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Group B, Poster #194, SCEC Community Models (CXM)

Crustal architecture across Southern California and its implications on San Andreas Fault development

Siyuan Sui, Weisen Shen, Jeonghyeop Kim, & William E. Holt
Poster Image: 

Poster Presentation

2023 SCEC Annual Meeting, Poster #194, SCEC Contribution #12966 VIEW PDF
Located on the boundary of the Pacific and North American Plates, Southern California is seismically and tectonically active. The present-day configuration of the San Andreas Fault is a result of the long-term tectonic activities with modulation of the lithospheric strength variations, which are affected by both temperature and chemical compositions. Previous studies indicate the seismic velocities and the ratios between P and S waves (Vp/Vs) provide constraints on the crustal physical and chemical properties, which further helps investigate the crustal thermal and compositional architecture.

In this study, we observe regional variations in Vp/Vs, silica content, crustal heat ...
generation and temperature. By employing a 2-layer H-k stacking method, we measure the crystalline crust Vp/Vs in Southern California after removing the possible sedimentary layer effects. Particularly for southernmost California, a structural dichotomy in crust thickness, Vp/Vs and strain rate is observed between the Peninsular Ranges (PR) and Southern San Andreas system. Additional modeling of the compositional and thermal structure in this region suggests that crustal compositional differences are the primary controlling factor influencing the crustal strain rate variations.

In detail, we combine the crystalline crust Vp/Vs with the previous published shear velocity (Vs) model to quantify the crustal silica abundance. With knowing the silica abundance, concentration of heat producing elements and associated heat generation can be calculated. We then combine the new crustal heat generation data with other thermal parameters to infer the crustal thermal structure, which contributes to the new reference model for Southern California.
Moreover, we propose that the high-Vp/Vs, low strain rates, and strong block-behavior of the PR diverted the Pacific-North American plate boundary eastward into the interior of Southern California where the low Vp/Vs and higher silica content in the crust is present. This eastward jump in the plate boundary system from west of the rheologically strong PR to east of PR is responsible for the “Big Bend” geometry of the San Andreas fault.