Topography and Low-velocity Taper Improve Accuracy of 0-1 Hz Wave Propagation Simulations in the San Francisco Community Velocity Model (SFCVM)

Kim B. Olsen, & Te-Yang Yeh

Submitted September 8, 2024, SCEC Contribution #13816, 2024 SCEC Annual Meeting Poster #172

We have simulated 0-1 Hz wave propagation for 3 M4 events in a 40 km by 40 km section of the northern Bay Area Community Velocity model (SFCVM, Hirakawa and Aagaard, 2022), just east of San Francisco, to quantify the effects of topography, a low-velocity taper (LVT), the velocity of a Gabbro unit, and an imposed minimum shear-wave velocity (Vs) in preparation for an upcoming CyberShake study. The fourth-order finite-difference code AWP-ODC with support for topography via curvilinear grids is used for the simulations, with a discontinuous mesh enabling a minimum shear-wave velocities of 80 m/s. We quantify the fit to data using 0.2-1.0 Hz Fourier Spectral Amplitude (FAS). We find that an LVT with depths of 600-1,200 m both below and outside basin areas significantly improve the FAS fit. Topography increases the amplitude of the secondary arrivals and adds duration, generally bringing simulations in better agreement with data, and significantly improving the FAS bias for the vertical component. Clamping Vs at 400 m/s, as planned for the upcoming northern California CyberShake Study, causes further underprediction at basin sites at frequencies 0.5-1.0 Hz. FAS at basin sites are underpredicted by 55% without LVT and by ~40% with LVT for zT=700m, compared to 26% underprediction including topography and LVT, without a Vs floor. The choice of Vs in UCVM (2,400 m/s or 3,500 m/s) for a local Gabbro unit and the choice of Vp/Vs ratio when applying the Vs floor have negligible effects on the FAS.

Citation
Olsen, K. B., & Yeh, T. (2024, 09). Topography and Low-velocity Taper Improve Accuracy of 0-1 Hz Wave Propagation Simulations in the San Francisco Community Velocity Model (SFCVM). Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)