Co- and Postseismic Slip on the West Napa Fault Seen with GPS after the South Napa Earthquake of August 24, 2014

At 3 am local time on August 24, 2014 the M_W 6.0 South Napa earthquake shook the northern California Bay Area, resulting in 200 injuries and damage estimated near $1 billion. The event occurred within an area of dense GPS instrumentation including continuous stations from the EarthScope Plate Boundary Observatory, Bay Area Regional Deformation Network, and other networks. Many of these stations telemeter data in real time or near real time, allowing for the rapid estimation of coseismic displacements and subsequent aseismic deformation.

We processed data from these networks as part of a mega-network solution from over 12,895 globally distributed stations using the GIPSY/OASIS software. We obtained 5 minute and 24 hour sample rate solutions using orbit products from the Jet Propulsion Laboratory with various latencies: ultra rapids (2 hour latency), rapids (next day latency) and finals (10 day latency). To assess the resolvability of deformation from M6 earthquakes and subsequent postseismic slip inside the dense California network we used these products to estimate coseismic offsets at the GPS stations at various times after the event.

The event was clearly detected as steps in the 5 minute rapid and 24 hour time series. The day following the event we measured significant coseismic displacements at 9 stations, with 3 stations having signal to noise ratio greater than 2 (accounting for scatter in the rapid 5 minute time series), and a maximum displacement of 28 mm. Displacements were significant up to 48 km from the event, and had a quad-lobe pattern typical of strike slip earthquakes on northwest striking vertical faults. During the following 30 days the resolution of the displacement field increased as more data were collected and the dataset became more complete. The maximum displacement increased significantly to 35 mm, consistent with continued postseismic afterslip on the west Napa fault.

The similarity between final solutions and positions obtained from rapid 5 minute time series suggests that rapidly estimated coseismic offsets from GPS are accurate enough to rapidly assess earthquake effects and monitor the evolution of afterslip. Future applications of rapid 5 minute time series could include, e.g., best-latency updates of static stress transfer during continued postseismic afterslip.