Coseismic Coulomb Stress Changes Research Articles

Stress Evolution of the Main Himalayan Thrust Fault Induced by the Mw 7.8 Gorkha Earthquake

ABSTRACT The Mw 7.8 Gorkha earthquake that occurred in 2015 is the largest thrust event in the middle of the main Himalayan thrust fault zone (MHT) in the past 81 yr. Its impact on the regional tectonic stress state and future seismic risks is a significant scientific issue worthy of in-depth analyses. In this study, we inverted the planar fault-slip model (the planar model), the flat-ramp fault-slip model (the flat-ramp model) and the double flat-ramp fault-slip model (the double flat-ramp model) to analyze the effect of the fault geometry, based on the steepest descent method (SDM) and the layered earth model. Compared with the flat-ramp model, the planar model exhibits a wider slip distribution in the down-dip direction of the main rupture zone, whereas the double flat-ramp model shows a larger coseismic slip on the middle ramp at the depth of 7.5–11.5 km. Those slip differences produce larger stress shadow areas in the above models, but for a blind thrust event induced by a low-dip thrust fault, this does not significantly change the distribution mode of coseismic Coulomb stress change (ΔCFS) in the three models. Namely, there is an obvious stress release in the main rupture but an obvious stress loading in the up-dip and down-dip directions of the main rupture zone. Based on the Burgers rheological model, we calculated the postseismic viscoelastic Coulomb stress change (V−ΔCFS) and the cumulative Coulomb stress change (C−ΔCFS) induced by the Gorkha earthquake in the flat-ramp model and comparatively analyzed the evolution pattern of coseismic and postseismic stresses. Our results indicate that the variation trend of postseismic stress in the lithosphere is opposite to that of the coseismic ΔCFS. The postseismic viscoelastic relaxation promotes the slip of the flat-ramp structure at the depth of 10–25 km, and the stress unloads in the shallow and deep part simultaneously. As a blind thrust event, the coseismic ΔCFS still plays a dominant role in the shallow part after 50 yr, whereas the loading C−ΔCFS in the deep part of the MHT is greatly weakened by the postseismic V−ΔCFS. Seismic risks still exist in the unruptured area on the west side of the mainshock and the shallow Main Frontal thrust.

Present-day crustal movement and seismic moment balance on major active faults around the easternmost segment of the Kunlun fault, Tibetan Plateau, China

After the 2021 Ms 7.4 Madoi, China earthquake located south of the eastern Kunlun fault, the coseismic Coulomb stress changes show that the earthquake potential is enhanced towards the easternmost segment of the Kunlun fault. Before the 2008 Mw 7.9 Wenchuan, China earthquake, the easternmost segment of the Kunlun fault is considered to being a region with the highest frequency occurrence of moderate-to-strong earthquakes in mainland China, where there are still at least three seismic gaps at present, indicating an urgent earthquake hazard. The tectonic deformation pattern is closely related to the development and occurrence of earthquakes. In this study, we collect all the available GPS data to obtain a crustal movement velocity field with high precision and high resolution by removing nontectonic effects within the study area. The GPS velocity profile and two-dimensional strain rates are then derived from the updated GPS velocities to analysis the characteristics of crustal deformation. Our results demonstrate that the Longriba fault zone slips predominantly right-laterally with the rate of 5.2 ± 0.5 mm/yr and the crustal shortening rate is ∼0.5 mm/yr across the Longriba and Longmenshan faults. Along with notable crustal extension and contraction, the principal strain rate field is distributed in a wide area on the northwest part of the Longmenshan fault, and decreases southwestward along the Longmenshan fault zone. In addition, we provide the time series of 8 campaign GPS stations which have never been published before. Moreover, the moment deficit estimate is a well-established way of assessing the earthquake hazard at a specific fault. Accompanying with the inverted slip rates and locking depth, we estimate the rates of moment accumulation on major faults and compare them with the seismic moment released on each fault derived from the Chinese historical earthquake catalog which extends for more than 2000 years in many regions. We find that the moment deficit is mainly concentrated on the Longriba fault with a value of 4.18 × 1017 Nm/yr. The Longriba fault have accumulated enough seismic energy to generate one Mw > 7.7 earthquake on it, or two Mw > 7.5 earthquakes on its two branches in the future. This study is an essential indicator for understanding the present-day tectonic deformation pattern in the Tibetan Plateau and is also helpful for seismic hazard assessment.

