Near Surface Seismic Velocity Measurement and Profiling of the East San Francisco Bay Shoreline

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: 

Seismic surface wave surveys were performed along the east San Francisco Bay shoreline to estimate shear wave velocity (V_S) versus depth. Near-surface surveys were conducted at 20 sites, deeper surveys at four. Near-surface surveys used combined active (MASW) and passive (microtremor) methods and had a target depth of 30 m. A typical recording spread was a linear array of 48 vertical geophones (4.5 Hz) with 2 m spacing. Products for each site include a velocity-depth profile, V_S30, and UBC site class. Deeper surveys were conducted using the passive method and provided V_S profiles to depths of 800 to 1500 m.

V_S30 at the near-surface sites ranged from 124 to 251 m/s, with a mean of 197 m/s and standard deviation of 33 m/s. Fourteen of the 20 sites lie in Class D (180 < V_S30 ≤ 360 m/s) and the remaining six in Class E (V_S30 ≤ 180 m/s). Observed V_S30 were clustered about the D-E boundary, with most in the range 158 - 198 m/s. Fundamental mode dispersion curves were dominant at most sites except sites with fill over mud, where higher modes were significant. Higher modes were included in dispersion curve modeling for eight of the 20 sites.

Deeper passive surveys were conducted using 2 Hz vertical geophones. Three surveys were performed to obtain 1D velocity profiles using 22 sensors with 100 m spacing. A fourth survey was executed to prepare a 2D V_S profile and to map depth to bedrock. Fifty seismographs were deployed in two overlapping patches, with about 2-3 hours recording time per patch. The total footprint of the survey was 3 x 1 km. Data were processed to prepare a 2D V_S profile 500 meters deep. The profile shows that lower-velocity sediments thicken with distance from the Coyote Hills. Horizontal to vertical spectral ratio (H/V) was measured using 2 Hz, 3-component geophones at four sites over the length of the survey area. An H/V spectral peak shifts from 1 Hz to 0.5 Hz with increasing distance from the Coyote Hills, indicating an increase in bedrock depth.

Presenting Author: Mitchell Craig

Authors