Normal Faulting Stress Regime Research Articles

Image analysis of acoustic data and interpretation of rock stress orientations for geothermal exploration in Gothenburg borehole GE-1, SW Sweden

Abstract This study contributes to geothermal exploration in 1660–1520 Ma old, reworked bedrock in Sweden. Our primary objectives are to constrain the orientation of horizontal stresses, and to discuss implications for geothermal exploration. High-resolution acoustic televiewer image data reveal the downhole distribution of stress indicators (borehole breakouts, drilling-induced fractures and petal centreline fractures) and pre-existing structures (natural fractures, foliation). About 135 m of stress indicators are measured from 0.2–1.0 km. The results suggest a uniform NNW–SSE mean maximum horizontal stress orientation. A total of 1525 pre-existing structures (natural fractures, foliation) are mapped in borehole GE-1. The prevailing stress regime controls whether natural fractures and foliation are well-oriented for stimulation. For strike-slip and normal faulting stress regimes, well-oriented fractures steeply dip towards the WSW. For a reverse faulting stress regime, shallow dipping fractures are well-oriented for stimulation. The downhole distribution of stress indicators and other stress measurements in the region and other parts of Fennoscandia tentatively suggest a strike-slip stress regime, but additional studies are needed to constrain the complete stress field at study depth and towards engineered geothermal systems reservoir target depths. Our secondary objective is to highlight that interpretation of high-resolution acoustic data, particularly in metamorphic crystalline rocks, is subjective and that more guidelines for data interpretation are needed. The interactive interpretation of the images is based on visual analyses of complex pre-existing structures and stress indicators with highly variable shapes. The application of three methods for data analyses in the GE-1 borehole proposes that drilling-induced fractures are little influenced by the method applied. Interpretations on individual borehole breakout azimuths may, however, result in over 10° differences in orientation.

Assessing the hazard of fault triggering by deep geothermal energy production in an active fault system via a 1D stress model and 3D fault mapping

Borehole image data and a 1D-stress model built on open hole logs, leak-off tests (LOT) and image logs are used to evaluate the potential of seismicity caused by fault triggering during geothermal heat production in the city of Vienna. Data were derived from a 4220 m deep geothermal exploration well that investigated the geothermal potential of fractured carbonates below the Miocene fill of the Vienna Basin. The well penetrated several normal faults of the Aderklaa Fault System (AFS) that offset Pleistocene terraces at the surface and hence are regarded as active. Stress-induced borehole failures and 1D geomechanical modeling proves that the potential reservoirs are in a normal fault stress regime with Sv > SHmax > Shmin. While stress magnitudes in the upper part of the well (down to about 2000 m) are significantly below the magnitudes that would trigger the rupture of critically oriented faults including the AFS, stresses in the lower part of the drilled section in the pre-Neogene basement (below about 3300 m) are not. Data evidence a rotation of SHmax for about 45° at a fault of the AFS at 3694 m to fault-parallel below the fault suggesting that the fault is active. Critical or near-critical stressing of the fault is corroborated by the stress magnitudes calculated from the 1D geomechanical model. The safety case to exclude unintended triggering of seismic fault slip by developing geothermal reservoirs in close vicinity to one of the branch faults of the AFS may therefore be difficult or impossible to make.Graphical

Numerical investigation of the stress regime effect on injection-induced fault reactivation and associated seismicity