Three-dimensional finite-element modeling of Coulomb stress changes on normal and thrust faults caused by pore fluid pressure changes and postseismic viscoelastic relaxation

Abstract The analysis of Coulomb stress changes has become an important tool for seismic hazard evaluation because such stress changes may trigger or delay subsequent earthquakes. Processes that can cause significant Coulomb stress changes include coseismic slip and transient postseismic processes such as poroelastic effects and viscoelastic relaxation. However, the combined influence of poroelastic effects and viscoelastic relaxation on co- and postseismic Coulomb stress changes has not been systematically studied so far. Here, we use three-dimensional finite-element models with arrays of normal and thrust faults to investigate how pore fluid pressure changes and viscoelastic relaxation overlap during the postseismic phase. In different experiments, we vary the permeability of the upper crust and the viscosity of the lower crust or lithospheric mantle while keeping the other parameters constant. In addition, we perform experiments in which we combine a high (low) permeability of the upper crust with a low (high) viscosity of the lower crust. Our results show that the coseismic (i.e., static) Coulomb stress changes are altered by the signal from poroelastic effects and viscoelastic relaxation during the first month after the earthquake. For sufficiently low viscosities, the Coulomb stress change patterns show a combined signal from poroelastic and viscoelastic effects already during the first postseismic year. For sufficiently low permeabilities, Coulomb stress changes induced by poroelastic effects overlap with the signals from viscoelastic relaxation and interseismic stress accumulation for decades. Our results imply that poroelastic and viscoelastic effects have a strong impact on postseismic Coulomb stress changes and should therefore be considered together when analyzing Coulomb stress transfer between faults.

Revisiting the seismic hazards of faults surrounding the 2022 Ms6.8 Luding earthquake, Sichuan, China

The occurrence of the Ms6.8 Luding earthquake on 5 September 2022 filled the seismic gap in the southeastern segment of the Xianshuihe fault zone and transferred stress to the surrounding faults, which attracted much more attention to the seismic hazard. In this study, we calculate the coseismic Coulomb stress changes caused by the Ms6.8 Luding earthquake and analyze its relationship with the relocated aftershocks, and further explore the stress evolution before the Luding earthquake and seismic hazards on the surrounding faults after it. The results show significant coseismic Coulomb stress increases in the northern Anninghe fault zone and the coseismic unruptured section of the southeastern Xianshuihe fault zone. Aftershocks are highly correlated with the positive coseismic Coulomb stress change, wherein eight out of eleven aftershocks of M ≥ 3.0 occurred in areas with significant stress increases ranging from 0.03 MPa to 0.44 MPa, indicating the triggering effect. The Moxi fault experiences co- and post-seismic Coulomb stress changes due to historical M ≥ 7.0 strong earthquakes since 1515, which help understand the seismogenic mechanism of the Luding earthquake. Moreover, the Luding earthquake advanced the potential earthquakes in the northern section of the Anninghe fault zone by approximately 60 years, further highlighting the potential seismic hazards.

SLIP SOURCE MODEL OF THE 1995 NEFTEGORSK EARTHQUAKE (NORTH SAKHALIN) FROM GEODETIC DATA

The May 27, 1995 Mw=7.0 Neftegorsk earthquake occurred in the north of Sakhalin Island, rupturing the Upper Piltun fault, a secondary feature of the main Hokkaido-Sakhalin regional fault zone. The fault geometry, coseismic slip model, and Coulomb stress changes in the earthquake focal area were calculated based on a finite fault modeling. We used near-field coseismic offsets at 24 points obtained by comparison between predating triangulation and GPS observations, which were collected before and after the earthquake. Our slip distribution model shows two major slip patches. Larger slip asperity (amplitude up to 6.36 m) was characterized by right-lateral strike-slip movements, which correspond to focal mechanism of the earthquake, whereas the northern segment has reverse fault mechanism with maximum slip of 2.64 m. The fault length and width, average slip and stress drop values are estimated at 78 km, 28 km, 1.91 m and 11.3 MPa, respectively. The estimated release moment is approximately 7.49×1019 N∙m equal to Mw=7.2, which is larger than that reported by the USGS and GCMT but consistent with the values reported by other researchers. The coseismic Coulomb stress changes enhanced the stress by more than 10 MPa on the southern segment of the Gyrgylaninsky fault and middle section of the Hokkaido–Sakhalin fault. Seismic risks on the nearest faults cannot be ignored in the future despite the fact that the earthquake with a magnitude of 5.8 occurred in 2010 near the Gyrgylaninsky fault. The recent GPS rates in the surroundings of the Neftegorsk surface rupture mean that the recurrence interval for similar earthquakes may be more than a thousand years.

