Viscoelastic stress change from the 1931 M_(w)7.8 Fuyun earthquake and its impacts on seismic activity around the Altai mountains

The 1931 M_(w)7.8 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 M_(w)7.2 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 predictedΔCFS suggests that the impact of the 1931 M_(w)7.8 Fuyun earthquake on seismic activity around the Altai mountains is still significant as to hasten occurrence of the 2003 M_(w)7.2 Chuya earthquake at a relatively far distance and to trigger its aftershocks in the near-field even after several decades of the mainshock.

Research on Frequency of the Aftershock Sequence of the Wenchuan Earthquake Based on Coseismic Coulomb Stress Change

By the aftershock frequency estimation method based on the calculation of coseismic static Coulomb stress changes and rate-and state-dependent fault constitutive law,we calculate the frequency of "direct "aftershocks of the Wenchuan earthquake related to coseismic static Coulomb stress changes in its aftershock zone and the areas nearby.It shows that the frequency is significantly lower than the truth in the main rupture zone,especially in the southern rupture zone,due to the decrease of stress level on the rupture plane of the main shock resulting from coseismic Coulomb stress change.The study also shows that the effect of the Coulomb stress change on the duration of aftershock activity is associated with the lower limit magnitude.The duration is about 15-16 months for aftershocks above ML4.0,and close to 60 months for aftershocks above ML3.5.In this period,the ratio of the"direct"aftershocks caused by coseismic Coulomb stress change ranges between 44.7% to48.6%,which suggests that,even in the "effective"period of coseismic Coulomb stress changes,about half of the aftershocks on the main shock rupture plane are independent of coseismic Coulomb stress changes.It is pointed out that those aftershocks may be related to the afterslip or the viscoelastic relaxation,which are time dependent cases.

Stress triggering" between different rupture events in several earthquakes

Most strong earthquakes have complex rupture processes. As an approximation, each earthquake can be described as two or more subevents of rupture with time interval of several seconds to several days. In order to discuss the relationship between different subevents, we investigated the rupture process of the 1966 Xingtai, the 1976 Tangshan, the 1990 Gonghe and the 1996 Lijiang earthquake by calculating the static Coulomb failure stress changes produced by the first subevent. The calculation of static stress changes produced by fault slip is based on the formulation of Okada (1992). The result suggests that the static Coulomb failure stress changes (ΔCFS) produced by the first subevent have 'triggering' effect on the subsequent subevents which locate in the region where the Coulomb stress change produced by the first event is positive, with the order of magnitude 10-2 [similar to] 10-1 MPa.

Regional seismicity triggered by the M_s=7.8 Tangshan event on July 28, 1976 and the stati cstress field change

We studied the seismicity before and after the M_s=7.8 Tangshan event on July 28, 1976 (39°28'N, 1 18° 11'E) and the results show that in 3 regions outside of the source zone, seismicity rate increasing were observed, which was significant in 0.99 significance level using Z-statistic test and was proposed to be triggered by the M_s=7.8 Tangshan earthquake. The magnitude of the greatest triggered event was 5.5. The epicenter distances of these earthquakes were several ten kilometers to 300 km. The static stress change △CFS of Coulomb failure was calculated using an elastic dislocation model in half space and the △CFS on the major rupture directions in these three regions were positive.

Crustal Stress Evolution over the Past 700 Years in North China and Earthquake Occurrence

