Estimating stress state along the San Jacinto and southern San Andreas faults on the eve of past ground-rupturing earthquakes

Emery O. Anderson-Merritt, Michele L. Cooke, & Katherine M. Scharer

Published September 10, 2023, SCEC Contribution #13161, 2023 SCEC Annual Meeting Poster #093

Estimating the evolving state of stress in a fault system can help us constrain the conditions that may have generated previous ground-rupturing earthquakes and provide initial conditions for dynamic rupture models of large earthquakes. We use quasistatic forward numerical models that allow us to incorporate a complex, three-dimensional configuration of active faults in southern California to estimate strike-slip shear tractions on the southern San Andreas and San Jacinto fault systems since 1000 CE. These tractions include the interseismic accumulation of traction due to tectonic loading, viscoelastic relaxation of stress within the crust between earthquakes, and effects of nearby earthquakes. To simulate the accumulation of shear tractions during the interseismic period we use a two-step back slip approach to estimate steady-state interseismic loading rate and subtract from it the effect of viscoelastic stress relaxation. We simulate ground-rupturing earthquakes by releasing the accumulated strike-slip shear traction along the rupture length taken from earthquake extents compiled by Scharer & Yule (2020). We use a Monte Carlo approach to explore how uncertainties in earthquake timing and effective viscosity of the upper crust impact traction accumulation. Elastic models and viscoelastic models with viscosity greater than ~10^20 Pa•s produce average slip per event that is generally consistent with Scharer & Yule (2020), suggesting that the modeled traction evolution adequately captures many of the processes that impact long-term stress evolution. Maximum pre-quake tractions in these models rarely exceed ~0.5 MPa and minimum tractions increase slightly with rupture length. This new modeling approach may provide estimates of shear tractions that are unavailable from direct measurements and improve accuracy of dynamic simulations of past earthquakes.

Key Words
stress, San Andreas, San Jacinto

Citation
Anderson-Merritt, E. O., Cooke, M. L., & Scharer, K. M. (2023, 09). Estimating stress state along the San Jacinto and southern San Andreas faults on the eve of past ground-rupturing earthquakes. Poster Presentation at 2023 SCEC Annual Meeting.

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