A California state-wide seismic velocity model extracted from the USGS National Crustal Model

Oliver S. Boyd, Robert W. Graves, & Evan T. Hirakawa

Published September 10, 2023, SCEC Contribution #13178, 2023 SCEC Annual Meeting Poster #181 (PDF)

The U.S. Geological Survey National Crustal Model (NCM) is being developed to improve estimates of site effects in seismic hazard assessments. Primary outputs of the NCM are continuous velocity and density profiles from the Earth’s surface to the mantle on a 1-kilometer grid across the conterminous United States. The NCM is defined by Biot-Gassmann and mineral physics theory applied to a (1) geologic framework, (2) thermal model, (3) petrologic and mineral physics database, and (4) rock age- and type-dependent porosity model. It is calibrated with over 2000 P- and/or S-wave velocity and density profiles less than 10 km deep from across the conterminous United States and southwest Canada. The NCM in California contains features you would expect in this region. Low velocities and densities are present within young basins including, but not limited to, the Great Valley and Los Angeles basin, decreasing to a VS of ~100 m/s, VP of ~300 m/s, and density near 1,100 kg/m3 approaching the Earth’s surface. In contrast, mountainous areas with igneous outcrops have much higher velocities and are denser with maximum VS near the Earth’s surface as high as 4,500 m/s, VP of 8,100 m/s, and density of 3,500 kg/m3. In this poster, we compare the NCM with existing Bay Area and Southern California velocity models, as well as measured velocity and density profiles and dispersion curves. In addition, we perform and compare ground motion simulations of selected earthquakes in these models.

Preliminary results from simulations using three velocity models of southern California (CVMH15.10, CVMS4.26.M01, and NCM v230802) for the 2014 M5.1 La Habra earthquake indicate that while there are significant differences in characteristics of the velocity models, the amplitudes and pattern of peak ground velocities (PGVs) are broadly similar. The simulations are performed up to 1 Hz with a minimum shear-wave velocity of 500 m/s. The resulting ground motions are amplified in the Los Angeles and surrounding basins, with PGVs of up to 20, 19, and 16 cm/s for CVMH15.10, CVMS4.26.M01, and NCM, respectively. The ground motions are generally larger beyond 100 km epicentral distance for the NCM, which may indicate greater continuity in the seismically slower sediments in this smoother model. The NCM also has higher ground motions to the south towards Dana Point, which is not seen as strongly in the other two models.

Key Words
velocity model, ground-motion simulation, site response

Citation
Boyd, O. S., Graves, R. W., & Hirakawa, E. T. (2023, 09). A California state-wide seismic velocity model extracted from the USGS National Crustal Model. Poster Presentation at 2023 SCEC Annual Meeting.

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