Coseismic Coulomb stress changes induced by a 2020–2021 MW > 7.0 Alaska earthquake sequence in and around the Shumagin gap and its influence on the Alaska-Aleutian subduction interface

Three MW > 7.0 earthquakes in 2020–2021 occurred in the Shumagin seismic gap and its adjacent area of the Alaska-Aleutian subduction zone, including the MW7.8 Simeonof thrust earthquake on July 22, 2020, the MW7.6 Sand Point strike-slip earthquake on October 19, 2020, and the MW8.2 Chignik thrust earthquake on July 29, 2021. The spatial and temporal proximity of these three earthquakes prompts us to probe stress-triggering effects among them. Here we examine the coseismic Coulomb stress change imparted by the three earthquakes and their influence on the subduction interface. Our results show that: (1) The Simeonof earthquake has strong loading effects on the subsequent Sand Point and Chignik earthquakes, with the Coulomb stress changes of 3.95 bars and 2.89 bars, respectively. The Coulomb stress change caused by the Sand Point earthquake at the hypocenter of the Chignik earthquake is merely around 0.01 bars, suggesting the negligible triggering effect on the latter earthquake; (2) The triggering effects of the Simeonof, Sand Point, and Chignik earthquakes on aftershocks within three months are not well pronounced because of the triggering rates of 38%, 14%, and 43% respectively. Other factors may have played an important role in promoting the occurrence of these aftershocks, such as the roughness of the subduction interface, the complicated velocity structure of the lithosphere, and the heterogeneous prestress therein; (3) The three earthquakes caused remarkable coseismic Coulomb stress changes at the subduction interface nearby these mainshocks, with an average Coulomb stress change of 3.2 bars in the shallow region directly inwards the trench.

Coseismic Coulomb stress changes on intraplate faults in the western Quebec seismic zone following three major earthquakes in the past century

There is currently no active fault map for the intraplate western Quebec seismic zone (WQSZ) in eastern Canada, and consequently, no detailed finite-fault source models, which are critical for seismic hazard assessments in this region with a rapidly growing population. While previous numerical stress modelling studies have shown that mostly NNW–SSE to NW–SE-striking faults exhibit the highest potential for reactivation under the present-day tectonic stress field, such modelling is unable to take into account the interaction of faults and earthquakes. This study attempts to identify possible future rupture zones using Coulomb stress analysis. We explore the static stress transfer caused by the 1935 M W 6.1 Témiscaming, 1944 M W 5.8 Cornwall–Massena, and 2013 M W 4.7 Ladysmith earthquakes, which are proximal to faults in the WQSZ that exhibit a relatively high reactivation potential, to determine whether these faults have an increased potential for failure. The significance of Coulomb stress changes (ΔCFS) observed on the nearby “receiver” faults varied widely. Among the events analyzed in this study, only the 1935 M W 6.1 Témiscaming earthquake caused extensively positive ΔCFS (≥0.1 bar) on its receiver fault. The areal extent of the receiver fault that has been promoted to failure suggests that earthquakes with a comparable magnitude to the 1935 event can be triggered. This work is the first attempt to provide a physical basis for seismic hazard assessment input parameters in the WQSZ based on the results of numerical stress modelling.

