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Ambient Noise Surface-Wave Tomography of the San Francisco Bay Area, California

Session: Advances in Seismic Interferometry: Theory, Computation and Applications

Type: Oral

Date: 4/22/2021

Presentation Time: 05:00 PM Pacific

Description: 

The characterization of 3D seismic wave propagation in accurate velocity models and the effects it has on strong ground motions from earthquakes is important for accurate estimates of seismic hazard utilizing wavefield simulations. The San Francisco Bay area in California, USA is a highly urbanized active tectonic region facing high seismic hazard and crisscrossed by numerous faults. Our study aims to improve the shear wave velocity model in this region using ambient noise derived surface-wave tomography. We use continuous waveform data recorded by permanent and temporary stations in specific epochs between 1995 and 2020. We derive fundamental mode Love-wave and Rayleigh-wave phase velocity dispersion measurements at periods ~3-16 s using automatic frequency-time analysis applied on multi-component noise cross-correlations. The measurements are used to invert for phase velocity maps using the 2D fast-marching tomography method. The phase velocity maps are used to invert for a 3D Vs model at a horizontal grid spacing of ~10 km. The Vs model clearly shows low velocities in the top ~1 km of known sedimentary basins ~ San Pablo Bay, Livermore basin, Santa Clara valley and the Sacramento-San Joaquin delta and strong velocity contrasts across the Hayward fault and the San Andreas Fault at depth ~2.5 km. Assuming prograde elliptical particle motion, we identify and incorporate dispersion measurements for the 1^st higher mode Rayleigh wave on the radial-vertical plane of the noise cross-correlations over the Sacramento-San Joaquin Delta, which is a deep sedimentary basin. Higher mode Rayleigh waves in the period range ~3-9 s provide further constraints on the velocity structure. We compare the resulting velocity model to the current US Geological Survey Bay area velocity model (version 8.3.0, year ~2010) in terms of velocities and geologic discontinuities. We forward model synthetic seismograms for a few virtual sources and the two 3D velocity models using a finite-difference based seismic wave propagation method and present waveform comparisons.

Presenting Author: Avinash Nayak

Student Presenter: No

Authors