Coseismic slip of the 2014 M w 6.1 Napa, California earthquake revealed by Sentinel-1A InSAR

On 24 August 2014, an Mw 6.1 earthquake occurred in the Napa Valley shook a large area of northern San Francisco Bay, California, USA. An interferometric pair acquired from the newly launched ESA’s Sentinel-1A satellite was used to map the coseismic displacements, and then combined with GPS coseismic displacements to determine its fault geometry and slip distribution. The Sentinel-1A interferogram shows that this earthquake produced significant ground displacements with maximum uplift and subsidence of 10 cm in the satellite light of sight (LOS). 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 ~1.1 m at a depth of ~4 km, suggesting that shallow slip deficit happened during the mainshock. The accumulative seismic moment is up to 1.5x1018 N.m, equivalent to a magnitude of Mw 6.1. The seismic energy estimated using InSAR and GPS is less than that obtained by InSAR only, which might be due to considerable postseismic displacements included in the Sentinel-1A coseismic interferogram. The fault is believed to be part of the West Napa fault system, but little attenetion has been paid before the quake. Summary