Exciting news! We're transitioning to the Statewide California Earthquake Center. Our new website is under construction, but we'll continue using this website for SCEC business in the meantime. We're also archiving the Southern Center site to preserve its rich history. A new and improved platform is coming soon!

A correlation between the crystalline crust Vp/Vs and strain rates in Southern California and its implication on the fault system formation

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

Submitted September 11, 2022, SCEC Contribution #12451, 2022 SCEC Annual Meeting Poster #257

The crustal deformation and its associated seismic hazards of Southern California have been under intense investigations. Among the investigations, seismic models that contain the elastic properties (e.g. compressional Vp and shear Vs seismic velocity) have been constructed and are often utilized to infer the geological history and the ongoing crustal deformation. The velocity ratio between the compressional and shear velocity (Vp/Vs) provides unique information of the crust as it reflects the rock composition, cracks, partial melting, water saturation, etc.
In this study, we apply a 2-layer H-κ stacking method to obtain the Vp/Vs and thickness for both the sedimentary and crystalline crustal layers for Southern California. The resulting crystalline crust Vp/Vs show a regional pattern related to the tectonic provinces, which can be explained mostly by the compositional difference (i.e. felsic crust: high Vp/Vs; mafic crust: low Vp/Vs). Particularly, for the Southernmost regions encompassing the Peninsular Ranges (PR) and Southern San Andreas system (SSA), the crystalline crust Vp/Vs and Moho depth vary significantly over a small lateral scale (~ 100 km). The PR displays thick crust (>30 km) with high Vp/Vs (>1.85) and the SSA displays thin crust (<20 km) with low Vp/Vs (<1.65). Additionally, the strain rates from a steady-state model inferred from GPS motions show that the felsic thin extended crust in the SSA have ~4 time the strain rates on average than the thick mafic crust in the PR.

The newly constrained structural variations define the PR as a distinctive crustal block, with thicker crust, higher Vp/Vs and Vs, and lower strain rate, indicative of being compositionally more mafic and rheologically stronger than the transform fault plate boundary zone to its east. The fundamental difference between the relatively rigid (lower strain rate) and mafic PR and the rapidly straining and more felsic SSA has implications for the evolution of the fault zone in Southern California that accommodates Pacific-North America relative motion. The result is consistent with the hypothesis that rather than slice through the PR, the rheologically stronger PR perhaps forced the transform system to migrate eastward. The resulting right-lateral transform system is responsible for the thinner crust in the SSA and this eastward migration ultimately led to the opening of the Gulf of California from 5 Ma onward. However, this hypothesis needs to be further tested.

Key Words
Vp/Vs, composition, strain rates, big bend

Sui, S., Shen, W., Holt, W. E., & Kim, J. (2022, 09). A correlation between the crystalline crust Vp/Vs and strain rates in Southern California and its implication on the fault system formation. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)