Fault interaction and earthquake occurrence have attracted much attention in seismological community during recent years. Many studies have shown that the rupture of one fault could encourage or discourage earthquake nucleation on a neighboring fault, depending on the relative geometry of the two faults and the earthquake rupture mechanisms. In this paper, we simulate the evolutionary process of cumulative Coulomb failure stress change （ CCFSC ） in North China since 1303, manifested by secular tectonic stress loading and occurrence of large earthquakes. Secular tectonic stress loading is averaged from crustal strain rates derived from GPS. Fault rupture parameters of historical earthquakes are estimated as follows： the earthquake rupture length and the amount of slip are derived based on their statistical relationships with the earthquake intensity distribution and magnitude, calibrated using parameters of instrumentally measured contemporary earthquakes. The earthquake rake angle is derived based on geologically determined fault orientational parameters and seismically estimated orientation of regional tectonic stresses. Assuming a layered visco-elastic medium, we calculate stress evolution resulting from secular tectonic loading and coseismic and postseismic deformation. On the eve of each large earthquake, the accumulated stress field is projected to the fault surface of that earthquake and the CCFSC is evaluated to assess the triggering effect of CCFSC. Forty-nine earthquakes with M≥6.5 have occurred in North China since 1303. Statistics shows that 39 out of the 48 subsequent events were triggered by positive CCFSC, yielding a triggering rate of 81.3%. If we use the accumulative stress field to evaluate the CCFSC for the M ≥ 5.0 earthquakes that occurred in North China since 1303, we find that 75.5% of those events were triggered. The triggering rate for the M ≥ 5.0 earthquakes after the 1976 Ninghe earthquake is up to 82.1%. The triggering rates can be higher if corrections are made for some aftershocks which were wrongly identified as occurring in stress shadow zones because of errors in parameter estimates of historical earthquakes. Our study shows a very high correlation between positive CCFSC and earthquake occurrences. Relatively high CCFSC in North China at present is concentrated around the Bohai Sea, the west segment of the Northern Qinling fault, the western end of the Zhangjiakou-Bohai Sea seismic zone, and the shiyuan basin, Shanxi graben, suggesting relatively higher earthquake potential in these areas.

Research on seismic stress triggering

This paper briefly reviews basic theory of seismic stress triggering. Recent development on seismic stress triggering has been reviewed in the views of seismic static and dynamic stress triggering, application of viscoelastic model in seismic stress triggering, the relation between earthquake triggering and volcanic eruption or explosion, other explanation of earthquake triggering, etc. And some suggestions for further study on seismic stress triggering in near future are given.

Study on Stress Triggering During the Activity Process of the Jiashi Strong Earthquake Swarm

The Bachu-Jiashi earthquake of MS6.8 occurred on February 24,2003,about 20km from the southeast of the 1997～1998 Jiashi seismic region in Xinjiang,and its aftershocks are rich and strong.Did the 1997～1998 Jiashi strong earthquake swarm trigger the Bachu-Jiashi MS6.8 earthquake? The Atushi earthquake of MS6.7 occurred in 1996,and the 1997～1998 Jiashi strong earthquake swarm occurred about 70km from the Atushi earthquake 10 months later.Did the Atushi earthquake of M-S6.7 encourage the 1997～1998 Jiashi strong earthquake swarm? There were 9 earthquakes with M-S6.0 from 1996 to 1997 in the Jiashi seismic region,how did they act on each other? To answer the above questions,the article studies the triggering effect of the activity process of the whole Jiashi earthquake swarm from the 1996 Atushi earthquake of M-S6.7,the 1997～1998 Jiashi strong swarm to the 2003 Bachu-Jiashi earthquake of M-S6.8,and analyzes the seismicity characteristics around the Jiashi region.The results show that the 1996 Atushi earthquake of M-S6.7 encouraged the 1997～1998 Jiashi strong swarm to some extent,the accumulative Coulomb stress change from the previous M-6.0 earthquakes of the Jiashi strong swarm had certain triggering effects on the following M-6.0 events,and the Coulomb stress change converted from the Jiashi strong swarm strongly encouraged the 2003 Bachu-Jiashi earthquake with M-S6.8.

Seismicity changes and numerical simulation of coseismic deformation following the 2022 M_(s)6.8 Luding earthquake in Sichuan,China

