Coulomb Failure Stress Change Research Articles

Role of Plasticity in Induced Seismicity Risk Mitigation: A Case of the Groningen Gas Field

Summary Earthquakes induced by fluid extraction from deep underground reservoirs are not well understood in rocks that are deforming plastically. The problem grows in importance when seismicity risk mitigation strategies, such as gas injection into a depleting hydrocarbon reservoir, attempt to reverse the declining pressure trend of a poromechanical system deforming irreversibly. This is the case at the Groningen gas field and similar fields worldwide. Poroplasticity associated with half a century of seasonally fluctuating gas production makes it challenging to predict Groningen’s state, especially with hundreds of faults compartmentalizing the reservoir. We provide new insights into the role of plasticity in depletion-induced seismicity and its mitigation via injection. The irreversibility of plastic deformation is key to predicting stress and fault stability when the pressure trend is reversed by fluid injection. The elastic deformation assumption predicts unrealistically high Coulomb failure stresses on the faults, implying a higher risk of induced seismicity than possible under plastic deformation. The inaccuracy in the elastic model’s predicted stress cannot be discerned from the reservoir pressure or subsidence measurements in the field. Therefore, rock’s plasticity must be considered in assessing and mitigating the risk of induced seismicity. The probability distribution of the change in Coulomb failure stress over 115 faults in the field reveals a multimodal shape that emerges from the stabilization and destabilization of different faults depending on the fault’s geometry and position relative to the wells.

Rupture behaviors of the southern Xianshuihe fault and seismicity around Mt. Gongga: Insights from the 2022 MW 6.6 Luding (China) earthquake sequence

The 2022 MW 6.6 Luding earthquake occurred on the Moxi segment of the Xianshuihe fault at the southeast margin of Tibetan Plateau, China. To assess the seismic potential of the Moxi segment, we examine the rupture process of the mainshock and aftershock sequence, along with historical seismicity. Our preferred slip model inverted from teleseismic body waves and regional GNSS static displacements shows a dominant southeastward rupture consisting of two distinct, prominent slip patches along strike extending by ∼15 km, with a peak slip of ∼2.8 m, approximately balancing the slip deficit since the last major earthquake in 1786. The northern section of the Moxi segment experienced minor coseismic slip, which, together with the significant slip deficits and positive Coulomb failure stress change induced by the 2022 mainshock indicates a high seismic potential. Several aftershock clusters are distributed along or near the Moxi segment, with strike-slip focal mechanisms around the downdip edge of the coseismic slip area at ∼8‐12 km. At the eastern flank of Mt. Gongga, another cluster of normal faulting aftershocks is located at shallower depths of ∼3‐7 km, with high seismicity rate over ∼9 months including two other M5 sequences in January and February 2023. Similar intense shallow normal faulting activity had occurred after the impoundment of the nearby Dagangshan reservoir in 2015. We speculate that some NW-SE trending normal faults were initially developed by the gravitational collapse of Mt. Gongga underneath the eastern flank, further weakened by fluid flow, as supported by the existence of hot springs and water impoundment, and reactivated by the tensional stress change induced by the 2022 mainshock. These results have important implications for assessing the seismic hazard in and around the Moxi segment, and the potential interplay between strike-slip fault and nearby mountain areas.

Not trench-parallel but trench-normal source fault of the 1994 Hokkaido Toho-oki earthquake as revealed by the aftershock relocation using HypoDD