The occurrence of many seismic events in energy production projects is caused by fluid injection-induced reactivation of critically stressed faults. The distribution of local stress regime has a significant influence on natural earthquakes, while its role in controlling injection-induced fault slip behavior and seismicity magnitude remains poorly understood. In this paper, we present a three-dimensional hydro-mechanical coupled fault reactivation study using unified pipe-interface element method (UP-IEM). The accuracy and reliability of the numerical method are evaluated and confirmed by numerical verification and actual observation data in Pohang EGS project, South Korea. The relationship between fault aseismic slip, seismic slip and seismicity magnitude is analyzed. Results show that both aseismic and seismic slip have different spatiotemporal distribution with the variation of stress regime. The seismic slip zone will exceed the pressurized area with the increase of initial slip tendency under the strike-slip-faulting stress regime (σH>σv>σh). Compared with fluid pressure, the seismic slip is mainly determined by a sudden increase of the shear stress at the aseismic slip zone boundary. In addition, the location of the microseismic events moves towards the fluid pressure propagation front. The larger slip area and seismicity magnitude are easier to be induced under strike-slip-faulting stress regime than thrust-faulting stress regime (σH>σh>σv). The tensile failure and higher permeability of the fault induced at injection phase for normal-faulting stress regime (σv>σH>σh) leads to a greater seismogenic hazard at the shut-in stage.

Constraint strategies for estimating in-situ stress from borehole measurements

In this study, we describe strategies to reduce uncertainties in the estimation of in-situ stresses. The strategies are based on the commonly used poro-elastic model with strain corrections for in-situ stress estimation. In addition to the calibration of the minimum horizontal stress magnitudes from measurement data, we further constrain the magnitude of maximum horizontal stress quantitatively with (i) a critical stress state model, (ii) (non)-occurrences of breakout and/or drilling-induced fracture observed from image logs, and (iii) qualitatively with elliptical borehole shapes observed from multi-arm caliper logging data. The methodology is demonstrated using wells near Dawson Creek, Northeast British Columbia and Northwest Alberta, in Western Canada Sedimentary Basin (WCSB). The uncertainty of the estimated maximum horizontal stress magnitude was reduced and the range of the maximum horizontal stress was narrowed after the application of these constraint strategies. It is also observed that in the east of the study area, the in-situ stress regime is a normal fault stress regime. The presence of a normal fault stress regime is unexpected since a strike–slip fault stress regime is typically considered for the entire region. Yet, analysis of caliper data in two horizontal wells has confirmed its presence.

Unraveling the Evolution of an Unusually Active Earthquake Sequence Near Sheldon, Nevada

One of most universal statistical properties of earthquakes is the tendency to cluster in space and time. Yet while clustering is pervasive, individual earthquake sequences can vary markedly in duration, spatial extent, and time evolution. In July 2014, a prolific earthquake sequence initiated within the Sheldon Wildlife Refuge in northwest Nevada, USA. The sequence produced 26 M4 earthquakes and several hundred M3s, with no clear mainshock or obvious driving force. Here we combine a suite of seismological analysis techniques to better characterize this unusual earthquake sequence. High-precision relocations reveal a clear, east dipping normal fault as the dominant structure that intersects with a secondary, subvertical cross fault. Seismicity occurs in burst of activity along these two structures before eventually transitioning to shallower structures to the east. Inversion of hundreds of moment tensors constrain the overall normal faulting stress regime. Source spectral analysis suggests that the stress drops and rupture properties of these events are typical for tectonic earthquakes in the western US. While regional station coverage is sparse in this remote study region, the timely installation of a temporary seismometer allows us to detect nearly 70,000 earthquakes over a 40-month time period when the seismic activity is highest. Such immense productivity is difficult to reconcile with current understanding of crustal deformation in the region and may be facilitated by local hydrothermal processes and earthquake triggering at the transitional intersection of subparallel fault systems.

In Situ Stress Distribution of Deep Coals and Its Influence on Coalbed Methane Development in the Shizhuang Block, Qinshui Basin, China.

