REGIONAL DEFORMATION BACKGROUND AND COSEISMIC DEFORMATION CHARACTERISTICS OF THE 2022 LUDING M S6.8 EARTHQUAKE
Situated as the eastern boundary of the Sichuan-Yunnan block,the Xianshuihe fault system exhibits a notably high left-lateral strike-slip rate,establishing itself as one of the most active regions for seismic activity in the Chinese mainland,profoundly influencing the occurrence of large earthquakes within the region.The fault zone and its surrounding area are relatively densely populated,intersecting with the famous Sichuan-Xizang National Highway No.317 and No.318 and serving as a significant focal point in the design of the Sichuan-Xizang railway.Given its substantial seismogenic capacity and associated earthquake risk,notable attention is warranted.Notably,on September 5,2022,a left-lateral strike-slip MS6.8 earthquake struck Luding County,Ganzê Prefecture,Sichuan Province,rupturing the Moxi fault of the Xianshuihe fault zone within the southeastern margin of the Qinghai-Xizang Plateau.Our study used Sentinel-1 SAR images to obtain both the interseismic deformation(2014-2020)and coseismic deformation resulting from the 2022 Luding M6.8 earthquake.Furthermore,we estimated the fault slip rate and locking depth during interseismic periods and inverted the coseismic slip distribution model.Utilizing the co-seismic dislocation model,we quantified Coulomb stress changes on surrounding fault planes induced by the Luding event.Finally,we provide an in-depth discussion on the seismogenic structure of the Luding earthquake and offer insights into the future seismic hazard implications associated with the Moxi fault and its adjacent faults.We collected Sentinel-1 SAR imagery data spanning from October 2014 to April 2020 for both the descending orbit T135 and ascending orbit T026,and calculated the Line-of-Sight(LOS)direction deformation during the interseismic period covering the Moxi Fault of the Xianshuihe fault zone.The InSAR-derived interseismic deformation presented in this study effectively captures the long-term slip behavior of the seismogenic fault associated with the 2022 Luding earthquake.Our analysis reveals an aestimated slip rate of(5.9±1.8)mm/yr along the Moxi Fault.Combined with the GNSS and InSAR deformation observations,we generated a fused three-dimensional deformation field characterized by high density and precision.Additionally,we calculated the strain rate field based on the three-dimensional deformation within the study area.Our findings indicate pronounced shear deformation near the Moxi Fault,with strain highly concentrated along the fault trace.Notably,the strain concentration in the southern section of the Moxi Fault surpasses that observed in the northern section before the earthquake event.Furthermore,our analysis suggests that the Moxi Fault was locked at shallow depths before the earthquake occurrence,indicating a predisposition for seismic activity.The Luding earthquake thus transpired within the context of a seismically active background associated with the Moxi Fault.Following the 2022 Luding 6.8 earthquake,we acquired InSAR coseismic deformation data within the seismic region,revealing predominantly horizontal surface displacements induced by the event.Employing the Most Rapid Descent Method(SDM),we conducted inversion of the fault plane slip distribution resulting from the earthquake.Our analyses indicate maximal dislocation quantities located south of the central earthquake zone,indicative of predominantly pure strike-slip movement.Dislocations are primarily observed at depths ranging between 5km to 15km,with the maximum left-lateral strike-slip dislocation measuring 1.71m and occurring at a depth of approximately 10km.In the north of the epicenter,fault slip manifests as predominantly sinistral strike-slip motion with a partial thrust component,exhibiting a progressively deepening slip pattern towards the northern region.Utilizing the coseismic slip distribution derived from the 2022 Luding MS6.8 earthquake,we conducted calculations to assess the Coulomb stress changes induced by the coseismic dislocation effects across the fault plane of the Moxi Fault and its surrounding major fault zones.These fault zones include the Xianshuihe fault zone(comprising the Moxi,Yalahe,Selaha,Zheduotang,and Kangding segments),the Anninghe fault zone(encompassing the Shimian-Mianning and Mianning-Xichang segments),the Zemuhe Fault zone,and the Daliangshan fault zone(comprising the Zhuma,Gongyihai,Yuexi,Puxiong,Butuo,and Jiaojihe segments).Our analysis reveals that the Luding earthquake caused a substantial decrease in Coulomb stress within its rupture section,resulting in the formation of a stress shadow area in the southern segment of the Moxi Fault.However,it significantly increased the Coulomb stress in the northern section of the Moxi Fault that was not ruptured in the earthquake.Concurrently,the Coulomb stress on the fault plane increases significantly in the southeast section of the Zheduotang fault,the northwest section of the Shimian-Mianning segment of the Anninghe fault zone,as well as the southeast section of the Zhuma segment,and the southeast section of the Gongihai segment of the Daliangshan fault zone.The seismogenic structure of the 2022 Luding earthquake is a part of the Moxi Fault of the Xianshuihe fault zone.However,the magnitude and rupture length of the earthquake are significantly smaller than that of the Moxi M734 earthquake in 1786,resulting in a less pronounced stress unloading effect.Additionally,the Luding earthquake triggered a noteworthy increase in Coulomb stress along the northern segment of the Moxi Fault.Consequently,the Luding earthquake did not ultimately reduce the seismic hazard within the Xianshuihe fault zone.Thus,greater attention should be directed towards the unruptured section of the Moxi Fault and its adjoining rupture with the background of large earthquakes.
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