The Xianshuihe fault is a major tectonic boundary between the Sichuan-Yunnan rhombic and Bayanhar blocks in Southwest China.With an average left-lateral strike-slip movement of 10–15 mm/yr,it is a fast-moving strike-slip continental fault.On September 5,2022,the Ms6.8 Luding earthquake occurred along the Moxi segment of the Xianshuihe fault,reaching a maximum intensity of IX and resulting in a significant number of casualties and severe property damage.This earthquake broke the long-standing seismic quiescence of the Xianshuihe fault,which lasted for more than 40 years,and was followed by a significant number of aftershocks.An outstanding question is how the behavior of the Xianshuihe fault and major earthquakes changed following this mainshock.In this study,we examined the changes in regional seismicity following the Luding earthquake and identified the potential for future strong earthquakes along the Xianshuihe fault.We used a finite element numerical method to simulate the environment of the seismogenic fault and its adjacent areas.In addition,we used the coseismic slip model of the Luding earthquake with the split-node method to calculate how the stress and strain fields in the surrounding area were affected by the2022 mainshock.Coulomb stress changes were resolved in the main faults,and the seismicity of adjacent faults was analyzed in conjunction with the observed seismic data.The results indicate that regional tectonic movement primarily occurred to the southeast along the Moxi segment.The stress field is approximately north-south in tension and east-west in compression.Variation in the stress field in the epicentral region of the Luding earthquake exceeded 1 MPa.The maximum displacement of the coseismic deformation field was concentrated between Moxi town and Tuanjie village,and the Coulomb stress of the fault zone in this region experienced the largest decrease.However,the b-value of the Gutenberg-Richter magnitude-frequency relationship at the epicenter and the surrounding area exhibited an abnormal pattern of decrease-decrease-increase,indicating that the regional stress may not be fully released.This earthquake increased the Coulomb stress in other segments of the Xianshuihe,Anninghe,and Daliangshan faults,whereas the Coulomb stress in the Longmenshan and Xiaojinhe fault zones decreased.In addition,it triggered a series of normal-fault,moderate-sized earthquakes in nearby areas.The Dagangshan reservoir,located~20 km from the epicenter of the Luding earthquake,received an increase of~5.3 MPa in the tensile stress along the NWW-SEE direction.The Xiluodu Reservoir,located approximately 225 km from the epicenter,was less affected by this earthquake,and the seismic activity near the reservoir remained relatively unchanged.In this study,post-earthquake seismicity in the vicinity of the Ms6.8 Luding earthquake was analyzed and predicted by numerical simulation,providing a scientific basis for earthquake prediction and disaster reduction.

Features of seismicity in Xinjiang and its possible reason after the Yutian M_S7.4 earthquake,2008

The paper discusses quantitatively the influence of the Yutian Ms7.4 earthquake of March 21, 2008 and Wuqia Ms6.9 earthquake of October 5, 2008 on regional seismicity in Xinjiang, and explains primarily the possible reason of earthquake activity feature in Xinjiang after the Yutian Ms7.4 earthquake by analyzing the static Coulomb failure stress change produced by the Yutian Ms7.4 earthquake and Wuqia Ms6.9 earth-quake, and the seismicity feature of Ms≥3 earthquakes in the positive Coulomb stress change region of Kashi-Wuqia joint region, the central segment of Tianshan Mountain and Kalpin block. The result shows that the Yutian Ms7.4 earthquake of March 21, 2008, may encourage the Wuqia Ms6.9 earth-quake of October 5, 2008, and the Yutian Ms7.4 earthquake and Wuqia Ms6.9 earthquake may change the seismicity state in the central segment of Tianshan Mountain, Kalpin block and Kashi-Wuqia joint region, and encourage the subsequent Ms≥3 earthquakes.

The Kangding earthquake swarm of November, 2014

There was an earthquake swarm of two major events of MS6.3 and MS5.8 on the Xianshuihe fault in November, 2014. The two major earthquakes are both strike-slip events with aftershock zone along NW direction.We have analyzed the characteristics of this earthquake sequence. The b value and the h value show the significant variations in different periods before and after the MS5.8earthquake. Based on the data of historical earthquakes, we also illustrated the moderate-strong seismic activity on the Xianshuihe fault. The Kangding earthquake swarm manifests the seismic activity on Xianshuihe fault may be in the late seismic active period. The occurrence of the Kangding earthquake may be an adjustment of the strong earthquakes on the Xianshuihe fault. The Coulomb failure stress changes caused by the historical earthquakes were also given in this article. The results indicate that the earthquake swarm was encouraged by the historical earthquakes since1893, especially by the MS7.5 Kangding earthquake in1955. The Coulomb failure stress changes also shows the subsequent MS5.8 earthquake was triggered by the MS6.3earthquake.