The in situ stress plays a crucial role in variations in coal permeability, hydraulic fracturing behavior, and accordingly coalbed methane (CBM) productivity. As the depth increases, the effects of in situ stress will become more prominent. In the Shizhuang block, present-day stress magnitude and permeability of coals at depths >800 m were measured with multiple-cycle hydraulic fracturing and injection falloff test, respectively. The results show that most seams are under pressure reservoirs with pressure gradient <0.9 MPa/100 m. Horizontal stress magnitudes and gradients tend to increase (800-1200 m) and then decrease (>1200 m) with increasing depth. Strike-slip fault stress regimes are predominant, while seams with depth >1400 m are subjected to a normal fault stress regime. Coal permeability tends to decrease gradually with depth and tends to be convergence to 0.01 mD. Considering extremely low permeability of these deep seams, hydraulic fracturing must be applied to create seepage channels for gas and water drainage. Although the high horizontal stress difference in deep seams is favorable for the generation of longer and simple hydraulic fractures, there is no obvious relations between fracture length and gas productivity as poor-support fractures and limited sand migration distance. The current hydraulic fracturing missed the variable stress regimes and permeability at various depth, but used the similar fracturing schemes, leading to significant reduction in gas productivity with depth. Using the high viscosity fracturing fluid, great sand volume, big injection rate, and low sand concentration are recommended for hydraulic fracturing. Single-branch horizontal well with staged fracturing show better applicability for deep CBM extraction.

In-situ stress prediction in ultra-deep carbonate reservoirs of Fuman Oilfield, Tarim Basin of China

The Fuman Oilfield in Tarim Basin has great potential for ultra-deep carbonate oil and gas resources, and is an important area for future storage and production increase. The present-day in-situ stress field is critical during the exploration and development. However, no systemic investigations have been carried out in this oilfield. Therefore, in this study, the present-day in-situ stress field in the Ordovician carbonate reservoir is predicted and analyzed based on well log calculation and geomechanical numerical modeling. The results indicate that, 1) NE-SW-trending is the dominant horizontal maximum principal stress (SHmax) orientation. The vertical principal stress is the maximum principal stress, showing the Ordovician reservoir is under a normal faulting stress regime. 2) The distribution of in-situ stresses in the Ordovician carbonate reservoir is heterogeneous, which is mainly controlled burial depth and fault/fracture development. High stress magnitudes in the Yingshan Formation are mainly in the southeastern part of Fudong area, Fuman Oilfield. The present-day horizontal differential stress mainly ranges from 27 MPa to 30 MPa in the Yingshan carbonate reservoir. iii) Natural fractures are generally stale under the present-day in-situ stress state. Fractures that parallel to the SHmax orientation with high fracture dip angle are easier to be reactivated. The results are expected to provide geomechanical references for further oil and gas development in the Fuman Oilfield of Tarim Basin.

Seismicity and state of stress in the north-central region of the Gulf of California, Mexico

We compile a catalog of well-located earthquakes to estimate focal mechanisms in the north-central region (27.5 N–31.5 N) of the Gulf of California (GoC) to determine the state stress. The catalog consists of 81 relocated earthquakes from 2016 to 2021 with magnitudes between M4 and M5.7 and depths <25 km. We selected 47 earthquakes recorded by the Gulf of California Broadband Seismological Network (RESBAN) and the National Seismological Service (SSN) to obtain focal mechanism solutions by waveform inversion using the ISOLA software (Sokos and Zahradnik, 2008). Our database consists of 80 focal mechanisms including 33 reported in other catalogs. The fault plane solutions indicate that 36.25% correspond to normal faulting, 33.75% strike-slip, 25% oblique-slip, and 5% thrust faulting. We formed 3 groups of earthquakes with known focal mechanisms to estimate the orientation of the principal stresses using the Bayesian method of right trihedral (BRTM). Each group contains similar focal mechanisms and that align with the main faults in the region. We find that in the northernmost region of the GoC predomine a normal faulting stress regime while in the central region persists a strike-slip faulting stress regime, typical of transform boundaries. We conclude that the state of stress is heterogeneous in the north-central region of the GoC; that the orientation of maximum horizontal stress (σHmax) is compatible with the world stress map (WSM) in some zones; and that the friction coefficient estimated indicates that the faults are strong in the central region of the GoC.

