2022年泸定M_(S)6.8地震同震形变特征及周边强震危险性

Coseismic deformation of the 2022 Luding M_(S)6.8 earthquake and seismic potential along adjacent major faults

2022年四川泸定6.8级地震发生于鲜水河断裂磨西段,为揭示此次地震的同震形变特征,收集了震中200 km范围内的连续全球定位系统(global positioning system,GPS)测站和震中50 km范围内的强震动数据,进行了高精度处理以提取测站的同震水平位移.此外,还收集了覆盖震中区域的Sentinel和ALOS-2降轨数据,通过干涉差分技术处理并获得了卫星视线向同震形变场.结果显示:水平同震形变呈四象限分布,表明泸定地震的震源机制为左旋走滑;距震中16 km的强震动台站记录到了明显的位移波形,其峰值位移达14 cm、永久变形约为12 cm;合成孔径雷达干涉测量(interferometric synthetic aperture radar,InSAR)显示断层近场30 km×30 km区域具有明显的同震变形,断层西侧卫星视线向最大位移达15 cm,挖角乡附近的局部形变超过15 cm;野外调查推测在“二台子-爱国村”间的断层段发育地表破裂,但从InSAR形变场上难以判定,同震是否破裂到地表仍需详细的野外考察予以确定.反演了川滇块体北东边界主干断裂在泸定地震前的闭锁分布,并定量计算了断层的地震矩亏损,结果表明,2022年泸定6.8级地震充分释放了磨西段自1786年磨西M73/4地震以来积累的地震矩.据此研判近期在原震区发生更大震级地震的可能性不大;安宁河、则木河、大凉山和龙门山南段等断裂已累积的地震矩等效于MW7.0~7.7地震;鉴于泸定地震对上述断裂段均造成了库仑应力加载作用(>0.01 MPa),且在安宁河断裂北段的库仑应力加载超过了0.1 MPa的经验触发阈值,川滇块体北东边界尤其是安宁河断裂的未来强震危险性需密切关注.

The 2022 Luding(Sichuan)M6.8 earthquake occurred on the Moxi segment of the Xianshuihe fault,one of the most seismically active faults in the mainland of China.Since 1900,six moderate to strong earthquakes(>M6.5)have ruptured different sections of the Xianshuihe fault.This Moxi segment has a complex tectonic setting as it forms an intersection zone of the Anninghe,Daliangshan,Xiaojin,and Longmenshan faults.In this study,we investigated the distribution of coseismic deformation of the 2022 Luding earthquake in order to improve understanding of the seismogenic fault and gain insight into the kinematic behaviors of regional faults and seismogeneity of future strong earthquakes.We collected continuous global positioning system(GPS)and strong motion data within 200 and 50 km from the epicenter,respectively,and performed high-precision processing to extract the coseismic deformation field of this earthquake.We also collected Sentinel-1 and Advanced Land Observing Satellite-2(ALOS-2)ScanSAR model descending data covering the epicenter area,and processed these data using differential interferometric synthetic aperture radar(D-InSAR)technology.The GPS measurements confirm that deformation from this earthquake occurred as far as 200 km from the fault.This coseismic deformation was distributed in four quadrants,indicating that the focal mechanism of the Luding earthquake was leftlateral strike-slip.A strong motion station 16 km from the epicenter recorded a significant displacement waveform;peak ground displacement was 14 cm and the permanent coseismic displacement was 12 cm.InSAR deformation maps show significant coseismic deformation in a 30 km×30 km area along the fault.The maximum line-of-sight displacement reached 15 cm to the west of the fault.In addition,significant local deformation(>15 cm)was observed near the Wajiao township.Field investigations suggest that surface ruptures developed along the fault segment between Ertaizi and Aiguocun.However,it is difficult to identify rupture information from the InSAR maps owing to a loss of coherence in the near-field of the seismogenic fault.Detailed field investigations are needed to determine if coseismic slip reached the surface.Finally,we inverted interseismic coupling along the northeastern boundary faults of the Sichuan–Yunnan block,and quantitatively calculated the seismic moment budget on these faults.Based on the historical earthquake distribution of the past 300 years and present-day fault locking conditions along the Xianshuihe fault,the Luding earthquake accommodated clockwise vertical-axis rotation of the expanding Qinghai-Tibetan Plateau relative to the South China Block.The earthquake fully released the strain accumulated on the Moxi segment since the 1786 Moxi M73/4 earthquake;as such,it is unlikely that large earthquakes will occur on the Moxi segment in the near future.However,we also performed a comprehensive investigation of seismic hazard along the Xianshuihe-Xiaojiang fault system based on the mapped seismic gap,asperity distribution,seismic moment deficit,and Coulomb failure stress caused by surrounding earthquakes.We found that the seismic moment accumulated on the Anninghe,Zemuhe,Daliangshan,and southern Longmenshan faults can be balanced by Mw7.0–7.7 earthquake.The Luding earthquake caused significant Coulomb stress loading(>0.01 MPa)on the Daliangshan,Anninghe,and southern Longmenshan faults,bringing those faults closer to failure.In particular,Coulomb stress loading at the northern end of the Anninghe fault exceeded the conventional triggering threshold(0.1 MPa)for earthquakes;therefore,the risk of future strong earthquakes on the Anninghe fault deserves special attention.