Estimating stress state along the San Andreas and San Jacinto faults on the eve of past earthquakes

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

In Preparation March 20, 2024, SCEC Contribution #13437

Estimating evolving stresses along a fault system may reveal the conditions that generated previous ground-rupturing earthquakes and provide realistic initial conditions for dynamic rupture models. We use quasistatic forward numerical models incorporating 3D complex configuration of active faults in southern California to estimate shear tractions along the southern San Andreas and San Jacinto fault systems since 1000 CE. These tractions include the interseismic accumulation due to tectonic loading, viscoelastic relaxation of stress within the crust between earthquakes, and coseismic effects of nearby earthquakes. To simulate interseismic shear traction accumulation we use a two-step back slip approach to estimate linear interseismic loading rate and subtract from it the effect of viscoelastic relaxation. We simulate ground-rupturing earthquakes using earthquake timing and extents compiled by Scharer & Yule (2020), testing two different assumptions about coseismic stress release: a complete stress drop, for which we release all of the accumulated shear traction, and an incomplete stress drop, for which we apply a tapered stress drop independent of the accumulated traction. Both models produce average slip per event that is generally consistent with Scharer & Yule (2020), but they produce different traction accumulation histories and pre-earthquake tractions. In the complete stress drop models, maximum pre-quake tractions rarely exceed ~0.5 MPa and minimum tractions increase slightly with rupture length; in the incomplete stress drop models, pre-quake tractions are more variable, and the minimum pre-quake tractions increase distinctly with rupture length. This new modeling approach may provide estimates of shear tractions that are unavailable from direct measurements.

Anderson-Merritt, E. O., Cooke, M. L., & Scharer, K. M. (2024). Estimating stress state along the San Andreas and San Jacinto faults on the eve of past earthquakes. Geosphere, (in preparation).