Rock Strength and In Situ Stress Analysis for Wellbore Instability, Fault Stress Regime Prediction, 3D Structural and Geomechanical Modelling in H-field Offshore of Rio Del Rey Basin, Southwest Region, Cameroon

The study involved the determination of strength parameters and in situ stresses in two wells pseudo-named H005 and H008 in a hydrocarbon field located offshore of Rio Del Rey basin. The work was aimed at revealing the stability of formation rocks and fault stress regimes associated with the study wells. The parameters determined employed only empirical correlations and tools since core samples were not available for laboratory experiments. The unconfined compressive strength for the two wells was relatively low varying from 1500 to 3300 psi in well H005 and from 1000 to 4000 psi in well H008. The friction angle varies from 21.0° to 23.0° in well H005 and from 21.0° to 27.0° in well H008. The results of the in situ stress analysis in well H005 indicate a reverse fault stress regime where maximum horizontal stress (SH) is greater than the minimum horizontal stress (Sh), which is in turn greater than the vertical stress (SV) in the shallow part of the well (1028.5–1249 m), while a normal fault stress regime where the vertical stress is greater than the two horizontal stresses was interpreted at a depth from 1250 to 1390 m. Unlike well H005, the entire depth of well H008 under interpretation revealed a normal fault stress regime. The outcome of the 3D geomechanical model closely agrees with the 1D model. It was concluded that the formation rocks are weak, and the wells are dominated by normal fault stress regimes.

Well trajectory optimization of ultradeep and high-pressure drilling engineering based on high in situ stress as the main control factor: A case study from the Ordovician carbonate reservoir in the Shunbei area of the Tarim Basin

With increasing oil and gas exploration and development, well trajectory optimization has gradually become the focus of the oil and gas industry. Considering the wellbore instability in the Shunbei area of the Tarim Basin, the well trajectory was scientifically optimized under the guidance of rock mechanics, drilling engineering, and mathematical methods, combined with actual geologic data, and with in situ stress as the main controlling factor. In this paper, the stress state of the wellbore is analyzed by linear elastic theory to establish the stress distribution model of the wellbore. The safety window model of wellbore stability is established using different rock failure criteria to calculate the collapse pressure and fracture pressure of the formation. Based on this, the safe mud density window is defined to achieve wellbore trajectory optimization. Finally, the influence factors of wellbore stability are discussed, and the applicability of different rock failure criteria is evaluated. The results indicate that under the normal faulting stress regime condition in the study area, the direction of horizontal minimum principal stress is the best drilling direction, where the borehole inclination angle of [Formula: see text] &gt; 50° is the optimal well trajectory. The wellbore stabilities of high-angle deviated wells and horizontal wells are better than those of low-angle deviated wells and vertical wells. The calculation results of the Mogi-Coulomb criterion can describe the conditions of the in situ stress field more accurately. The safe windows for different well trajectories are obtained directly by the numerical method, which is very practical for optimizing well trajectories and improving wellbore stability.

Three-dimensional stress state above and below the plate boundary fault after the 2011 Mw 9.0 Tohoku earthquake

