Coseismic slip of the 2014 Mw6.1 Napa, California Earthquake revealed by Sentinel-1A InSAR

On August 24, 2014, a M(w)6.1 earthquake occurred in the Napa Valley and shook a large 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-1A satellite first obtained the coseismic deformation field by two times imaging before and after Napa earthquake on August 7th and August 31st. The Sentinel-1A satellite 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 10 cm 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 degrees. The maximum slip of similar to 1.1 m at a depth of similar to 4 km suggests that shallow slip deficit happened during the mainshock. The accumulative seismic moment was up to 1.5 X 10(8) N . m, equivalent to a magnitude of M(w)6.1. 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.