In the southwestern area of the Kurile Islands subduction zone, three large earthquakes including the Mw8.2 1969 and the Mw8.3 1994 Hokkaido Toho-oki earthquakes and the Mw7.8 1975 tsunami earthquake occurred in almost the same location. The three main shocks and their aftershocks were re-determined simultaneously using a double-difference earthquake location method, HypoDD. As a result, the 1969 and the 1975 main shocks and their aftershocks were concentrated near the upper surface of the subducting Pacific plate, indicating that both events were plate boundary megathrust earthquakes as shown by previous studies. On the other hand, the 1994 main shock and its aftershocks were distributed within the Pacific plate indicating that the 1994 event was an intraslab event as reported by previous studies. The result that interplate earthquakes and intraslab earthquakes could be appropriately distinguished is strong evidence that the accuracy of hypocenter determination is sufficiently high. The centroid moment tensor solution of the 1994 main shock has two nodal planes which are parallel or normal to the Kurile Trench. According to the high-resolution hypocenter distribution obtained in the present study, the aftershocks appear to locate on the nodal plane normal to the trench, suggesting that the source fault ruptured by the 1994 main shock was not parallel but normal to the Kurile Trench. Moreover, we found that the Coulomb failure stress change (ΔCFS) produced by the trench-normal fault plane is more likely to trigger the largest aftershock than ΔCFS produced by the trench-parallel fault plane.Graphical abstract

Investigation of the 2011 Yingjiang, Yunnan, China Ms. 5.8 Earthquake Sequence: Seismic Migration, Seismogenic Mechanism, and Hazard Implication

On March 10, 2011, an Ms. 5.8 earthquake struck Yingjiang City, western Yunnan, China, causing destructive damage. Due to the very sparse distribution of seismic stations on the southwestern border of China, its seismogenic structure and mechanism remain controversial. In this study, with the aid of machine-learning-based detection and location workflow and template matching technique, we detect 10,356 events ranging from December 1, 2010, to April 30, 2011. The high-precision earthquake catalog shows that the foreshocks initiated in the extensional stepover connecting the northeast and middle segments of the Dayingjiang fault and then bilaterally extended northeast and southwest, with migration fronts that can be simulated by fluid diffusion model with diffusivities of 0.8 m2/s and 0.19 m2/s, respectively. The mainshock occurred at the southwest end of the foreshock sequence and then probably activated the northwest-trending blind fault. In addition, we determine the full moment tensor solutions for the mainshock, six large foreshocks, and one aftershock, with magnitudes ranging from 3.03 to 5.80, in which the mainshock was characterized by an obvious negative isotropic (ISO) component. The static Coulomb failure stress change caused by five Mw ≥ 4.0 foreshocks on the mainshock fault plane is ∼24 kPa, reaching the typical static triggering threshold. Therefore, we suggest that both the fluid diffusion and stress perturbation contribute to triggering the mainshock. This study advances our understanding of the spatiotemporal evolution, seismogenic mechanism, and hazard implication for the Yingjiang Ms. 5.8 earthquake and provides additional evidence of natural fluid-triggered seismicity in western Yunnan.

Thermoporoelastic Stress Perturbations from Hydraulic Fracturing and Thermal Depletion in Enhanced Geothermal Systems (EGS) and Implications for Fault Reactivation and Seismicity

Hydraulic fracturing then fluid circulation in enhanced geothermal system (EGS) reservoirs have been shown to induce seismicity remote from the stimulation – potentially generated by the distal projection of thermoporoelastic stresses. We explore this phenomenon by evaluating stress perturbations resulting from stimulation of a single stage of hydraulic fracturing that is followed by thermal depletion of a prismatic zone adjacent to the hydraulic fracture. We use Coulomb failure stress to assess the effect of resulting stress perturbations on instability on adjacent critically-stressed faults. Results show that hydraulic fracturing in a single stage is capable of creating stress perturbations at distances to 1000 m that reach 10-5-10-4 MPa. At a closer distance, the magnitude of stress perturbations increases even further. The stress perturbation induced by temperature depletion could also reach 10-3-10-2 MPa within 1000 m - much higher than that by hydraulic fracturing. Considering that a critical change in Coulomb failure stress for fault instability is 10-2 MPa, a single stage of hydraulic fracturing and thermal drawdown are capable of reactivating critically-stressed faults at distances within 200 m and 1000 m, respectively. These results have important implications for understanding the distribution and magnitudes of stress perturbations driven by thermoporoelastic effects and the associated seismicity during the simulation and early production of EGS reservoirs.

