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Shallow Geophysical Investigations following the South Napa Earthquake

​In response to the M 6.0, 24 August 2014 South Napa Earthquake, California, shallow shear wave (Vs) IMASW data and acoustic wave (Vp) 2D tomography data were acquired at nearby seismic stations and locations with mapped surface rupture.

23 Apr 2015
Jamey Turner, David Trench, Bruno Protti, Cooper Brossy, Zachery Mayo


Vs data will be used to develop 1D depth-averaged 30 meter (Vs30) measurements and 0.5m resolution Vs-depth profiles for each alignment. These site-specific Vs data provide constraints on recorded ground motions and calibration of ground conditions at seismic instrumentation sites in support of California’s statewide compilation of Vs30 data for mapped geologic units. These data could be used to improve attenuation relationships for this region. 2D Vp tomography data provide a high-resolution subsurface map of Vp structure along each 92 meter alignment, and can be used to map the depth to water table and identify subsurface faulting based on juxtaposed velocity structure. Vs-based NEHRP site classification criteria combined with Vp constraints on water table depth provide inputs to inform liquefaction susceptibility mapping. For this investigation, Vs and Vp data were acquired at Green Valley (ID NGVB), Huichica Creek (NHC), and No. 3 Napa Fire Station (ID 1765) seismic monitoring stations. The Green Valley Road site provides a Vs30 measurement of the Sonoma Volcanics, a widespread volcanic unit in the North Bay area, which is identified by the CGS as a key data gap in the statewide geologic unit-Vs30 dataset. Data were acquired with 4 m spacing using a 24-channel acquisition unit, 4.5 Hz geophones, with a hammer and strike plate active source. Data processing is currently in progress; Vs processing is being completed using the IMASW approach (O’Connell and Turner, 2011), and Rayfract software for the Vp 2D tomography processing. Preliminary results show Vs-depth is well-constrained to 60 m at the Fire Station Site, to 100 m at Huichica Creek, and 50-100 m at Green Valley.


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