Sub-regional Site Response for the San Francisco Bay Area
Description:
This study aims to develop a site response model specific to Northern California, focusing on the San Francisco Bay Area (SFBA). Whereas current NGA-West2 ground motion models, such as Boore et al. (2014; BSSA14), were developed for application across broad regions (such as California), they may be biased for application in particular sub-regions. We investigate whether the unique geology in the SFBA contribute to sub-regional site response features that can be modelled conditional on VS30, 1.0 km/s isosurface depths (z1.0), and broad geologic descriptors.
Typical site conditions in the SFBA include Quaternary sediments of variable thicknesses over Franciscan Complex bedrock west of the Hayward Fault, whereas east of the Hayward fault outcropping bedrock includes Late Jurassic through Cretaceous Great Valley Sequence formations and Tertiary sedimentary rock such as the Orinda formation. In contrast to the deep sedimentary basins that inhabit large areas in Southern California, the SFBA Quaternary sediments are generally characterized by smaller basins (e.g., San Jose, Livermore) and shallow sedimentary-shelf deposits, typically with z1.0 < 200 m. Based on these observations, we classify major site types, from rock to soil as: Franciscan and Great Valley Sequence rock, Tertiary rock, shelf deposits, valleys, and basins. These distinct geological provinces provide a basis for investigating systematic variations of site response across the region.
Preliminary findings suggest that the ergodic VS30-based site response model in BSSA14 generally performs well across the region as a whole, but that adjustments are needed for soft and stiff sites and stiff sites (> 760 m/s). Specifically, the data support VS30-scaling saturation for low-velocity sites (< 200 m/s), small adjustments to VS30-scaling for moderate site conditions, and reduced de-amplification for VS30 > 760 m/s. Ongoing work is underway to examine depth effects, which will be conditioned on type of geologic province.
Session: Why Ignore the Structure? Soil-structure Interaction and Site Response at Local and Regional Scales - I
Type: Oral
Date: 4/17/2025
Presentation Time: 11:00 AM (local time)
Presenting Author: Shako
Student Presenter: Yes
Invited Presentation:
Poster Number:
Authors
Shako Mohammed Presenting Author Corresponding Author eng.shako@gmail.com University of California, Los Angeles |
Rashid Shams rashidsh@usc.edu University of Southern California |
Tristan Buckreis tristanbuckreis@ucla.edu University of California, Los Angeles |
Chukwuebuka Nweke chukwueb@usc.edu University of Southern California |
Scott Brandenberg sjbrandenberg@ucla.edu University of California, Los Angeles |
Jonathan Stewart jstewart@seas.ucla.edu University of California, Los Angeles |
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Sub-regional Site Response for the San Francisco Bay Area
Session
Why Ignore the Structure? Soil-structure Interaction and Site Response at Local and Regional Scales