Coseismic and Postseismic Deformation from the August 2014 Mw 6.0 South Napa Earthquake Measured with InSAR Time Series

The Mw 6.0 South Napa Earthquake struck Napa County of California on 24 August 2014 with extensive surface ruptures mapped in the field and with satellite and airborne interferometric synthetic aperture radar (InSAR). The Italian Space Agency's (ASI) has been acquiring COSMO-SkyMed™ (CSK) synthetic aperture radar (SAR) images of the Napa area since June 2013 with two different look directions (satellite moving south looking west, and satellite moving north and looking east). The Canadian Space Agency and partner MDA have been acquiring RADARSAT-2 (RS2) SAR images over the San Francisco area for several years, with partial coverage of the southern part of Napa County. Preliminary time series analysis of the CSK and RS2 SAR image time series before the 2014 earthquake shows moderate rates of surface deformation likely related to variations in ground water levels. By the end of September 2014, there were 6 CSK scenes on the descending track (first on 27 August 02:08 UTC) and two scenes on the ascending track (first on 3 September 13:55 UTC) acquired after the earthquake. We use GIAnT to extend the time series analysis across the time of the earthquake and calculate a better estimate of the coseismic deformation as a step function in the time interval between the date of the last pre-quake scene and first post-quake scene, plus a postseismic deformation time function. With the 6 CSK descending track scenes, the time series estimate of the coseismic deformation has much less of the atmospheric effects that are present in single interferograms, but the 2 CSK ascending track scenes do not provide as much improvement. The postseismic single interferograms are useful for seeing the early postseismic deformation. We observe rapid afterslip on both the ascending and descending interferograms, concentrated at shallow depth on the southern part of the main coseismic rupture, extending about 4 km north of the epicenter. We also observe what appears to be poroelastic rebound in the area west of the southern part of the rupture, which needs confirmation from additional observations and modeling.