Poster #091, San Andreas Fault System (SAFS)

Estimating tractions along the San Andreas and San Jacinto faults prior to ground-rupturing earthquakes of the last four centuries

Emery Anderson-Merritt, & Michele Cooke
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Poster Presentation

2021 SCEC Annual Meeting, Poster #091, SCEC Contribution #11583 VIEW PDF
Understanding the state of stress in a fault system prior to previous ground-rupturing earthquakes can help us understand the conditions that generate these events and constrain initial conditions for dynamic rupture models of large earthquakes. We use forward numerical models that incorporate 3D complex configuration of active faults in southern California to estimate shear and normal tractions on the San Andreas and San Jacinto fault systems within the Cajon Pass and San Gorgonio Pass regions. These tractions include stresses accumulated during the interseismic period as well as the effects of several ground-rupturing earthquakes in the region (1857, ~1740, & ~1726 San Andreas; 1800 Jacinto; 1812 & ~1760 SAF/SJF earthquakes). By summing the interseismic loading and effects of nearby earthquakes since the last ground-rupturing event we estimate fault tractions immediately prior to each of these six events. To simulate the interseismic loading we use a two-step back slip approach and the time since last event taken from the Sharer & Yule (2020). To simulate the ground-rupturing earthquakes we use rupture extents from Scharer & Yule (2020) and assign a uniform stress drop along the length of the rupture. We constrain the stress drop by testing several stress drop scenarios for each event and comparing the resulting strike-slip surface displacement to offset measurements from field studies. A stress drop of 0.75 MPa produces surface slip distributions that match field data for nearly all sites along the rupture traces of the six earthquakes. These stress drops are similar to the accumulated tractions we calculate on the pre-rupture fault surfaces. The elastic model provides maximum estimates of fault tractions that do not consider relaxation of stresses during the interseismic period. Additionally, the stress accumulation calculations assume that shear stress drop is complete on ground-rupturing events, which may underestimate absolute shear tractions on the faults. Both shear and normal tractions estimated from models may inform earthquake genesis.