The Integrated Ocean Drilling Program conducted Expedition 343 and 343T, named the Japan Trench Fast Drilling Project (JFAST), to drill through the plate boundary fault that ruptured during the 2011 Mw 9.0 Tohoku earthquake in the area with the largest fault slip displacement near the Japan trench. Analyses of breakouts observed from borehole C0019B produced postearthquake stress states above the plate boundary fault between the subducting Pacific plate and overriding North American plate. To supplement the lack of stress data below the rupture zone of the earthquake, we conducted core-based three-dimensional stress measurements by the anelastic strain recovery (ASR) method using four whole-round core samples of sediments, of which three samples were located above, but one sample was located below the plate boundary fault in borehole C0019E. As a result of the stress measurements, the postearthquake three-dimensional stress magnitudes at ∼802 and ∼828 meters below seafloor (mbsf) across the plate boundary fault at ∼820 mbsf reveal a normal faulting stress regime. The differences between the three-dimensional intermediate principal stress and the minimum principal stress at the two depths are less than 1 MPa, suggesting a complete release of horizontal tectonic stresses that accumulated before the earthquake. In addition, the maximum horizontal stress SHmax azimuth N115°E at ∼828 mbsf below the plate boundary fault from ASR measurements shows consistency with the SHmax azimuth N139 ± 23°E (mean ± standard deviation) at ∼550–810 mbsf from breakout analyses above the fault. Taken together with the similar stress magnitudes at ∼802 and ∼828 mbsf, we interpret that the postearthquake stress states are almost the same in the sediments above and below the plate boundary fault. In other words, the stress state in terms of both orientation and magnitude is continuous across the fault. At a shallower depth of ∼177 mbsf in the slope sediments, the ASR stress data reveal a “stress state at rest”, which is likely free from tectonic effects of plate subduction, suggesting that the stress state was reset by the great coseismic displacement of ∼50 m slipped during the Tohoku earthquake.

Extensional stress accompanied by Miocene near-trench magmatism in the southern Kii Peninsula, SW Japan

Voluminous near-trench magmatism began in the Kumano area, SW Japan, at ca. 15 Ma. The magmatism formed batholiths, cauldrons, and numerous dikes. It has been known that The dikes are oriented perpendicular to the Nankai Trough, and are hosted by young formations that are folded with axes parallel to the trench. Therefore, it has been considered that the magmatism was accompanied by trench-perpendicular compression during the period shortly after backarc opening in the Japan Sea. For this study, we obtained orientation data from 14 felsic and 151 mafic dikes in the early to middle Miocene Kumano Group and Shionomisaki Igneous Complex. Inversion of both datasets yields a normal-faulting stress regime with NE–SW extension. This orientation meets at a high angle with that of the trench. We also obtained zircon fission-track ages of 14.5 ± 1.5 and 13.9 ± 1.4 Ma from two felsic dikes. These ages and existing radiometric and micropaleontological data show that both the groups of dikes were formed at ca. 14.5 Ma. Large- and meso-scale structures indicating trench-perpendicular shortening in igneous rocks suggest that the folding took place after the magmatism. We interpret that the extensional stress resulted from uplift caused by the emplacement of a batholith under the Kumano area. The inferred normal-faulting stress regime raises doubts about popular tectonic models of SW Japan that rely on a transition from extension to compression at ca. 15 Ma.

Role of the Nyainrong Microcontinent in Seismogenic Mechanism and Stress Partitioning: Insights from the 2021 Nagqu Mw 5.7 Earthquake

The Nyainrong microcontinent carries key information about the ongoing evolution of the central Tibetan Plateau. The 2021 Mw 5.7 Nagqu earthquake is the largest instrumentally recorded event inside this microcontinent, which provides an ideal opportunity to elucidate the influence of this ancient microcontinent on the seismogenic mechanisms, stress heterogeneity and strain partitioning across the Tibetan Plateau. Here, we constrain the seismogenic fault geometry and distributed fault slip using Interferometric Synthetic Aperture Radar (InSAR) observations. By using the regional focal mechanism solutions, we invert the stress regimes surrounding the Nyainrong microcontinent. Our analysis demonstrates that the mainshock was caused by a normal fault with a comparable sinistral strike-slip component on a North-West dipping fault plane. The Nyainrong microcontinent is surrounded by a dominant normal faulting stress regime to the northeast and a dominant strike-slip stress regime to the southwest. Moreover, the clockwise rotation of the maximum horizontal stress (SHmax) from the southwest to the northeast is ~20°. This indicates that the Nyainrong microcontinent is involved in the mainshock occurrence as well as regional stress heterogeneity, and strain partitioning. Our results highlight the significance of the ancient microcontinent in the tectonic evolution of the Tibetan Plateau.