Seismogenic structures and spatiotemporal seismicity patterns of the 2022 Ms6.0 Maerkang earthquake sequence, Sichuan, China

The 2022 Ms6.0 Maerkang earthquake sequence, Sichuan, China, occurred in an unexpected area with historically rare seismicity in the Bayan Har block. Here we relocated the earthquake sequence, inverted for the focal mechanisms of the larger events, and calculated the rupture directivity of the earthquake sequence to reveal the seismogenic structures and mechanisms of this sequence. The high-precision relocations indicate that the seismogenic structures consist of several clusters that are generally parallel to the nearby NW-trending Songgang fault, and relatively small-scale conjugate faults are also identified. The seismicity migrated from cluster one in the south to cluster two in the north during the sequence. Furthermore, the hypocenters were largely located at 5–10 km depth, thereby highlighting that the seismogenic structures are buried. The vertical fault planes of the seismogenic structures are consistent with the high-dip focal mechanism solutions from seven events. A stress field inversion based on the focal mechanisms indicates that the sequence occurred in a strike-slip environment that was controlled by a NNW–SSE-striking principal compressive stress. The different rupture directivities of theMs5.8 (southwestward) andMs6.0 (southeastward) events prove the existence of conjugate faults. TheMs5.8 event induced a coseismic Coulomb stress change of 1.6 MPa where theMs6.0 event subsequently occurred, thereby highlighting that theMs5.8 event triggered theMs6.0 event and produced the spatiotemporal seismicity pattern of the sequence. We therefore conclude that the seismogenic structures of the 2022 Ms6.0 Maerkang earthquake sequence are previously unknown concealed conjugate structures associated with the main Songgang fault. The complex seismogenic structures and their potential to generate large earthquakes warrant the need to better understand the seismogenesis of this area and the seismic risks that may be present.

Interaction between historical earthquakes and the 2021 Mw7.4 Maduo event and their impacts on the seismic gap areas along the East Kunlun fault

On May 21, 2021 (UTC time), an Mw7.4 earthquake struck Maduo County, Qinghai Province, China. The rupture of this typical strike-slip event and its aftershocks along the Kunlun-Jiangcuo fault (JCF) propagated approximately 170 km from the epicenter. In this study, we calculated the coseismic and postseismic Coulomb stress changes induced by 14 historical earthquakes and investigated their impacts on the 2021 Maduo source area. We found that the JCF is in the stress shadow of these historical events with a combined ΔCFS range of approximately − 400 to − 200 kPa. Since the seismogenic fault of the 1937 event is nearly parallel and close to the JCF, the rupture of the 1937 event had the greatest inhibitory effect on Maduo source area. We hypothesize that the actual loading rate at the depth of the seismogenic layer in the Maduo source area is much higher than the simulated value (0.3 kPa/a). Consequently, the Maduo earthquake still occurred despite the considerable delaying effect of these historical earthquakes (especially the 1937 event). Our findings also indicate that the tectonic stress in the eastern Bayanhar block is still rapidly accumulating and adjusting. Our investigation further reveals the enhanced stress induced by the historical and Maduo events with ΔCFS values of approximately 30–300 kPa and 50–300 kPa on the Xidatan-Dongdatan segment (XDS) and the eastern end of the East Kunlun Fault (EKF), respectively, not only on the Maqin-Maqu segment (MMS) but also at the eastern end of each branch segment of the EKF. Hence, considering the accumulation of tectonic stress, we suggest that the seismic hazard in these two regions has been promoted.Graphic

The 2019 September 24, Mw = 6, Mirpur earthquake, NW Himalaya: Geodetic evidence for shallow, near-horizontal décollement rupture of the Main Himalayan Thrust

We present a source model for the 2019 Mw = 6 Mirpur earthquake, NW Himalaya using Interferometric Synthetic Aperture Radar (InSAR) measurements. Bayesian inversion of InSAR data from both ascending and descending orbits suggests that the earthquake ruptured a shallow (Depth ~ 5 km), near-horizontal (Dip ~2.5°) up-dip portion of the décollement of the Main Himalayan Thrust (MHT). The distributed slip model suggests a compact rupture terminating the up-dip end at the base of the Main Frontal Thrust (MFT) with higher slip (> 0.4 m) around the hypocentre, equivalent to a moment magnitude of Mw = 6. A shallow up-dip rupture of the MHT through a moderate magnitude earthquake is atypical as Himalayan earthquakes generally originate at the down-dip portion of the MHT and propagate towards south. We estimate a low effective coefficient of friction of 0.06 ± 0.02 from the slip model and suggest that high pore fluid pressures and/or a weak, lubricated portion of décollement could have caused a local, near-horizontal rupture at the base of the MFT. The 2019 Mirpur earthquake released a small fraction of the accumulated strain energy since the 1555 Kashmir earthquake. Coseismic Coulomb stress change analysis suggests a significant increase in stress on the locked, up-dip portion of the MHT and the frontal fold-thrust system. These findings compel a revisit of the seismic hazard assessment of the northwestern Himalaya.

