Three-Dimensional Deep S-Wave Velocity Model of the South and East San Francisco Bay Area Obtained From Microtremor Array Measurements and Three-Component Microtremor Measurements

Session: Near-Surface Effects: Advances in Site Response Estimation and Its Applications [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

We estimated three-dimensional (3D) deep S-wave velocity (Vs) model of the south and east San Francisco Bay area using microtremor array measurements (MAM) and three-component (3C) microtremor measurements. Data acquisition included large scale MAM at 16 sites and single station 3C microtremor measurements at approximately 60 sites. The large-scale MAM uses two to twenty long-period accelerometers or 2 Hz geophones. Maximum distance between stations ranged from several hundred meters to several kilometers, depending on the site. A spatial auto-correlation (SPAC) method calculates phase velocities from the vertical component of seismic ambient noise. Minimum frequency ranged from 0.2 to 2 Hz, depending on the site, corresponding to maximum wavelengths of 10 to 1 km. An inversion was used to estimate Vs profiles to a depth of 500 to 3000 m, depending on the site. Three-component microtremor measurements use a single 3C long-period accelerometer or 3C 2 Hz geophone. Horizontal to vertical spectra ratio (H/V) was calculated from 3C seismic ambient noise. The peak frequency of H/V ranged from 0.15 to 2 Hz depending on the site. We compiled all Vs profiles and H/V to estimate an approximate 3D Vs model to a depth of 3000 m. Depth to bedrock of Vs = 2000 m/s is the deepest (> 2000 m) to the west (Cupertino) and north (San Jose - Milpitas) of downtown San Jose. The peak frequency of H/V is less than 0.2 Hz in this region. Bedrock depth appears to be relatively shallow (700 to 1500 m) in the middle of the south Bay, from downtown San Jose to San Jose Airport and Alviso. This high-velocity ridge in the south Bay appears parallel to the Silver Creek Fault and may continue north to Coyote Hills. The peak frequency of H/V ranged from 0.3 to 1.5 Hz in the region. The bedrock depth is relatively deep (1500 to 2000 m) along shoreline between Coyote Hills and Hayward Fault in the east Bay. The peak frequency of H/V ranged from 0.3 to 0.6 Hz in the region and implies that bedrock depth in the east Bay is probably shallower than the Cupertino or San Jose - Milpitas region in the south Bay.

Presenting Author: Koichi Hayashi

Authors