2014年美国加州纳帕M_W6.1地震断层参数的Sentinel-1A InSAR反演

Coseismic slip of the 2014 M_W6.1 Napa,California Earthquake revealed by Sentinel-1A InSAR

2014年8月24日,在美国加州旧金山海湾北部的纳帕地区发生了MW6.1地震.发震断层是西纳帕断裂系统中的一部分,但是该断层之前并未被足够重视.本文利用欧洲空间局最近发射成功并刚刚投入使用的Sentinel-1A卫星获取的第一对同震干涉像对(20140807-20140831),得到了该地震的地表同震形变场,结合震后24h内区域GPS同震形变资料作为约束条件,反演了纳帕地震的断层几何参数以及滑动分布.Sentinel-1A干涉结果表明,此次地震造成了明显的地面形变,视线向最大抬升和最大沉降量均达到了10cm.联合反演结果表明,该发震断层的走向为344°,倾角为80°.主要破裂以右旋走滑为主,平均倾滑角为-146.5°,最大倾滑量达到了1.1m,位于地表下约4km,存在明显的滑动亏损现象.此次地震,累计释放地震矩达1.5×1018 N·m,约合矩震级MW6.1.该结果略小于InSAR单独约束结果,可能与Sentinel-1A像对中包含的快速震后形变分量有关.

On August 24,2014,aMW6.1earthquake occurred in the Napa Valley and shookalarge area of the northern San Francisco Bay,California,USA.The causative fault is believed to be part of the West Napa fault system,but little attention has been paid before the quake.The earthquake was the strongest earthquake in California USA San Francisco Bay Area in the past 25 years and caused serious damage to construction and road at Napa area and the bay area.The epicenter was located in the northern area of San Francisco Bay.It is just 40 km far away from San Andreas Fault（SAF）which is a transform plate boundary between the Pacific plate and the North American plate.On August 7,2014,Sentinel-1A reached the predetermined orbit and Napa earthquake occurred on August 24,2014.Sentinel-1Asatellite first obtained the coseismic deformation field by two times imaging before and after Napa earthquake on August 7th and August 31 st.The Sentinel-1Asatellite exhibited very high imaging quality and interferometry ability.We developed the format conversion module for Sentinel-1A data and used the JPL/Caltech Repeat Orbit Interferometry PACkage（ROI_PAC,V3.1Beta）software to process the next InSAR operation.The interferogram shows that the earthquake produced significant ground displacements with maximum uplift and subsidence of～10cm in the satellite line of sight（LOS）respectively.Using a two-step inversion strategy,the InSAR data were combined with GPS coseismic displacements to determine the fault geometry and slip distribution.The results show that the fault parameters agree well with the observed data.The best-fit slip model in the joint inversion suggests that the major coseismic slip is concentrated on a right-lateral fault with a strike of 344°,a dip of 80°and an average rake angle of-146.5°.The maximum slip of～1.1mat a depth of～4km suggests that shallow slip deficit happened during the mainshock.The accumulative seismic moment was up to 1.5×1018 N·m,equivalent to a magnitude of Mw6.1.There were subtle differences in the inversion results whether the GPS data was combined with InSAR data or not,probably because the GPS data was dominated by the coseismic deformation and the InSAR data also contained significant postseismic deformation signals.The overall deformation of the earthquake fault is characterized by the right lateral strike slip motion,which is consistent with the tectonic deformation characteristics of the main large fracture zones in this region.This large fault zone includes San Andreas Fault,Rodgers Creek Fault and Green Valley Fault.Napa earthquake plays a positive role on adjustment of the regional tectonic deformation and the seismic safety of the San Francisco Bay area.