Evidence for strong lateral seismic velocity variation in the lower crust – upper mantle beneath the California margin

Voon Hui Lai, Robert W. Graves, Shengji Wei, & Donald V. Helmberger

Published August 29, 2016, SCEC Contribution #6712, 2016 SCEC Annual Meeting Poster #022

Regional seismograms from earthquakes in Northern California show a systematic difference in arrival times across central and southern California where long period (30 – 50 seconds) SH waves arrive up to 15 seconds earlier at stations near the coast compared with sites towards the east at similar epicentral distances. We attribute this time difference to heterogeneity of the velocity structure at the crust − mantle interface beneath the California margin. To explain these observations, we propose a relatively simple 3D model consisting of a fast seismic lid with thickness growing westward from the San Andreas fault, along with a thicker and slower continental crust to the east. Synthetics generated from such a model are able to better match the observed timing of SH waveforms throughout most of central and southern California compared with the 3D models currently available in the SCEC UCVM. The presence of a strong lid buttressed against a weaker crust has a major influence in how the boundary between the Pacific plate and North American plate deforms and is consistent with the observed asymmetric strain rate across the boundary.

Key Words
Lid, lithosphere, California, plate deformation, velocity structure, San Andreas fault

Citation
Lai, V., Graves, R. W., Wei, S., & Helmberger, D. V. (2016, 08). Evidence for strong lateral seismic velocity variation in the lower crust – upper mantle beneath the California margin . Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)