Contemporary state of stress in a stable plate interior (northern Poland): The integration of satellite geodesy, borehole and seismological data

In our study we examined the possibility of using strain rate data, determined from geodetic satellite measurements for estimations of magnitude of contemporary stress in a relatively stable area of northern Poland, comprised of fragments of the East European Craton, the Teisseyre-Tornquist Tectonic Zone and the Palaeozoic Platform. The strain rates and their principal directions were determined from 63 GNSS stations lacking special stabilization. The method for filtrating the unstable stations allowed us to eliminate half of these stations from the final strain rate calculations that included data averaging in a correlation radius of 150 km. A comparison of the horizontal strain directions determined by geodetic methods with the directions of contemporary stress based on borehole data (four of which were determined in this study) showed a high degree of convergence. Also the trends of strain rate variations showed consistency with integrated lithospheric strength of main tectonic units. The strain rates obtained are consistent with those published for the adjacent areas. Stress magnitudes were estimated by calibrating the timing of elastic strain accumulation by earthquakes in the Sambia Peninsula (Kaliningrad Oblast) and assuming a transient state of elastic strain on the craton. Our results indicate that the crystalline upper crust in the analysed part of the craton is in the strike-slip faulting stress regime prevailing over the normal faulting stress regime. The present state of an elastic strain has been accumulated over the last few million years causing stress being far from the critical state in the Mohr-Coulomb frictional criteria, except of the seismically active Sambia Peninsula.

Contemporary background stress field in the eastern Tibetan Plateau: Insights from 3D geomechanical modeling

Knowledge of the crustal stress state is important for understanding tectonic processes and seismic hazard assessment as well as for various practical applications. In this study, we present a model describing the contemporary 3D crustal background stress field of the eastern Tibetan Plateau. The model incorporates a complex fault system and an appropriate initial stress state that matches both model-independent stress observations and kinematic data well. Our results show that the modeled maximum horizontal stress SH is generally oriented in NW-SE direction. The Bayan Har Block and the Chuan-Dian Block are dominated by a strike-slip faulting stress regime. However, in some local areas, the crust is governed by a normal faulting regime. We infer that the local normal faulting stress regime in the Bamei-Kangding area is caused by the clockwise rotation of the strike of the Xianshuihe fault at Kangding. This normal faulting stress regime would promote the ascent of intrusions and provide the space necessary for the emplacement of the Zheduoshan granite. The stress state on the Longmen Shan fault zone is rather inhomogeneous. For the Yingxiu-Beichuan fault, the northeastern segment has a lower normal stress but a higher shear stress than the southwestern segment. This inhomogeneous stress state on the Yingxiu-Beichuan fault could provide an explanation for the northeastward propagation of the ultra-long unilateral rupture in the Wenchuan Ms8.0 earthquake. According to the stress state on the Xianshuihe fault, we conclude that the Kangding-Shimian segment has a high potential to produce a strong earthquake due to its high normal stress and a fast slip rate and should be carefully monitored. On the Bamei-Kangding segment, which is governed by a normal faulting stress regime and may be lubricated by fluids, slip seems to be facilitated favoring more frequent small to moderate magnitude earthquakes to occur on this segment.

Integrated Mechanical Earth Modeling for Predicting Sand Production: A Case Study