Rupture Process of the 2020 Mw7.0 Samos Earthquake and its Effect on Surrounding Active Faults

AbstractOn October 30, 2020, a Mw7.0 normal faulting earthquake struck the eastern Aegean Sea, causing casualties, and substantial damage at Samos island and Izmir province. We constrained the rupture history of the main shock by jointly inverting GPS static offsets, high‐rate GPS recordings, strong motion waveforms, and teleseismic broadband data. The results revealed that the slip distribution was dominated by a slip patch which spans a depth range of 3–13 km and occupies ∼30 km along strike. The rupture propagated toward SW and updip and the maximum slip amplitude reached 4.6 m at 7 km depth. The remarkably low aftershock productivity within the main asperity suggests a near complete stress release. Coseismic Coulomb stress changes effectively explain the entire aftershock sequence implying absence of short‐term postseismic stress transfer mechanisms. The static stress change calculations also suggest that active faults around Sıgacık and Kusadasi Bays have been brought closer to rupture.

AutoCoulomb: An Automated Configurable Program to Calculate Coulomb Stress Changes on Receiver Faults with Any Orientation and its Application to the 2020 Mw 7.8 Simeonof Island, Alaska, Earthquake

AbstractCoulomb stress change is the change in resultant force of shear stress and friction imposed on a receiver fault plane. The resulting stress change is often computed using the Coulomb 3.4 and the postseismic Green’s functions and postseismic components (PSGRN-PSCMP) programs. Notwithstanding both preferences, both have incomplete optimally oriented failure planes (OOPs) and are inconvenient to resolve Coulomb stress changes on various fault planes placed in varying depths. Here, we present an alternative program termed AutoCoulomb. It leverages the shell command-line tool to automatically batch-process Coulomb stress changes on all sorts of receiver fault planes. We first validate the program. We then apply it to the 2020 Mw 7.8 Simeonof Island, Alaska, earthquake, as a case study. Our results show that Coulomb stress changes resolved on fixed receiver faults, using the three programs, are in line with each other. So are those resolved on 3D OOPs using the PSGRN–PSCMP and the AutoCoulomb programs. Nevertheless, Coulomb stress changes on 2D OOPs, generated by the AutoCoulomb program, always outweigh those done by the Coulomb 3.4 program, indicating that 2D OOPs constrained by the latter are not the most optimal. Some nonoptimal 2D OOPs result in the reversal of the signs of Coulomb stress changes, posing a risk of misleading stress shadows with negative Coulomb stress changes. For the case study, the 28 July 2020 Mw 6.1 aftershock received a positive coseismic Coulomb stress change of ∼3.5 bars. In contrast, the compounded coseismic Coulomb stress changes at the hypocenters of the 1946 Mw 8.2, the 1948 Mw 7.2, and the 2020 Mw 7.8 earthquakes are within a range from −1.1 to 0.1 bar, suggesting that coseismic Coulomb stress changes promoted by preceding mainshocks alone are not responsible for these mainshocks. Other factors, such as postseismic viscoelastic relaxation, afterslip, and slow slip, may contribute to promoting their occurrence.

The 2019 Mw 5.9 Torkaman chay earthquake in Bozgush mountain, NW Iran: A buried strike-slip event related to the sinistral Shalgun-Yelimsi fault revealed by InSAR

On 7 November 2019, an Mw 5.9 earthquake with a left-lateral strike-slip mechanism struck the Bozgush mountain in NW Iran, marking the largest event in the mountain area in the past 44 years. Because no surface rupture associated with the earthquake has been reported, the causative fault of the earthquake is unknown. This event provides an opportunity to gain insight into the active tectonics of the region. Here, we acquire the coseismic deformation associated with this earthquake through Sentinel-1A interferometric synthetic aperture radar (InSAR) data and further invert the fault geometry and the detailed coseismic slip of this event based on the planar fault model. By a joint analysis of the inversion results, aftershocks sequence and local tectonics, we propose that the buried branch fault splaying from the Shalgun-Yelimsi fault (SYF) at depth may be responsible for this earthquake. Both the uniform and distributed slip models infer a smaller fault rupture size than predicted by classical fault scaling relation. We suggest that the subsurface fault geometry of the SYF may stop the rupture propagation further down to southeast at depth, thus resulting in high slip and stress drop at such a small fault plane. Considering the proximity of the epicenter to the SYF, we also invert for the distributed slip model on an anti-listric fault with the purpose to explore the possible complexity of rupture fault. However, due to the lack of obvious improvement to fit the observed data and the limited slip in the deep fault segment, the planar fault model is still regarded as the optimal one. According to the calculated coseismic Coulomb stress changes, we find that 55 % of aftershocks are triggered by the statics stress and that the northern segment of SYF and the eastern segment of SBF have a potential rupture risk. The occurrence of this earthquake, together with the prominent offset feature and slip style of surrounding active fault, indicates an active conjugate fault system, which may accommodate the small-scale northeastward lateral extrusion.

