Poster #085, San Andreas Fault System (SAFS)

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

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

2021 SCEC Annual Meeting, Poster #085, SCEC Contribution #11164 VIEW PDF
Understanding the mechanical conditions that lead to complexities in earthquake observations is important to seismic hazard analysis. In this study, we simulate physics-based multicycle dynamic models of the southern San Andreas fault (Cajon Pass to Parkfield) and San Jacinto fault (Claremont and Clark strands). We focus on the fault geometry provided by the SCEC Community Fault Model (CFM) and its effect over earthquake cycles. Using geodetically derived strain rates (Smith-Konter and Sandwell, 2009), we validate the models against long-term geologic slip rates and recurrence intervals at various paleoseismic sites. We find that the interactions between fault geometry, dynamic ruptures and ...interseismic stress accumulation produce stress heterogeneity, leading to rupture segmentation and variability in earthquake recurrence intervals. Our models produce earthquakes with rupture extents similar to a recent comprehensive paleoseismic catalog (Scharer and Yule, 2020). The reproduced earthquake ruptures include 1) ruptures of the whole simulated fault system, 2) ruptures that break the Carrizo to Big Bend sections, 3) ruptures involving the simulated San Andreas fault and the Claremont segment of the San Jacinto fault, 4) 1857 Fort Tejon-like events, 5) a 1812 Wrightwood-like event, 6) ruptures that involve a small portion of the southern San Andreas fault near the Big Bend, and 7) ruptures that involve a small portion of the Clark segment of the San Jacinto fault. The Big Bend and Cajon Pass ‘earthquake gates’ occasionally impede dynamic ruptures. The angle of compression, which is the subtraction of the maximum shear strain rate direction (Smith-Konter and Sandwell, 2009) from the local fault strike (CFM), can better assess the impedance of restraining bends to dynamic ruptures. The Big Bend has an angle of compression of ~20 degrees while it is a ~40 degrees restraining bend measured by the fault trace. As a result, ruptures that traverse the Big Bend, like the 1857 Fort Tejon earthquake, are more frequent than would be expected based on the empirical relation, where a ~ 40 degree restraining bend tends to terminate most of the ruptures. 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, similar to that of the 1857 event, providing critical information for ground shaking hazard assessment in the region.