Dynamically Triggered Earthquakes in the Geysers Region following the 2014 M6.0 South Napa Earthquake

The 08/24/2014 M6.0 South Napa earthquake is the largest seismic event to strike the San Francisco Bay Area since the 10/17/1989 M6.9 Loma Prieta earthquake. The South Napa event caused severe damage near the epicenter. Based on the Northern California Seismic Network (NCSN) catalog, we find a clear increase of seismicity near the Geysers Geothermal Field following the South Napa event, which is located along its rupture directivity path ~50 km NNW from the hypocenter. Visually inspecting 10 Hz high-pass filtered waveforms at seismic stations near Geysers, we can identify many local earthquakes during the surface waves of the mainshock event that are missing from the NCSN catalog. To obtain a more complete catalog, we apply a recently developed matched filter technique to detect new events within continuous seismic recordings from 74 seismic stations near the Geysers. We use 4000 local earthquakes listed in the NCSN catalog from 06/01/2014 to 09/10/2014 as templates and systematically scan continuous data within ±7 days from the South Napa mainshock. As a result, we detect ~10 times more earthquakes than in the NCSN catalog, and the magnitude of completeness reduces from 0.75 to -0.6. Of the 8091 new events, 28 occurred within the mainshock wavetrain. Depending on the filter used, the first triggered event has an inferred magnitude in the range 3.6-4.0. The intensive seismic activity near the Geysers gradually decays with a p-value of ~0.7 and returns to pre-shock level in about one day. We fit the seismicity rate in the week prior to the South Napa event with the Epidemic Type Aftershock Sequence (ETAS) model and extrapolate to obtain a post-mainshock rate. The observed post-mainshock seismicity rate clearly deviates from the ETAS prediction, which suggests that not all increased seismicity near the Geysers can be explained as aftershocks of the first triggered event. Instead these new events may be associated with stress transients (e.g. creep) or fluid migration. Moreover, the spatial-temporal pattern of the detected events shows possible NW migration, which supports the creep or fluid migration hypothesis. Our next step is to repeat this analysis using events from the more complete Enhanced Geothermal System (EGS) catalog as templates to detect additional events. Our updated results will be presented at the meeting.