Source model for the Mw 6.0 earthquake in Jiashi, China on 19 January 2020 from Sentinel-1A InSAR data

On January 19, 2020, an Mw 6.0 earthquake occurred in Jiashi, Xinjiang Uygur Autonomous Region of China. The epicenter was located at the basin-mountain boundary between the southern Tian Shan and the Tarim Basin. Interferometric Synthetic Aperture Radar (InSAR) is used to obtain the coseismic deformation field from both ascending and descending Sentinel-1A satellite images of the European Space Agency. The results showed that the coseismic deformation is distributed between the Kalping fault and the Ozgertaou fault. The earthquake produced significant deformation over an area of approximately 40 km by 30 km. The maximum and minimal displacements along the line of sight (LOS) are 5.3 cm and − 4.2 cm for the ascending interferogram and are 7.2 cm and − 3.0 cm for the descending interferogram, respectively. The fault geometry from the Multi peak Particle Swarm Optimization computation indicates that the seismogenic fault is a shallow low-dipping planar fault that is 4.58 km depth underground. The finite slip model inverted by the Steepest Descent Method implies that the rupture is dominated by a thrust fault. The slips are concentrated in a depth of 5–7 km with a maximum slip of 0.29 m. The estimated total seismic moment is 1.688 × 1018 Nm, corresponding to a magnitude of Mw 6.1. The seismogenic fault is the Kalping fault which has a listric structure. The coseismic deformation only occurred on the décollement layer and did not involve the ramp segment. The coseismic Coulomb stress changes have enhanced the stress on the deep margin of the Jiashi earthquake rupture area, indicating that there is still the possibility of strong earthquakes in this region in the future.

Driving stress and seismotectonic implications of the 2013 Mw5.8 Awaji Island earthquake, southwestern Japan, based on earthquake focal mechanisms before and after the mainshock

A driving stress of the Mw5.8 reverse-faulting Awaji Island earthquake (2013), southwest Japan, was investigated using focal mechanism solutions of earthquakes before and after the mainshock. The seismic records from regional high-sensitivity seismic stations were used. Further, the stress tensor inversion method was applied to infer the stress fields in the source region. The results of the stress tensor inversion and the slip tendency analysis revealed that the stress field within the source region deviates from the surrounding area, in which the stress field locally contains a reverse-faulting component with ENE–WSW compression. This local fluctuation in the stress field is key to producing reverse-faulting earthquakes. The existing knowledge on regional-scale stress (tens to hundreds of km) cannot predict the occurrence of the Awaji Island earthquake, emphasizing the importance of estimating local-scale (< tens of km) stress information. It is possible that the local-scale stress heterogeneity has been formed by local tectonic movement, i.e., the formation of flexures in combination with recurring deep aseismic slips. The coseismic Coulomb stress change, induced by the disastrous 1995 Mw6.9 Kobe earthquake, increased along the fault plane of the Awaji Island earthquake; however, the postseismic stress change was negative. We concluded that the gradual stress build-up, due to the interseismic plate locking along the Nankai trough, overcame the postseismic stress reduction in a few years, pushing the Awaji Island earthquake fault over its failure threshold in 2013. The observation that the earthquake occurred in response to the interseismic plate locking has an important implication in terms of seismotectonics in southwest Japan, facilitating further research on the causal relationship between the inland earthquake activity and the Nankai trough earthquake. Furthermore, this study highlighted that the dataset before the mainshock may not have sufficient information to reflect the stress field in the source region due to the lack of earthquakes in that region. This is because the earthquake fault is generally locked prior to the mainshock. Further research is needed for estimating the stress field in the vicinity of an earthquake fault via seismicity before the mainshock alone.

