Three-dimensional deep S-wave velocity model of the South San Francisco Bay Area obtained from horizontal to vertical spectral ratio and microtremor array measurements

Abstract:

We estimated three-dimensional (3D) deep S-wave velocity (Vs) model of the South San Francisco Bay Area using horizontal to vertical spectral ratio (H/V) and microtremor array measurements (MAM). Data acquisition included single station 3C microtremor measurements at approximately 90 sites and MAM at approximately 20 sites. The 3C microtremor measurements used a single 3C 2 Hz geophone or 3C long-period accelerometer. We recorded 20 minutes to one-hour seismic ambient noises at each site. Horizontal to vertical spectral ratio (H/V) was calculated from 3C seismic ambient noise. The peak frequency of H/V ranged from 0.15 to 2.0 Hz depending on the site. The MAM used two to twenty 2 Hz geophones or long-period accelerometers. Maximum distance between stations ranged from several tens of meters to several kilometers, depending on the site. We recorded 20 minutes to one-hour seismic ambient noises at each array. A spatial auto-correlation (SPAC) method calculated phase velocities from the vertical component of ambient noise. Minimum frequency of dispersion curves ranged from 0.2 to 2 Hz, depending on the site, corresponding to maximum wavelengths of 10 km to 200 m. A joint inversion of a H/V spectrum and a dispersion curve consisting of a non-linear least squares method and/or a Genetic Algorithm estimated Vs profiles to a depth of 100 to 3000 m, depending on the site. We compiled all H/V and Vs profiles and estimated an approximate 3D Vs model to a depth of 3000 m. Figure 1 shows H/V peak frequencies obtained from 3C microtremor measurements. Depth to bedrock of Vs = 2000 m/s is the deepest (> 2000 m) to the Southwest (Cupertino) and northeast (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. The peak frequency of H/V ranged from 0.3 to 2.0 Hz in the region. This high-velocity ridge in the South Bay appears parallel to the Silver Creek Fault and may continue west to Mountain View.