Observations and Implications of Fault-Zone Trapped Waves From the 2014 M6 South Napa Earthquake, California

Abstract:

From aftershocks of the 24 August 2014 Mw 6.0 South Napa earthquake, we observe prominent fault-zone trapped waves (FZTWs) along two 1.9-km-long seismic arrays that were deployed across the northernmost projection (array 1) and the southern part (array 2) of the surface rupture of the West Napa Fault Zone (WNFZ). We also observe FZTWs on an array (array 3) deployed south of the Carquinez Straits across the intersection of the Franklin and Southhampton faults, which appear to be southward continuations of the WNFZ. Arrays 1 and 2 consisted of 20 three-component, short-period seismographs, and array 3, which was 900 m long, consisted of 10 of the same seismographs. We analyzed waveforms from 55 aftershocks in both time and frequency to characterize the fault damage zone associated with the Mw 6.0 South Napa earthquake. Post-S-wave durations of the FZTWs increase with epicentral distances and focal depths from the recording arrays, suggesting a low-velocity waveguide along the WNFZ at depths in excess of 5-6 km. Locations of the aftershocks that generated the FZTWs and 3-D finite-difference simulations demonstrate that the subsurface rupture zone has a remarkable velocity reduction of ~40-50% between arrays 1 and 2, coincident with the ~14-km-long mapped surface rupture zone and a ~500-m-wide deformation zone. The low-velocity waveguide associated with the WNFZ extends further southward to at least array 3, but with a more moderate velocity reduction of 30-35%. The waveguide effect appears to have localized and amplified ground shaking along the WNFZ and along faults farther to the south, consistent with the observations of Baltay and Boatwright (2015). Thus, we suggest that during future major earthquakes, the wave-guide effect may result in localized amplification, extended ground shaking, and increased damage along the WNFZ and its extensions further to the south, even if the surface rupture is limited to only a portion of the WNFZ. Broad-band recordings near the WNFZ for teleseismic earthquakes are in waveform analysis for more information on the deep structure of fault damage zone.