Earthquake Interactions in Central Taiwan: Probing Coulomb Stress Effects Due to ML≥ 5.5 Earthquakes From 1900 to 2017

AbstractThough large efforts have been made in studying earthquake generation processes, earthquake triggering in central Taiwan is still not well understood. The occurrence of 14 mainshocks with ML≥ 6.0 from 1900 to 2017 and the presence of eight active fault systems provide excellent data sets for studying earthquake interaction in central Taiwan. We examined the coseismic Coulomb stress changes (ΔCFSs) due to mainshocks in the sequence and other surrounding earthquakes comprising 3 smaller magnitude mainshocks with 5.5 ≤ML≤ 6.0 and 25 aftershocks with ML≥ 5.5. When considering only triggering effects from mainshocks in the sequence, the compounded ΔCFSs are significant for all 13 mainshocks with 8 being promoted (ΔCFS &gt; 0.1 bar) and 5 being inhibited (ΔCFS &lt; −0.1 bar). After adding effects from other surrounding earthquakes, we found that five mainshocks were promoted and seven were inhibited. Yet it is questionable whether the 2016 mainshock was promoted, because the calculated uncertainties at the hypocenter might reduce ΔCFS below the 0.1 bar triggering threshold. In addition, significant positive ΔCFSs (&gt;2 bars) on the Chukou, Chaochou, and Chishan faults and flat decollement of central Taiwan suggest failure promotion on those active faults. Contrarily, significant negative ΔCFSs (&lt;−0.2 bar) on the Changhua, Shungtung, Chelungpu, and Hsinhua faults suggest that ruptures on those faults might be inhibited. We also conducted sensitivity studies indicating uncertainty level of 33–38% of the calculated ΔCFSs. Our findings indicate that preceding earthquakes have affected nucleation and rupture propagation of large earthquakes in central Taiwan during the past 120 years.

A hybrid source mechanism of the 2017 Mw 6.5 Jiuzhaigou earthquake revealed by the joint inversion of strong-motion, teleseismic and InSAR data

As occurred during the tourist season, the 2017 Mw 6.5 Jiuzhaigou (China) earthquake led to destructive damages. The seismogenic fault of this event was merely speculated to be the northern extension of the Huya fault, while no apparent surface ruptures were discovered in the field investigation. Previous studies and released moment tensor solutions indicated the Jiuzhaigou earthquake was dominated by left-lateral strike slip with partial normal-slip component. It seems unintelligible that the normal slip occurred in this event as the epicenter was located near the boundary of convergent blocks. Hence, a reasonable and elaborate source rupture model is necessary to investigate such a blind fault and the role it plays in the complex fault system of this region. Combining space-based geodetic, teleseismic or regional seismic observations can provide detailed information about earthquake ruptures. We first attempted to determine the fault geometry using the Bayesian approach with synthetic aperture radar interferograms (InSAR). And then the fault geometry was refined based on the relocated aftershock distribution, and a two-segment fault model was constructed. Based on the two-segment model, we resolved the source rupture process of the 2017 Jiuzhaigou earthquake through the joint inversion of strong-motion, teleseismic body-wave and InSAR data. The inversion results reveal a hybrid source mechanism, in which normal and thrust slips coexist besides the strike-slip component. We suggest that the eastward motion with the extrusion of the lower crustal flow in the northeastern margin of the Bayan Har block is responsible for such a faulting behavior. The co-seismic Coulomb stress changes show a significant stress loading in the western segment of the Tazang fault, increasing its seismic hazard. Due to the lack of aftershocks to the southeast of the seismogenic fault, the probably enhanced seismicity in the northern segment of the Huya fault is also worth further attention.

