Southern San Andreas‐San Jacinto fault system slip rates estimated from earthquake cycle models constrained by GPS and interferometric synthetic aperture radar observations

Abstract

We use ground geodetic and interferometric synthetic aperture radar satellite observations across the southern San Andreas (SAF)‐San Jacinto (SJF) fault systems to constrain their slip rates and the viscosity structure of the lower crust and upper mantle on the basis of periodic earthquake cycle, Maxwell viscoelastic, finite element models. Key questions for this system are the SAF and SJF slip rates, the slip partitioning between the two main branches of the SJF, and the dip of the SAF. The best‐fitting models generally have a high‐viscosity lower crust (η = 1021 Pa s) overlying a lower‐viscosity upper mantle (η = 1019 Pa s). We find considerable trade‐offs between the relative time into the current earthquake cycle of the San Jacinto fault and the upper mantle viscosity. With reasonable assumptions for the relative time in the earthquake cycle, the partition of slip is fairly robust at around 24–26 mm/a for the San Jacinto fault system and 16–18 mm/a for the San Andreas fault. Models for two subprofiles across the SAF‐SJF systems suggest that slip may transfer from the western (Coyote Creek) branch to the eastern (Clark‐Superstition hills) branch of the SJF from NW to SE. Across the entire system our best‐fitting model gives slip rates of 2 ± 3, 12 ± 9, 12 ± 9, and 17 ± 3 mm/a for the Elsinore, Coyote Creek, Clark, and San Andreas faults, respectively, where the large uncertainties in the slip rates for the SJF branches reflect the large uncertainty in the slip rate partitioning within the SJF system.