Updates for San Francisco Bay Sediments in the USGS SF-CVM and Simulation of Long-Duration Shaking in the East Bay Plain

Evan T. Hirakawa, & Brad T. Aagaard

Published August 12, 2021, SCEC Contribution #11326, 2021 SCEC Annual Meeting Poster #011

We update seismic velocities for Quaternary sediments in the Central Bay Block of the USGS San Francisco Bay Region 3D Seismic Velocity Model (SF-CVM), with the goal of more accurately simulating long-duration reverberations in the East Bay Plain as observed during Hayward Fault earthquakes. Very low velocity Bay Mud and other SF Bay sediments trap seismic energy and are most likely responsible for the intense reverberations recorded, which are not reproduced in simulations using the current SF-CVM (Hirakawa and Aagaard, in review). We divide the existing Quaternary zone of the SF-CVM into subzones corresponding to sediment types (e.g., clay, sand, alluvium) based on surface geology (Witter et al., 2006). We compile 41 one-dimensional velocity profiles from the Central Bay Block (Boore, 2003) and classifying each layer based on sediment-type label from corresponding geologic logs. We fit a power law function to this data to obtain new velocity-depth relations for each sediment type, which are then applied to each subzone. The result shows significantly lower velocities throughout the Quaternary zone, especially in Bay Mud, which in turn amplifies synthetic ground motions to be closer to those recorded. We quantify accuracy and improvement of synthetic ground motions by simulating eight Hayward Fault earthquakes (M3.6–4.5) using a seismic wave propagation code (SW4). We compare peak ground velocity and cumulative absolute displacement for the synthetics with observations. We find that the updates to the shallow velocities improve the efficiency in our modeling the of trapping seismic energy and produce reverberations that match observations more closely than those for the current SF-CVM.

Key Words
velocity, sediments, velocity model, San Francisco Bay Area, simulation, ground motion

Citation
Hirakawa, E. T., & Aagaard, B. T. (2021, 08). Updates for San Francisco Bay Sediments in the USGS SF-CVM and Simulation of Long-Duration Shaking in the East Bay Plain. Poster Presentation at 2021 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)