Sentinel-1 observation of 2019 Mw 5.7 Acipayam earthquake: A blind normal-faulting event in the Acipayam basin, southwestern Turkey

Rapid disaster response will provide significant information to relevant responding agencies. On 20 March 2019, an Mw 5.7 earthquake struck the Acipayam basin in southwestern Turkey, which is the largest event to occur in the basin. Here, we use Sentinel-1A interferometric synthetic aperture radar (InSAR) data to investigate the cosesimic deformation related to the Acipayam earthquake. The ascending and descending interforegrams suggest the normal faulting during this earthquake, with the maximum line of sight (LOS) displacements of 4.6 cm and 4.8 cm, respectively. The fault geometric parameters and a detailed coseismic distributed slip model of this event are retrieved by using geodetic data for the first time. Though the coseismic interferograms don’t show enough polarity changes to ascertain the dip orientation of the rupture fault, we regard the northeast-dipping fault as the causative fault by a joint analysis of the residuals plots, 2.5 dimensional (2.5D) deformation field, field investigation results, regional geomorphic feature and overall distribution of aftershock (4.0 < Mw < 4.9). The inversion results show that coseismic rupture was mainly dominated by normal faulting with minor sinistral strike-slip components. The rupture fault is a blind normal fault and the main slip is concentrated in the depth range of 3∼9.5 km with a maximum slip of approximately 0.33 m at a depth of ∼6 km. The InSAR-determined geodetic seismic moment is 3.96 × 1017 Nm corresponding to Mw 5.7. Our model resolution test supports these interpretations for the Acipayam earthquake. Based on the coseismic Coulomb stress change, we find that this 2019 event increases the stress loading in the northwestern and southeastern segment of the rupture fault plane and possibly triggers the subsequent Mw 4.8 and Mw 4.9 earthquakes. The mechanism of the determined seismogenic fault may be helpful to evaluate seismic hazards and understand the evolution of the Acipayam basin.

Co-seismic Coulomb stress changes on the northern Tanlu fault zone caused by the Tohoku-Oki MW9.0 earthquake*

Based on the spherical earth dislocation theory and a fault slip model of the Tohoku-Oki MW9.0 earthquake, the co-seismic Coulomb failure stress changes (ΔCFS) on the northern Tanlu fault zone at depths of 0–40 km are calculated. By comparing two sets of results from the spherical earth dislocation theory and the semi-infinite space one, the effect of earth curvature on the calculation results is analyzed quantitatively. First, we systematically summarize previous researches related to the northern Tanlu fault zone, divide the fault zone as detailed as possible, give the geometric parameters of each segment, and establish a segmented structural model of the northern Tanlu fault zone. Second, we calculate the Coulomb stress changes on the northern Tanlu fault zone by using the spherical earth dislocation theory. The result shows the Coulomb stress changes are no more than 0.003 MPa, which proves the great earthquake did not significantly change the stress state of the fault zone. Finally, we quantitatively analyze the disparities between the results of semi-infinite space dislocation theory and the spherical earth one. The average disparity between them is about 7.7% on the northern Tanlu fault zone and is 16.8% on the Fangzheng graben, the maximum disparity on this graben reaches up to 25.5%. It indicates that the effect of earth curvature can not be ignored. So it’s necessary to use the spherical earth dislocation theory instead of the semi-infinite space one to study the Coulomb stress change in the far field.

The 2007 Ning'er Mw 6.1 Earthquake: A Shallow Rupture in Southwest China Revealed by InSAR Measurements

AbstractThe Ning'er Mw 6.1 earthquake on 3 June 2007 occurred in the southwest to the Sichuan‐Yunnan rhombic block with complicated seismic structures. The focal depth determined by the U.S. Geological Survey is only 5 km, but the field investigation did not find obvious surface ruptures in the epicenter area. The magnitude obtained by inverting the broadband waveforms is Mw 6.4 that is much larger than the Mw 6.1 calculated by U.S. Geological Survey, indicating this event is complicated. The Interferometric Synthetic Aperture Radar is used to study the fault geometry, coseismic slip model, and Coulomb stress changes around the seismic region. It shows that this event is mainly dominated by a right‐lateral strike‐slip fault with a slight thrust component. The fault plane is 28‐km long along the strike and 14‐km wide along the dip. The maximum slip is up to 1.02 m at a depth of 3.6 km and the released moment is 2.57 × 1018 Nm, corresponding to a magnitude of Mw 6.25. The shallow source of this event is related to inelastic off‐fault deformations along the fault or magma chamber. The lack of obvious surface ruptures indicates the strain is not completely released. The coseismic Coulomb stress changes suggest this earthquake has enhanced the stress on the Puwen fault and Mohei fault. The future seismic risks of these two faults and its adjacent regions cannot be ignored since aftershocks are fewer and the residual strain unreleased by the Ning'er event.