Updating the USGS San Francisco Bay Area 3D Seismic Velocity Model: Special Focus on the North Bay

Evan T. Hirakawa, & Brad T. Aagaard

Published August 6, 2020, SCEC Contribution #10291, 2020 SCEC Annual Meeting Poster #175

We present the latest developments on our evaluation and adjustments to the USGS San Francisco Bay Area 3D Velocity Model, with the goal of more closely replicating observed seismic records with 3D ground-motion simulations. In previous work, we accomplished this in the East Bay by modifying some of the empirical rules used to assign elastic properties to the USGS Bay Area Geologic Model [Hirakawa and Aagaard, 2019]. One notable adjustment was to a large portion of the Napa-Sonoma fault block that extends into the East Bay and was mislabeled as Franciscan. We found that in the East Bay it is more likely Great Valley Sequence or Cenozoic rock, which has significantly lower seismic velocities. However, north of San Pablo Bay, Franciscan rocks do in fact exist in this fault block, bounded by the St. John Mountain fault. Resolving the appropriate location and velocity contrast associated with this geologic boundary is one goal of the current work.
We simulate motions from ten Mw 3.5-4.5 earthquakes whose seismic waves propagated through the North Bay. Some of these events are on the Hayward Fault while others are on faults in the Napa area. To compare with our synthetics, we use data from broadband and strong motion stations in local networks within ~50 km of the earthquake epicenters. We employ two metrics to quantify the agreement between synthetic and observed waveforms. One assesses the travel time by computing a time lag between synthetic and observed arrivals and calculates a relative seismic velocity error. The other computes the ratio of cumulative absolute velocity between synthetic and observed records, which captures the agreement in later reflected arrivals and long duration coda reverberations, thereby providing insight into material contrasts and complex path effects. These two metrics were successfully employed in our previous modifications to the East Bay. The goal is to implement changes that lead to significant improvements in these two metrics compared with the previous version of the seismic velocity model in the North Bay.

Key Words
velocity model, San Francisco Bay Area

Hirakawa, E. T., & Aagaard, B. T. (2020, 08). Updating the USGS San Francisco Bay Area 3D Seismic Velocity Model: Special Focus on the North Bay. Poster Presentation at 2020 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)