Summary Sand production is a serious problem in oil and gas wells, and one of the main concerns of production engineers. This problem can damage downhole equipment and surface production facilities. This study presents a sand production case and quantifies sanding risks for an oil field in Iraq. The study applies an integrated workflow of constructing 1D Mechanical Earth Modeling (MEM) and predicting the sand production with multiple criteria such as shear failure during drilling, B index, and critical bottomhole pressure (CBHP) or critical drawdown pressure (CDDP). Wireline log data were used to estimate the mechanical properties of the formations in the field. The predicted sand production propensity was validated based on the sand production history in the field. The interpretation results of some wells anticipated in this study showed that when a shear failure occurs during drilling, the B index is around 2 × 104 MPa or less and the CBHP is equal to the formation pore pressure. For this case, sand control shall be carried out in the initial stage of production. On the other hand, when the shear failure does not exist, the B index is always greater than 2 × 104 MPa, and the CBHP is mostly less than the formation pore pressure. In this case, implementing sand control methods could be postponed as the reservoir pressure undergoes depletion. However, for the anticipated field, sand control is recommended to be carried out in the initial stage of well production even when the CBHP is less than the formation pore pressure since sanding will be inevitable when the reservoir pressure depletes to values close to the initial reservoir pressure. The tentative evaluation of the stress regime showed that a normal fault could be the stress regime for the formations. For a normal fault stress regime, the study explained that when the reservoir permeability is isotropic, an openhole vertical wellbore has less propensity for sand production than a horizontal wellbore. Moreover, when the wellbore azimuth is in the direction of the minimum horizontal stress, the CBHP will be lower than in any other azimuth, and sanding will take place at higher wellbore inclination angles. For the anticipated field, because of the casedhole well completion and the anisotropic reservoir permeability, a horizontal well drilled in the direction of minimum horizontal stress with oriented perforation in the direction of maximum horizontal stress is an alternative method for controlling sand production.

20 July 2017 Bodrum-Kos Earthquake (Mw:6.6) in southwestern Anatolia, Turkey

In the Aegean Sea, the western part of Gökova Gulf, Kos and Bodrum were struck by a 6.6 (Mw) earthquake on July 20, 2017. The fault plane solution for the main shock shows an E-W striking normal type fault with approximately N-S (N4°E) tensional axis (T-axis). Fault plane solutions of 33 aftershocks show two groups of normal type fault with E-W and NE-SW to ENE-WSW orientations. The inversion of the focal mechanisms of the aftershocks yields two different normal faulting stress regimes: one is characterized by an approximately N-S (N5°E) σ3 axis (minimum horizontal stress axis). This extension is obtained from 13 focal mechanisms of aftershocks with approximately E-W direction. The other is characterized by approximately NW-SE (N330°E) σ3 axis. The latter is calculated from 21 seismic faults of aftershocks with approximately NE-SW direction. These aftershocks occurred on relatively small-scale faults that were directed from NE-SW to ENE-WSW, and possibly contributed to expansion of the basin in the west. The 24 focal mechanisms of earthquakes which occurred since 1933 in and around Gökova Basin are introduced into the inversion analysis to obtain the stress state effective in a wider region. The inversion yields an extensional stress regime characterized by an approximately N-S (N355°E) σ3 axis. The E-W directional metric faults, measured in the central part of Gökova Fault Zone bordering the Gökova Gulf in the north, also indicate N-S extension. The NE-SW extension obtained from NE-SW aftershocks appears to be more local and is responsible for the expansion of the western part of the asymmetric Gökova Basin. This N-S extension which appears to act on a regional-scale may be attributed to the geodynamic effects related to the combined forces of the southwestward extrusion of Anatolia and the roll-back process of African subduction beneath Anatolia.

Geomechanical Response Induced by Multiphase (Gas/Water) Flow in the Mallik Hydrate Reservoir of Canada

