Observation-constrained multicycle dynamic models of the southern San Andreas and San Jacinto faults: addressing complexity in paleoseismicity with realistic fault geometry

Dunyu Liu, Benchun Duan, Katherine M. Scharer, & Doug Yule

In Preparation March 10, 2021, SCEC Contribution #10942

Understanding mechanical conditions behind observed earthquake complexities are critical to develop models with better predicting power. In this study, we use physics-based multicycle dynamic models, with an emphasis on realistic fault geometry and its interactions with dynamic ruptures and interseismic stress accumulation and relaxation, to explore the rupture complexities observed on the fault system consisting of the southern San Andreas and San Jacinto faults. With geodetically derived strain rates and fault geometries from the Community Fault Model as input, the simulated results are validated against long-term geologic slip rates and compared to statistics of recurrence intervals at various paleoseismological sites. We find that realistic fault geometry and its interactions with dynamic ruptures and interseismic stress accumulation form a feedback loop of evolving stress heterogeneities, which determines rupture segmentation and high coefficient of variation of earthquake timing from paleoseismicity. Our models can reproduce rupture scenarios with diverse rupture extents derived from a recent comprehensive paleoseismological summary of the faults. We find that the Big Bend and the Cajon Pass earthquake gates act as conditional barriers to ruptures. A concept of effective fault geometry is developed to assess the impedance of restraining bends to dynamic ruptures, given the fact that near-fault maximum strain directions tend to adapt to local fault geometry. The Big Bend should be a much stronger restraining bend given its >40 degrees change in strike, while its effective angle of compression is only about 20 degrees, resulting in more through-going rupture like the 1857 Fort Tejon than expected. Our models indicate that large ruptures that involve the whole simulated fault system tend to initiate north of the Big Bend and to propagate southwards, providing critical information for ground shaking hazard assessment in the region.

Citation
Liu, D., Duan, B., Scharer, K. M., & Yule, D. (2021). Observation-constrained multicycle dynamic models of the southern San Andreas and San Jacinto faults: addressing complexity in paleoseismicity with realistic fault geometry. Journal of Geophysical Research, (in preparation).