Coseismic Slip and Downdip Afterslip Associated with the 2021 Maduo Earthquake Revealed by Sentinel-1 A/B Data

On 22 May 2021, an earthquake (98.3° E and 34.59° N) struck Maduo town in Qinghai province, occurring along a relatively obscure secondary fault within the block. We utilized 105 archived Sentinel-1A/B acquisitions to investigate the coseismic deformation and the evolution of postseismic displacements in both the temporal and spatial domains, as well as the associated dynamic mechanisms of the 2021 Maduo earthquake. The interference fringes and coseismic deformation revealed that the primary feature of this event was the rupture along a left-lateral strike-slip fault. The released seismic moment was close to 1.88 × 1020 N·m, which is equivalent to an Mw 7.45 event. Simultaneously, the maximum coseismic slip reached approximately 4 m along the fault plane. The evolution of postseismic displacements in both the temporal and spatial domains over 450 days following the mainshock was further analyzed to explore the underlying physical mechanisms. Generally, the patterns of coseismic slip and afterslip were similar, although the postseismic displacements decayed rapidly over time. The modeled afterslip downdip of the coseismic rupture (at depths of 15–40 km) effectively explains the postseismic deformation, with a released moment estimated at 4.57 × 1019 N·m (corresponding to Mw 7.04). Additionally, we found that regions with high coseismic slip tend to exhibit weak seismicity, and that afterslip and aftershocks are likely driven by each other. Finally, we estimated the Coulomb Failure Stress changes (ΔCFS) triggered by both coseismic rupture and aseismic slip resulting from this event. The co- and postseismic ΔCFS show similar patterns, but the magnitude of the postseismic ΔCFS is much lower (≤0.01 MPa). We found that ΔCFS notably increased on the Yushu segment of the Garze-Yushu-Xianshuihe Fault (GYXF), as well as the Maqin–Maqu and Tuosuo Lake sections of the East Kunlun Fault (EKF). Therefore, we infer that these fault segments may have a higher potential seismic risk and should be carefully monitored in the future.

Detecting sub-instability before three Mw > 6.0 earthquakes on Chinese mainland in 2020–2022 with load/unload response ratio

The sub-instability refers to a stress state that exists between the maximum stress the rock can withstand and its final failure. This stress state should be an indicator of impending large earthquakes, as it occurs in the final stage of earthquake nucleation. We apply the Load/Unload Response Ratio (LURR) method to explore sub-instability in the source media before large earthquakes. The earthquake-related observation data such as the groundwater, crustal deformation, electromagnetism and so on were adopted as data input. The load and unload phases at each observation station are discriminated by the change of Coulomb failure stress caused by earth tides along the tectonically optimal sliding direction. Retrospective studies of this approach on the 2020 Mw6.0 Jiashi, Xinjiang, 2022 Mw6.6 Menyuan, Qinghai, and Mw6.6 Luding, Sichuan earthquakes show that the LURR time series derived from the observation stations near the epicenter exceed 2 standard deviations of the mean within several days to years before the mainshocks. Additionally, as the time of mainshock approaches, the distance between the stations detecting LURR anomaly and epicenter decreases. The LURR anomalies indicate the presence of sub-instability prior to a large earthquake, and their spatio-temporal evolution may suggest the weakening processes in the source media.

Fluids-Triggered Swarm Sequence Supported by a Nonstationary Epidemic-Like Description of Seismicity

Abstract The variation in Coulomb failure stress (CFS) plays a crucial role in either increasing or decreasing seismic activity. In cases in which the standard epidemic-type aftershock sequence (ETAS) model does not adequately fit seismicity data, the potential deviations from empirical laws are explored. These deviations may arise from stress changes imparted by aseismic transients that lead swarm-like earthquake sequences to occur. The time-dependent background rate of seismicity serves as an indicator for detecting changes in CFS or the presence of transient aseismic forcing. We investigate seismic anomalies in the slow deforming Mt. Pollino, Italy seismogenic area, where a 4-yr-long swarm-like sequence partially filled a previously hypothesized seismic gap. The primary process of this seismic swarm is still under debate. Employing a nonstationary ETAS model on a new template-matching high-resolution catalog, we suggest a slow-slip event and fluid interplay as the main aseismic forces in triggering and developing this swarm-like sequence.

