Three-dimensional S-wave velocity model of the East San Francisco Bay Area obtained from three-component microtremor measurements and microtremor array measurements

Abstract

We estimated a three-dimensional (3D) S-wave velocity (Vs) model of the East San Francisco Bay Area, from San Leandro to Fremont, using three-component (3C) single station microtremor measurements and microtremor array measurements (MAM). Data acquisition included 3C microtremor measurements at approximately 60 sites and MAM at approximately 10 sites. The 3C microtremor measurements yielded horizontal to vertical spectral ratio (H/V) and microtremor array measurements yielded one-dimensional (1D) Vs profiles. We compiled Vs data from several different sources: H/V and 1D Vs profiles, a 3D ambient noise survey on the east side of the Coyote Hills, VS30 information from USGS and UCLA databases, and a USGS geology-based 3D Vs model. These data sources were used to prepare a new 3D Vs model to a depth of 1000 m. The peak frequency of H/V ranged from 0.3 to 10.0 Hz in the investigation area. The peak frequency of H/V is highest (> 1 Hz) around the Coyote Hills and northeast side of the Hayward fault. The peak frequency is about 0.5 Hz at Union City to Hayward and getting lower toward northwest (San Leandro). We summarized relationship between H/V peak frequencies and shallow and intermediate bedrock depths and proposed linear equations. A preliminary 3D Vs model was estimated based on the equations. Depth to the shallow bedrock with Vs of 700 m/s seems to be shallow (150 m) at Union City and getting greater (200 m) toward northwest from Hayward to San Leandro.