Finite element simulation of deformation and stress changes of Kalpin-Kemin fault system in the Southwest Tianshan Orogenic Belt

Under the shadow of the far-field effect of the India-Eurasia collision,the Tianshan orogenic belt underwent tectonic re-activation in the Cenozoic,accompanied by strong tectonic deformation and frequent large earthquakes.Bounded by two rigid cratonic blocks located in its north and south,a series of marginal foreland fold-and-thrust belts are developed within the Tianshan orogenic belt and continue to develop to the bilateral pull-apart basins.Meanwhile,the faults in the orogenic belt are reactivated.The deformation caused by thrust-related structure accounts for larger than 50%of the total convergence of the Tianshan Mountains,which results in the most active structure with large earthquakes in the Tianshan area.Therefore,it is of great significance to study the dynamic process of the newly generated and reactivated thrust-nappe structures in Tianshan orogen via numerical modeling.This paper selects a classical cross-section profile in the western segment of the Southwest Tianshan Mountains,which contains the Kalpin-Maidan-Nalati-Kemin fault system from the south to the north.We attempt to establish a two-dimensional plane strain,viscoelastic finite element model,by treating the regional faults as a whole fault system and considering the topography,fault geometry,and GPS data.The displacement and stress fields of the model are retrieved,the short-term cumulative deformation field of the overall fault system is analyzed,and the rate of Coulomb failure stress change of each fault is also considered.The results show that the deformation is concentrated in the middle and southern parts of the Southwest Tianshan Mountains.In contrast,the deformation of the Kemin fault in the north is relatively small.According to the Coulomb failure stress changes of these four faults and the historical earthquake catalog,the potential seismicity of each fault is qualitatively analyzed.Our preliminary results suggest that the possibility of large earthquake occurrence is higher in the Kalpin fault,Maidan fault,and Nalati fault but lower in the Kemin fault in the near future。

Influence of Surrounding Large Earthquakes on Moderate Earthquake Activity within Xinjiang

Based on the statistical results of the relationship between the large earthquakes surrounding Xinjiang and moderate earthquakes within Xinjiang since 1950,this article calculates the Coulomb failure stress change produced by the three groups of large earthquakes surrounding Xinjiang during the large earthquake active period outside Xinjiang and the quiet period inside Xinjiang from 1976 to 2007,and analyzes Xinjiang's moderate earthquake activity features in the three years after the large earthquakes surrounding Xinjiang. It then discusses the influence of large earthquake activity surrounding Xinjiang on seismic activity within Xinjiang. The research results show that the large earthquake activity surrounding Xinjiang may to a certain extent slow down the preparation process of moderate earthquakes in Xinjiang.

Static Stress Triggering Effects Related with M_s8.0 Wenchuan Earthquake

In this article, firstly, we calculated and analyzed the patterns of Coulomb stress changes induced by a sequence of strong earthquakes that occurred in Songpan （松藩）, Sichuan （四川） Province in 1973 and 1976, and discovered that the Ms8.0 Wenchuan （汶川） earthquake of 2008 was epicentered in a relevant Coulomb stress triggering zone. This suggests that the Coulomb stress on the middle and southern segments of the Longmenshan （龙门山） fault zone increased after the Songpan sequence of strong earthquakes, and the stress increment might cause the 2008 Wenchuan earthquake having al- ready occurred somewhat ahead of time. Further, we calculated and analyzed Coulomb stress changes coinduced by both the Songpan sequence and the Ms8.0 Wenchuan mainshock. The result shows that the Ms6.4 Qingchuan （青川） earthquake of May 25, 2008 on the northeastern segment of the Longmenshan fault zone was triggered by the Wenchuan mainshock, and that the southwestern segment of the fault zone is also in the stress triggering zone. Besides, the Maoxian （茂县）-Wenchuan fault （i.e., the back-range fault of the Longmenshan fault zone）, which extends parallel to the seismogenic fault of the Wenchuan earthquake, is in a shadow zone of the Coulomb stress changes, and therefore, its potential hazard for producing a strong or large earthquake in the near future could be reduced relatively.