SCEC Award Number 22030 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Estimating stress state along the San Jacinto and southern San Andreas faults on the eve of past ground rupturing earthquakes and implications for current stress state
Investigator(s)
Name Organization
Michele Cooke University of Massachusetts Amherst
Other Participants Emery Anderson-Merritt
SCEC Priorities 1c, 1d, 2a SCEC Groups SDOT, SAFS, Geology
Report Due Date 03/15/2023 Date Report Submitted 01/05/2024
Project Abstract
 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 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 southern San Andreas and San Jacinto fault systems since 1000 CE. We 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 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 stress relaxation. We simulate ground-rupturing earthquakes by assigning a tapered 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 to available geologic data. We combine these models to estimate evolved tractions assuming overall ~0.75 MPa stress drops for large ground rupturing earthquakes, which is consistent with slip per event data. Over a long timeframe, stress relaxation reduces traction uncertainties arising from earthquake timing and stress drop uncertainty. Uncertainties of viscosity have larger impact than stress drop or earthquake timing. Larger ruptures are associated with greater accumulation of pre-earthquake traction. This new modeling approach provides estimates of shear tractions that are unavailable from direct measurements.
Intellectual Merit In this study, we estimate the evolving and pre-earthquake along-strike shear tractions along the San Andreas and San Jacinto faults since ~1000 CE. Models with overall stress drop of ~0.75 MPa produce slip per event that are consistent with geologic data and models with up-per crustal viscosity of 1020 Pa-sec or greater produce traction histories that are consistent with geologic data. We also investigate the impacts of uncertainty in earthquake timing, upper crustal viscosity and stress drop on shear traction estimates. Uncertainties in crustal vis-cosity have the greater impact on shear tractions followed by stress drop uncertainties. While the earthquake timing uncertainties are large, they do not impact the shear traction estimates as much as other uncertainties. Estimates of the fault shear tractions through time and over several earthquake cycles reveal potential conditions that preceded previous ground-rupturing earthquakes and can provide initial conditions for dynamic rupture models. Our find-ings show that longer ruptures are associated with greater accumulated shear traction prior to the earthquake.
Broader Impacts The pre-earthquake tractions along the San Andreas and San Jacinto faults can be used as initial conditions for dynamic rupture models. This product would be a significant refinement for models that simulate past earthquake events and yield insight into the conditions that generate damaging earthquakes. This project supports both a UMass PhD candidate, Emery Anderson-Merritt, who is transgender, and a part-deaf female PI, Cooke.
Exemplary Figure Figure 3: Pre-earthquake tractions from Monte Carlo simulations using elastic and inelastic rheology incom-plete stress drop scenarios. Minimum pre-earthquake tractions increase with rupture length.