Estimating evolving tractions and uncertainties on the San Andreas and San Jacinto fault systems over the last ~1000 years

Emery O. Anderson-Merritt, & Michele L. Cooke

Published September 11, 2022, SCEC Contribution #12104, 2022 SCEC Annual Meeting Poster #252 (PDF)

Estimating the evolving state of stress in a fault system can help us constrain the conditions that may have generated previous ground-rupturing earthquakes, assess present-day fault tractions and their uncertainties, 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 tractions on the San Andreas and San Jacinto fault systems within the Cajon Pass and San Gorgonio Pass regions over the past ~1000 years. These tractions include the accumulation of traction due to tectonic loading, long-term viscoelastic relaxation of stress within the crust, 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 linear interseismic loading rate and subtract from it the effect of viscoelastic stress relaxation. We simulate ground-rupturing earthquakes by assigning a uniform stress drop along the rupture length based on the earthquake extents from Scharer & Yule (2020), estimating the magnitude of this stress drop by best fit of these coseismic simulations to available offset data. We use these models to estimate evolved tractions in two different ways: one assuming that earthquake stress drops are complete, and one subtracting a uniform stress drop from the accumulated traction. Envelopes of the maximum and minimum evolved tractions based on earthquake timing uncertainty show that the complete stress drop assumption results in less present-day uncertainty due to the precisely-known timing of historical earthquakes, but greater uncertainty prior to the historical period. The uniform stress drop assumption results in greater present-day traction uncertainty because the uncertainty in the timing of the start of traction accumulation is not eliminated by complete stress drops. Over a long modeled timeframe, stress relaxation may reduce this source of uncertainty. This new modeling approach may provide estimates of shear tractions that are unavailable from direct measurements.

Key Words
Cajon Pass, San Gorgonio Pass, fault tractions

Citation
Anderson-Merritt, E. O., & Cooke, M. L. (2022, 09). Estimating evolving tractions and uncertainties on the San Andreas and San Jacinto fault systems over the last ~1000 years. Poster Presentation at 2022 SCEC Annual Meeting.

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