Normal fault reactivation induced by hydraulic fracturing: Poroelastic effects

Numerous surface-felt earthquakes have been spatiotemporally correlated with hydraulic-fracturing operations. Because large deformations occur close to hydraulic fractures (HFs), any associated fault reactivation and resulting seismicity must be evaluated within the length scale of the fracture stages and based on the precise fault location relative to the simulated rock volumes. To evaluate the changes in Coulomb failure stress (CFS) with injection, we conduct fully coupled poroelastic finite-element simulations using a pore-pressure cohesive zone model for the fracture and fault core in combination with a fault-fracture intersection model. The simulations quantify the dependence of CFS and the fault reactivation potential on the host rock and fault properties, spacing between the fault and the HF, and the fracturing sequence. We find that fracturing in an anisotropic in-situ stress state does not lead to fault tensile opening but rather dominant shear reactivation through a poroelastic stress disturbance over the fault core ahead of the compressed central stabilized zone. In our simulations, poroelastic stress changes significantly affect fault reactivation in all the simulated scenarios of fracturing 50–200 m away from an optimally oriented normal fault. Asymmetric HF growth due to the stress shadowing effect of the adjacent HFs leads to (1) a larger reactivated fault zone following the simultaneous and sequential fracturing of multiple clusters compared with single-cluster fracturing and (2) a larger unstable area ([Formula: see text].1) over the fault core or a higher potential of the fault slip following sequential fracturing compared with simultaneous fracturing. The fault reactivation area is further increased for a fault with lower conductivity and a higher opening-mode fracture toughness of the overlying layer. To reduce the risk of fault reactivation by hydraulic fracturing under the reservoir characteristics of the Barnett Shale, Fort Worth Basin, it is recommended to (1) conduct simultaneous fracturing instead of sequential and (2) maintain a minimum distance of approximately 200 m for HF operations from known faults.

The Stress State before the MS 6.8 Luding Earthquake on 5 September 2022 in Sichuan, China: A Retrospective View Based on the b-Value

On 5 September 2022 (BJT), Luding, located in southwestern Sichuan Province, China, experienced an MS 6.8 earthquake. This earthquake occurred within the historical rupture zone of the 1786 MS 7.75 event, part of the southern section of the Xianshui He Fault belt. Given the average 155-year recurrence interval for strong earthquakes in this area, the 236 years since the last event made this earthquake somewhat expected. However, prior to this event, we did not detect any anomalies indicating low surface b-values, which are often indicative of a high-stress state in the source area before strong earthquakes, as highlighted by numerous studies. Our research focused on the northern section of the eastern boundary of the Sichuan–Yunnan sub-block, encompassing the Xianshui He, Anning He, Zemu He, and Daliang Shan fault belts. We meticulously located earthquakes of ML ≥ 1.5 from 2009 to May 2022. The catalog was divided into two periods: 2009–2014 and 2015–May 2022. Using an AIC-constraint method, we analyzed the changes in b-values (Δb) in the latter period compared to the former. Our findings revealed a significant abnormal Δb zone (Δb < −0.3), with a radius of approximately 50 km, when ΔAIC ≥ 2 was selected. Intriguingly, the epicenter of the recent Luding MS 6.8 earthquake fell within this abnormal zone. Furthermore, we calculated the b-value cross-section for the southern section of the Xianshui He fault belt using a directory of precisely located small earthquakes. This revealed that the location, scale, and shape of the abnormally low-b-value area corresponded with the large displacement co-seismic area of the main earthquake, affirming the b-value’s effectiveness in identifying asperities. The b-value’s temporal evolution prior to the mainshock exhibited a nearly decade-long continuous decrease, signifying a long-term stress-loading process akin to that observed before many strong earthquakes. The b-value anomalies observed from different profiles before the Luding earthquake underline the necessity of a comprehensive, multi-dimensional analysis of such anomalies. Finally, our analysis indicates that nine earthquakes with MS ≥ 6.5, including the Luding MS 6.8 event, have contributed to increased Coulomb Failure Stress change (ΔCFS) in the Daofu (DF)–Kangding (KD) section of the Xianshui He fault belt and the northern section of the Anning He fault belt south of Shimian (SM), with amplitudes surpassing the 0.01 MPa threshold. This suggests the potential for strong earthquakes in these zones.

