Surface deformation before, during and after the 2014 South Napa, California, earthquake from a spatially dense network of survey and continuous GPS site

The South Napa, California, earthquake occurred on the West Napa Fault within a dense network of established GPS sites. Continuous sites from the Plate Boundary Observatory (PBO), Bay Area Regional Deformation (BARD) and other publicly-available networks lie mostly in the far-field, greater than 15 km from the epicenter. The near-field is covered by two networks of survey GPS sites, one observed by the University of California, Riverside (UCR) and MIT, and the other by the US Geological Survey (USGS). First, we present the pre-earthquake GPS velocity solution from the combination of these networks, covering the entire Pacific-North America plate boundary zone north of San Francisco Bay. We take 1-D fault-perpendicular profile and block model approaches to fit the GPS velocities, both with and without an explicit West Napa Fault, to update previous estimates of slip rates on faults to the east of the Rodgers Creek Fault, which include the West Napa and Green Valley Faults. Second, we present results from the survey GPS field response to the South Napa earthquake. 13 survey sites within 20 km of the epicenter were re-observed within 15 hours of the earthquake by a UCR-MIT group with assistance from UC Berkeley. An additional two sites to the north of the rupture were re-observed within 36 hours. A USGS-led group re-observed 17 sites within 25 km of the epicenter, as well as sites further afield. In total, 35 survey-mode GPS instruments were deployed to observe post-earthquake motions for up to four weeks after the event. Maximum displacements of greater than 20 cm are observed at two survey GPS sites within 2 km and either side of the surface rupture, in agreement with visual inspection of surface rupture offsets. Other observed survey sites within 20 km show at least 2 cm of displacement and 22 continuous GPS sites show displacements that are statistically significant at the 2-sigma level. Further, we show post-earthquake displacements over time as a result of this event relative to our precise pre-earthquake velocity solution.