Summary Natural gas hydrate (NGH) is regarded as an important alternative future energy resource. In recent years, a few short-term production tests have been successfully conducted with both permafrost and marine sediments. However, long-term hydrate production performance and the potential geomechanical problems are not very clear. According to the available geological data at the Mallik site, a more realistic hydrate reservoir model that considers the heterogeneity of porosity, permeability, and hydrate saturation was developed and validated by reproducing the field depressurization test. The coupled multiphase and heat flow and geomechanical response induced by depressurization were fully investigated for long-term gas production from the validated hydrate reservoir model. The results indicate that long-term gas production through depressurization from a vertically heterogeneous hydrate reservoir is technically feasible, but the production efficiency is generally modest, with the low average gas production rate of 4.93 × 103 ST m3/d (ST represents the standard conditions) over a 1-year period. The hydrate dissociation region is significantly affected by the reservoir heterogeneity and reveals a heterogeneous dissociation front in the reservoir. The depressurization production results in significant increase of shear stress and vertical compaction in the hydrate reservoir. The response of shear stress indicates that the potential region of sand migration is mainly in the sand-dominant layer during gas production from the hydraulically heterogeneous hydrate reservoir (e.g., sand layers interbedded with clay layers). The maximum subsidence is approximately 78 mm and occurred at the 72nd day, whereas the final subsidence is slowly dropped to 63 mm after 1-year of depressurization production. The vertical subsidence is greatly dependent on the elastic properties and the permeability anisotropy. In particular, the maximum subsidence increased by approximately 81% when the ratio of permeability anisotropy was set at 5:1. Furthermore, the potential shear failure in the hydrate reservoir is strongly correlated to the in-situ stress state. For the normal fault stress regime, the greater the initial horizontal stress is, the less likely the hydrate reservoir is to undergo shear failure during depressurization production.

The inferences on the Aegean geodynamic context from 30 October 2020 Samos earthquake (Mw:6.8)

On October 30, 2020, an earthquake with a magnitude of 6.8 (Mw) struck the northern coast of Samos Island in the Kuşadası Gulf. The solution to the focal mechanism indicates that the earthquake of 30 October 2020 occurred on a normal fault with nodal planes of E-W strike; thus, indicating extension in N-S direction. The fault plane solutions show a N-S trending extension for normal faults, which are obtained by inverting the moment tensor waveforms of 23 earthquakes and the P-wave first motion polarities of 11 aftershocks. A normal fault stress regime of approximately N-S (N6°E) σ3 axis is given by the inversion of slip vectors measured at sites located on land in Kuşadası. The mean R value is 0.84, suggesting that the stress regime is triaxial extensional stress state. The inversion of the focal mechanism of earthquakes occurring on land and in the Gulf of Kuşadası describes an extensional stress regime active today, characterized by an approximately N-S (N9°E) σ3 axis. The calculated R value of 0.31 indicates a triaxial stress state. For the 30 October 2020 earthquake (Mw:6.8), the Coulomb failure stress change analysis shows a substantial reduction in stress in the N-S direction supporting the kinematic results. The N-S extension in Western Anatolia-Aegean is largely influenced by the relatively fast movement of the Hellenic trench southwards, related to the sinking of the African plate beneath Aegean.

Global Frictional Equilibrium via Stochastic, Local Coulomb Frictional Slips

AbstractBased on the assumption that fault slip dominates in the stress relaxation in the brittle crust, Coulomb theory allows for the crustal stress estimation with an empirical frictional coefficient. However, natural variability of fault friction and slip uncertainty exist in the Earth's crust. To address the extent to which heterogeneous frictional slips influence crustal stress and its evolution, we establish a quasi‐static, 2D model to represent the fractured crustal rock mass. The model consists of randomly oriented fractures with heterogeneous distribution of frictional coefficients. The global mechanical response is quantitatively related to the cascades of local frictional slips under specific boundary conditions. The temporal evolution of stress is explicitly modeled by an iterative process where a simple slip law is assumed for critical fractures. We particularly illustrate the stress evolution in a normal faulting stress regime, considering different distributions of frictional coefficients. All cases indicate that the decrease in differential stress manifests as a self‐organized process, eventually leading to the frictional equilibrium of the fractured rock mass. The final stress state upon equilibrium jointly depends on the orientation and frictional coefficient of all fractures therein. The model informs that the global stress state of a stochastic system can depart substantially from a deterministic estimation via an empirical frictional coefficient. This model quantitatively corroborates and extends the notion of frictional equilibrium, and reveals far more profound influence of system heterogeneity on the local and global stress evolution.