Fault Kinematics of the 2023 Mw 6.0 Jishishan Earthquake, China, Characterized by Interferometric Synthetic Aperture Radar Observations

Characterizing the coseismic slip behaviors of earthquakes could offer a better understanding of regional crustal deformation and future seismic potential assessments. On 18 December 2023, an Mw 6.0 earthquake occurred on the Lajishan–Jishishan fault system (LJFS) in the northeastern Tibetan Plateau, causing serious damage and casualties. The seismogenic fault hosting this earthquake is not well constrained, as no surface rupture was identified in the field. To address this issue, in this study, we use Interferometric Synthetic Aperture Radar (InSAR) data to investigate the coseismic surface deformation of this earthquake and invert both ascending and descending line-of-sight observations to probe the seismogenic fault and its slip characteristics. The InSAR observations show up to ~6 cm surface uplift caused by the Jishishan earthquake, which is consistent with the thrust-dominated focal mechanism. A Bayesian-based dislocation modeling indicates that two fault models, with eastern and western dip orientations, could reasonably fit the InSAR observations. By calculating the coseismic Coulomb failure stress changes (∆CFS) induced by both fault models, we find that the east-dipping fault scenario could reasonably explain the aftershock distributions under the framework of stress triggering, while the west-dipping fault scenario produced a negative ∆CFS in the region of dense aftershocks. Integrating regional geological structures, we suggest that the seismogenic fault of the Jishishan earthquake, which strikes NNE with a dip of 56° to the east, may be either the Jishishan western margin fault or a secondary buried branch. The optimal finite-fault slip modeling shows that the coseismic slip was dominated by reverse slip and confined to a depth range between ~5 and 15 km. The released seismic moment is 1.61 × 1018 N·m, which is equivalent to an Mw 6.07 earthquake. While the Jishishan earthquake ruptured a fault segment of approximately 20 km, it only released a small part of the seismic moment that was accumulated along the 220 km long Lajishan–Jishishan fault system. The remaining segments of the Lajishan–Jishishan fault system still have the capability to generate moderate-to-large earthquakes in the future.

Constraining Parameter Uncertainties in the Coulomb Rate–State Approach: A Case Study of Seismicity in the Longmenshan Region after the 2008 Mw 7.9 Wenchuan Earthquake, China

Abstract The rate–state frictional law, coupled with the Coulomb failure stress changes (ΔCFS), is one of the most popular physics-based models to forecast seismicity rate changes following a major earthquake. However, its effectiveness is hampered by parameter uncertainties. To seek possible solutions for such uncertainties, this article carried out retrospective forecasts of the decade-long seismicity in the Longmenshan region, China, after the 2008 Mw 7.9 Wenchuan earthquake, and proposed methods to constrain parameter uncertainties. First, we derived spatially variable ta and Aσ from fault-slip rates. This method not only provides observational constraints for these two parameters but also reflects spatial variations of fault rate–state properties. Second, although both complete and declustered background catalogs are common in Coulomb rate–state forecasts, this study demonstrated that declustering avoids false alerts of seismicity rate increase that resulted from temporary seismicity fluctuations in the background catalog. Finally, we extended the model from its typical application based on a stress step (the coseismic stress change) to a calculation that allows a more complex stress evolution (the postseismic viscoelastic stress change). With these methods to constrain parameter uncertainties, we are able to obtain more reliable forecasts.

