Synthesis of current seismicity and tectonics along the 1857 Mw7.9 Fort Tejon earthquake rupture and the southernmost San Andreas Fault, California, USA

Egill Hauksson, Lucile M. Jones, Joann M. Stock, & Allen L. Husker

Under Review April 12, 2024, SCEC Contribution #13446

We evaluate seismicity and tectonics along the San Andreas Fault (SAF) in southern California to elucidate ongoing near field crustal deformation processes. The principal slip surfaces (PSSs) or the fault core that accommodate major earthquakes, form the boundary between the tectonic plates. We analyze seismicity catalogs extending back to 1857, 1932, and 1981 with progressively improved magnitude of completeness and spatial resolution. The 1857 to present state-wide catalog that is complete at M5.5+ shows minimal aftershock activity for the Mw7.9 1857 and 1906 Mw7.85 San Francisco earthquakes. The higher quality 1932 and 1981 catalogs show that the PSSs (the rupture zone) of the 1857 Mw7.9 Fort Tejon earthquake exhibits remarkable seismic quiescence both in the core and in the adjacent seismicity zone. Further south, the fault core is still aseismic but the shape of the SAF is more complex, and the rate of adjacent seismicity is much higher. This fault complexity and the seismicity rate are larger the more the strike of the SAF deviates from the Pacific plate velocity-vector direction. The focal mechanisms of the SAF adjacent earthquakes are also heterogeneous and rarely have strike and dips that are consistent with slip on the nearby PSSs. We infer that the southern SAF is locked, and the applied shear stress at the core of the fault has not recovered since 1857. Future aftershock sequences of major earthquakes will likely have below average productivity.

Key Words
Seismicity, seismotectonics, rheology, stress, strength, San Andreas fault

Citation
Hauksson, E., Jones, L. M., Stock, J. M., & Husker, A. L. (2024). Synthesis of current seismicity and tectonics along the 1857 Mw7.9 Fort Tejon earthquake rupture and the southernmost San Andreas Fault, California, USA. Journal of Geophysical Research, (under review).