Full Waveform Inversion Tomography of Central and Northern California for Improved Ground Motion Simulations

Arthur J. Rodgers

Submitted September 7, 2025, SCEC Contribution #14527, 2025 SCEC Annual Meeting Poster #TBD

We report a new three-dimensional seismic wavespeed model of Central and Northern California derived from full waveform inversion of moderate earthquakes. The domain covers a large region (540 km west-east by 420 km south- north) spanning the Pacific Ocean to the Basin and Range and Big Sur (Monterey County) to the northern California coast (Mendocino County). Earthquake source parameters are taken from the University of California Berkeley Seismology Lab moment tensor catalog for the period 1993-present with moment magnitudes, MW 4.0-5.5 and include both seismicity on the San Andreas plate boundary and Walker Lane (eastern California & Nevada). We considered about 250 events recorded at broadband stations from permanent and temporary networks in the region and selected 100 events for the inversion and 42 events for model validation. We inverted time-frequency phase measurements on three-component waveforms with a multiscale approach starting in the period band 15-40 seconds and reducing the minimum period to 8 seconds in 5 stages and 57 total iterations. The starting model was a variation of the GIL7 model of Dreger and Romanowicz (1994) with a transitional Moho spanning the known crustal thickness across the region. Inversions solved for isotropic shear and compressional wavespeeds (VS and VP, respectively). The resulting tomography model reveals low VS in the sedimentary geology of the Central Valley with lowest and deepest sediments imaged in the Sacramento-San Joachim Delta. High VS is imaged in the Sierra Nevada. Shallow wavespeeds are discontinuous across Calaveras-Hayward-Rodgers Creek fault systems consistent with known geology and previous studies (high VS Franciscan in the west and low VS Great Valley sequence in the east). The ratio of compressional-to-shear wavespeeds (VP/VS) reveals possible compositional heterogeneity with high VP/VS in shallow sedimentary geology and in the middle and lower crust beneath the Geysers geothermal area, while high VP/VS is imaged in the batholithic Sierra Nevada. The resulting model, CENOCA, greatly improves waveform fits compared to the starting model for both the inversion and validation data sets. The current model could be used for improved source inversions and long-period ground motion simulations or could be used as a basis for further waveform inversion iterations fitting shorter periods including known shallow basin and geotechnical structure.

Key Words
Waveform inversion, grsound motion simulations, tomography

Citation
Rodgers, A. J. (2025, 09). Full Waveform Inversion Tomography of Central and Northern California for Improved Ground Motion Simulations. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Community Earth Models (CEM)