The 2022 Goesan earthquake of the moment magnitude 3.8 along the buried fault in the central Korean Peninsula

AbstractOn October 28, 2022, a moment magnitude (Mw) 3.8 earthquake occurred in Goesan, South Korea, typically characterized as a stable continental region. Herein, we analyze 42 earthquakes, including the Mw 3.8 earthquake, the largest foreshock (Mw 3.3), which preceded the mainshock by 17 s, and the largest aftershock (Mw 2.9). The primary aim of this study is to identify interactions among the seismic events. To this end, we utilized the permanent seismic networks with the closest station at 8.3 km from the epicenter, and the temporary network deployed eight hours after the mainshock’s occurrence. Relocation results delineate that the mainshock occurred at the southeastern tip of the hypocenter distribution of three foreshocks, trending west-northwest–east-southeast. The aftershocks form an overall spatially diffused seismic pattern that propagates toward both ends of the inferred lineament in the downdip direction. The rupture directivity of the mainshock, along with waveform similarity across the mainshock and foreshocks, confirms the inferred geometry, corresponding well with the focal mechanisms of the mainshock and the largest foreshock. We demonstrate that the change in Coulomb failure stress (ΔCFS) by the largest foreshock was positive where the mainshock occurred and that the mainshock generated ΔCFS capable of triggering the propagation of the aftershocks.

Stress triggering and future seismic hazards implied by four large earthquakes in the Pamir from 2015 to 2023 revealed by Sentinel-1 radar interferometry

SUMMARY The Mw 6.8 Murghob earthquake is the third earthquake in an Mw 6.4+ sequence occurring in the Pamir initiated by the 2015 Sarez Mw 7.2 earthquake. It is of great significance to investigate their interactions and to assess future seismic hazards in the region. In this paper, we use Sentinel-1 radar interferometric data to retrieve coseismic deformation, invert for the slip distributions of the four events, and then investigate their interactions. The cumulative Coulomb failure stress changes (ΔCFS) suggest that the 2023 Murghob earthquake was promoted by the three prior earthquakes in the sequence. Pre-stress from historical earthquakes is a key factor in explaining the triggering mechanism of the two 2016 Mw 6.4+ earthquakes. Stress loading and unloading effects on major faults in the region indicate that future attention should be paid in (1) the segment of the Sarez-Karakul fault north of the Kokuibel Valley, (2) the segment of the Sarez-Murghab thrust fault west of the Sarez-Karakul fault and (3) the east segments of the Pamir thrust fault system, all with a large positive ΔCFS.

Resolving a ramp-flat structure from combined analysis of co- and post-seismic geodetic data: an example of the 2015 Pishan Mw 6.5 earthquake

SUMMARY Previous studies have shown that it is difficult to determine whether the 2015 Pishan earthquake occurred on a uniform fault or a ramp-flat fault with variable dip angles due to the similar goodness of data fit to coseismic and afterslip models on these two fault models. Here, we first present the InSAR deformation obtained from both ascending and descending orbits, covering the coseismic period and cumulative 5-yr period after the 2015 Pishan earthquake. We then determine the preferred fault geometry by the spatial distributions between the positive Coulomb failure stress change triggered by main shock and the afterslip. Based on the preferred fault model, we finally use a combined model to determine the contributions of elastic and viscoelastic deformation in the post-seismic deformation. We find that the Pishan earthquake prefers to occur on a ramp-flat fault, and the coseismic slip is mainly distributed at a depth of 9–13 km, with a maximum slip of about 1.3 m. The post-seismic deformation is primarily governed by afterslip, as the poroelastic rebound-induced deformation fails to account for the observed post-seismic deformation and the contributions from the viscoelastic relaxation mechanism can be considered negligible in the combined model. Moreover, the modelled stress-driven afterslip and observed kinematic afterslip have good consistency, and the difference between the root mean square error of the two afterslip models is only 4.3 mm. The results from the afterslip model indicate that both of the updip and downdip directions distribute the afterslip, and slip in the updip direction is greater than that of the downdip direction. Meanwhile, the maximum cumulative afterslip after 5 yr is approximately 0.26 m which is equivalent to a released seismic moment of a Mw 6.47.

