Stress States on the Eve of Past Earthquakes Inform Earthquake Rupture Through Fault Complexity Along the San Andreas and San Jacinto Faults

ABSTRACT Estimating the evolving state of stress along active fault systems can provide insight into the conditions that generated past ground-rupturing earthquakes and influenced their ability to propagate through areas of geometric complexity, such as fault branches and stepovers. We use quasi-static forward numerical models that incorporate the 3D complex configuration of active faults in southern California to estimate shear tractions on the geometrically complex southern San Andreas and San Jacinto faults from 1000 to 1900 C.E. These tractions include interseismic accumulation of traction due to tectonic loading, viscoelastic relaxation of shear stress within the upper crust between earthquakes, and effects of other earthquakes on the fault network. We simulate ground-rupturing earthquakes based on the along-strike earthquake extents modeled by Scharer and Yule (2020), assuming that stress drop is complete in each earthquake. We use Monte Carlo simulations to estimate uncertainty in evolving shear tractions due to uncertainties in earthquake timing and in upper-crustal viscosity. Pre-earthquake shear tractions typically do not exceed ∼2 MPa. Although ruptures with length <200 km have pre-earthquake shear tractions that range from near zero to ∼1.75 MPa, these tractions are not less than ∼0.4 MPa for earthquakes with rupture length >200 km. Earthquakes with long (>200 km) ruptures occur only in the single-stranded part of the system, whereas those with short (<125 km) rupture length and high pre-earthquake shear traction occur near fault stepovers and branches. This suggests that high accumulated shear traction encourages longer rupture propagation, but may not be sufficient to overcome geometric complexities. This modeling approach informs our understanding of rupture propagation and provides estimates of fault shear tractions that are unavailable from direct measurements.