and frictional properties. Information about these parameters can be very limited, since 3D fault geometries are often only extrapolated from geological surface data. Furthermore, stress data is usually sparse, only available pointwise and unevenly spatially distributed. Geomechanical-numerical modelling can be used to derive a spatially comprehensive description of all six independent components of the stress tensor from the available stress data.   

For Germany, an estimate of the stress tensor is provided by the geomechanical-numerical model by Ahlers et al. (2022). Furthermore, fault geometries as part of geological models of the German federal states are available for large parts of Germany. We use both the stress data derived from the geomechanical-numerical model and the fault geometry data from the federal state models to calculate slip tendencies for more than 10.000 faults and fault segments. The resulting slip tendency is generally the highest in the northern Upper Rhine Graben area where it routinely reaches values of 0.7 and more. In the Alpine and Alpine Foreland region the slip tendency is generally the lowest with values only very rarely exceeding 0.3. In North Germany slip tendency values range mainly between 0.3 and 0.6 but with both higher and lower values being fairly common. In general, faults striking in NNE-SSW direction and NW-SE direction display the overall highest slip tendencies whereas faults striking in ENE-WSW direction show very low slip tendencies. With increasing depth slip tendencies generally decrease strongly. However, there are still major areas in Germany where either no fault geometries or only insufficient fault geometries are available. Furthermore, pore pressure has a major influence on the slip tendency. For our calculations, we assume hydrostatic pore pressure. While overpressured pore fluid is documented for example for the Molasse Basin in South Germany, no spatially comprehensive pore pressure data set is currently available for the whole of Germany.

How to cite: Röckel, L., Ahlers, S., Morawietz, S., Müller, B., Reiter, K., Heidbach, O., Hergert, T., Ziegler, M., Henk, A., and Schilling, F.: Slip tendency analysis of 3D faults in Germany, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12847, https://doi.org/10.5194/egusphere-egu23-12847, 2023.

X2.206
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EGU23-13718
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TS1.3
Karsten Reiter, Oliver Heidbach, Moritz Ziegler, Silvio Giger, Rodney Garrard, and Jean Desroches

The upper Earth crust is increasingly used by mankind, to extract, transport, store or dispose materials, energy, or waste. Regardless of the objective, long term safety and stability is essential and thus, the contemporary stress state of the upper crust is one of the key variables. To estimate a continuous description of the 3‑D stress tensor, geomechanical numerical models are used. The most important parameters to set up such models are the knowledge of the underground structures, the distribution of rock properties as well as the stress data, on which the models are calibrated. In the model, the vertical stress results from the gravitational volume forces due to the density distribution and the horizontal stresses from the Poisson effect as well as appropriate lateral displacement boundary conditions. The latter are determined by finding a best-fit with respect to given stress magnitude data of the maximum and minimum horizontal stress SHmax and Shmin, respectively.

A unique dataset of stress magnitude data has been recently acquired within the exploration phase for deep geological repository of radioactive waste in Switzerland. Nine cored boreholes in three potential siting areas have been drilled and besides a wide range of logging runs, and laboratory tests of rock properties, more than 120 Mini-Hydraulic Fracturing (MHF) and Sleeve Re-Opening (SR) tests were successfully performed in different stratigraphic units to estimate the magnitudes of Shmin and SHmax

Here, we present a 3‑D geomechanical-numerical model that shows both, the best-fit to the measured stress magnitudes as well as the range of stress magnitude variability in the volume of the different stratigraphic units. This variability results from MHF/SR measurements uncertainties and from the variation of rock properties within the lithologies. Furthermore, one has to assess how representative each MHF/SR measurement is for a larger rock volume. To represent the stress variability within the lithologies, many model simulations that cover the distribution of possible rock parameters were performed. The distribution is given by the cumulative density function (CDF) for the Youngs modulus and the Poisson number for each stratigraphic unit. Based on the range of model simulations we visualize the variation of the stress components along virtual well paths in analogy to the statistical variation. Such plots allow to quantify and visualize the potential variation of the present-day stress state within the stratigraphic column because of the petro-physical variability within the stratigraphic units. Furthermore, using the CDF, we can assign to each model simulation a probability that allows us also to estimate a probability distribution of the stress variability in the different units.

How to cite: Reiter, K., Heidbach, O., Ziegler, M., Giger, S., Garrard, R., and Desroches, J.: If you get the stress data, you've always asked for, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13718, https://doi.org/10.5194/egusphere-egu23-13718, 2023.

Posters virtual: Mon, 24 Apr, 16:15–18:00 | vHall TS/EMRP

Chairperson: Armin Dielforder
vTE.1
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EGU23-14316
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TS1.3
Silvia Martín-Velázquez, David Gomez-Ortiz, Tomás Martín-Crespo, Cristina De Ignacio, José Arnoso, and Fuensanta G. Montesinos

The Canary Islands are an archipelago of eight islands and several islets in the Atlantic Ocean that have been built up by intraplate magmatism. The more recent subaerial eruption took place in La Palma Island during the last four months of 2021 (September 19th to December 13th). This volcanic activity formed the Tajogaite volcanic vent and several minor vents following an eruptive fissure roughly trending N310ºE. The eruption was preceded by intense shallow (<12 km depth) volcanotectonic activity that continued during the whole eruptive process, reaching more than 11,000 earthquakes located. After the first shallow pre-eruptive seismic swarm, the seismicity was mainly located at two different depth levels with hypocenters located at 10-15 and 30-40 km depth.

Seismicity record in the island for previous historical eruptions is very scarce and we have used this seismic episode to explore the lithospheric strength in this intraplate geodynamic setting corresponding to an old (~156 Ma) oceanic lithosphere. Geotherms and brittle and ductile rheological laws with different thermo-mechanical properties have been used to calculate strength envelopes. We have combined the study of the lithospheric strength and the vertical distribution of the seismicity from that period to estimate the extension of the brittle mechanical layer that conditioned the hypocentral locations.

How to cite: Martín-Velázquez, S., Gomez-Ortiz, D., Martín-Crespo, T., De Ignacio, C., Arnoso, J., and Montesinos, F. G.: Lithospheric rheology and strength in La Palma Island (Canary archipelago), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14316, https://doi.org/10.5194/egusphere-egu23-14316, 2023.