InSAR, Gravity, and Geomagnetic Observations Suggest the 2020 Mw 6.5 Stanley Earthquake Occurred on a Blind Strike-Slip and a Normal Fault

ABSTRACT The 2020 Mw 6.5 Stanley earthquake occurred in western North America and is the largest earthquake in Idaho state in the past ∼40 yr. The hypocenter is located at the western margin of the Centennial Tectonic Belt (CTB), which is a subprovince of the Basin and Range and characterized by a series of normal faults. In this study, the coseismic Interferometric Synthetic Aperture Radar observations, geophysical data, and aftershocks are jointly collected to estimate the seismogenic fault of the 2020 Stanley earthquake. The best-fitting fault models suggest the earthquake occurred on two blind faults. Slip on the main, west-dipping, seismogenic fault was sinistral and on the second fault predominantly normal slip with a minor component of right-lateral strike slip. Geophysical observations including gravity and geomagnetic data are consistent with the fault interpretations. The Coulomb failure stress (CFS) change indicates that the rupture of the main seismogenic fault had a positive triggering effect on the slip of the second fault and may have increased the rupture risk of the Sawtooth fault. Moreover, a potential seismic hazard risk zone with a notably low occurrence of aftershocks is found in the west of the central segment of the main seismogenic fault.

Viscoelastic stress change from the 1931 MW7.8 Fuyun earthquake and its impacts on seismic activity around the Altai mountains

The 1931 MW7.8 Fuyun earthquake occurred around the Altai mountains, an intracontinental deformation belt with limited active strain-rate accumulation. To explore whether seismic activity in this deformation belt was affected by stress interaction among different active faults, we calculate the Coulomb failure stress change (ΔCFS) induced by the Fuyun earthquake due to coseismic deformation of the elastic crust and postseismic viscoelastic relaxation of the lower crust and upper mantle. Numerical results show that the total ΔCFS at a 10-km depth produced by the Fuyun earthquake attains approximately 0.015–0.134 bar near the epicenter, and just before the occurrence of the 2003 MW7.2 Chuya earthquake, which distances about 400 km away from the Fuyun earthquake. Among the increased ΔCFS, viscoelastic relaxation from 1931 to 2003 contributes to approximately 0.014–0.131 bar, accounting for >90% of the total ΔCFS. More importantly, we find that for the recorded seismicity in the region with a radius of about 270 km to the Fuyun earthquake from 1970 to 2018, the percentage of earthquakes that fall in positive lobes of ΔCFS resolved on the NNW-SSE Fuyun strike-slip fault, on the NWW-SEE Irtysh strike-slip fault, and on the NW-SE Kurti reverse fault is up to 67.22%–91.36%. Therefore, the predicted ΔCFS suggests that the impact of the 1931 MW7.8 Fuyun earthquake on seismic activity around the Altai mountains is still significant as to hasten occurrence of the 2003 MW7.2 Chuya earthquake at a relatively far distance and to trigger its aftershocks in the near-field even after several decades of the mainshock.

Coseismic Slip Distribution and Coulomb Stress Change of the 2023 MW 7.8 Pazarcik and MW 7.5 Elbistan Earthquakes in Turkey

On 6 February 2023, the MW 7.8 Pazarcik and the MW 7.5 Elbistan earthquakes occurred in southeastern Turkey, close to the Syrian border, causing many deaths and a great deal of property destruction. The Pazarcik earthquake mainly damaged the East Anatolian Fault Zone (EAFZ). The Elbistan earthquake mainly damaged the Cardak fault (CF) and the Doğanşehir fault (DF). In this study, Sentinel-1A ascending (ASC) and descending (DES) orbit image data and pixel offset tracking (POT) were used to derive surface deformation fields in the range and azimuth directions induced by the Pazarcik and Elbistan earthquakes (hereinafter referred to as the Turkey double earthquakes). Utilizing GPS coordinate sequence data, we computed the three-dimensional surface deformation resulting from the Turkey double earthquakes. The surface deformation InSAR and GPS results were combined to invert the coseismic slip distribution of the EAFZ, CF, and DF using a layered earth model. The results show that the coseismic ruptures of the Turkey double earthquakes were dominated by left-lateral strike-slips. The maximum slip was 7.76 m on the EAFZ and about 8.2 m on the CF. Both the earthquakes ruptured the surface. The Coulomb failure stress (CFS) was computed based on the fault slip distribution and the geometric parameters of all the active faults within 300 km of the MW 7.8 Pazarcik earthquake’s epicenter. The CFS change resulting from the Pazarcik earthquake suggests that the subsequent Elbistan earthquake was triggered by the Pazarcik earthquake. The Antakya fault experienced an increase in CFS of 8.4 bars during this double-earthquake event. Therefore, the MW 6.3 Uzunbağ earthquake on 20 February 2023 was jointly influenced by the Turkey double earthquakes. Through stress analysis of all the active faults within 300 km of the MW 7.8 Pazarcik earthquake’s epicenter, the Ecemis segment, Camliyayla fault, Aadag fault, Ayvali fault, and Pula segment were all found to be under stress loading. Particularly, the Ayvali fault and Pula segment exhibited conspicuous stress loading, signaling a higher risk of future seismic activity.

Potential Poroelastic Triggering of the 2020 M 5.0 Mentone Earthquake in the Delaware Basin, Texas, by Shallow Injection Wells

ABSTRACT The Delaware basin in Texas, one of the largest oil and gas production sites in the United States, has been impacted by widespread seismicity in recent years. The M 5.0 earthquake that occurred in March 2020 near the town of Mentone is one of the largest induced earthquakes recorded in this region. Characterizing the source parameters and triggering mechanism of this major event is imperative to assess and mitigate future hazard risk. A former study showed that this event may be attributed to the deep injection nearby. Interestingly, the earthquake is in proximity to shallow injection wells with much larger total injection volume. In this study, we investigate the role of these shallow injection wells in the triggering of the M 5.0 event despite their farther distance from the mainshock. We perform source-parameter inversion and earthquake relocation to determine the precise orientation of the south-facing normal-fault plane where the mainshock occurred, followed by fully coupled poroelastic stress modeling of the change of Coulomb failure stress (ΔCFS) on the fitted fault plane caused by shallow injection in the region. Results show that shallow wells caused up to 20 kPa of ΔCFS near the mainshock location, dominated by positive poroelastic stress change. Such perturbation surpasses the general triggering threshold of faults that are well aligned with the local stress field and suggests the nonnegligible role of these shallow wells in the triggering of the mainshock. We also discuss the complex effect of poroelastic stress perturbation in the subsurface and highlight the importance of detailed geomechanical evaluation of the reservoir when developing relevant operational and safety policies.

The 2023 Mw 7.8 and 7.6 Earthquake Doublet in Southeast Türkiye: Coseismic and Early Postseismic Deformation, Faulting Model, and Potential Seismic Hazard

Abstract On 6 February 2023, Mw 7.8 and 7.6 earthquakes struck southeast Türkiye and northwest Syria. They are the largest earthquakes in Türkiye in over 80 yr, causing significant damage and fatalities. We used Advanced Land Observation Satellite-2 and Sentinel-1 Synthetic Aperture Radar images to obtain near-field coseismic displacements by Differential Interferometric Synthetic Aperture Radar (DInSAR) and pixel offset tracking (POT). Discontinuities of the surface deformation suggest that the Mw 7.8 event ruptured ∼320 km along the East Anatolian fault, and the Mw 7.6 event ruptured ∼150 km near Elbistan, southern Türkiye. We inverted these earthquakes' fault geometry and slip distribution based on Global Navigation Satellite Systems, DInSAR, and POT displacements. The estimated fault slip model shows that the first Mw 7.8 event ruptured a steeply southeast-dipping fault, and the seismogenic fault of the second Mw 7.6 event is north-dipping with complex geometry. The dip angle of subfaults of the Mw 7.6 earthquake decreases from east to west. Faults responsible for the two earthquakes are dominated by left-lateral strike-slip motion, with the maximum slip of ∼9.1 m. Early postseismic deformation within two months exhibits displacement discontinuities in the Amanos and Pazarcık segments and the Çardak fault, suggesting that the afterslip partially compensated the coseismic slip deficit at the shallow depths. Furthermore, static Coulomb failure stress changes induced by the two earthquakes indicate that the southwestern Pütürge segment of the East Anatolian fault has a high risk of future rupture.