SCEC Award Number 15199 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title A finite-element model of southern California lithosphere deformation for the SCEC CSM
Investigator(s)
Name Organization
Elizabeth Hearn Consulting Geophysicist
Other Participants
SCEC Priorities 2d, 1b SCEC Groups SDOT, Geodesy
Report Due Date 03/15/2016 Date Report Submitted 03/16/2016
Project Abstract
 During 2015, I worked on both kinematic and dynamic models of southern California deformation. For the kinematic modeling, my efforts centered on implementing slip rate estimates via a Markov Chain Monte Carlo (MCMC) method to supplement the least squares Monte Carlo method I usually use. I have also corrected the GPS velocity field used in my kinematic models for a long-wavelength viscoelastic perturbation resulting from large earthquakes on the 1857 rupture segment of the SAF, and inverted the corrected velocity field for slip as a first step in assessing the importance of this transient to inferred slip rates. By varying locking depths (based on UCERF3 data) and accounting for viscoelastic seismic cycle effects, I am working to resolve or minimize geologic-geodetic slip rate discrepancies in the kinematic model, so I can obtain a meaningful estimate of off-fault deformation. For my current model suite, off-fault deformation appears to account for about 35% of the total strain energy accumulation in southern California, in line with other estimates. I have downloaded SHmax orientations from the SCEC Community Stress Model (CSM) and compared them with my dynamic and kinematic model results (for individual elements and at the SCEC CSM coordinates). This is the first step toward implementing model calibration to SHmax data (and other stress quantities), principally for dynamic modeling.
Intellectual Merit This project contributes to several SCEC science goals, directly and indirectly, by providing estimates of stress rates and stresses for the southern California lithosphere, as well as fault slip rates and rates of “off-fault” deformation. Kinematic models provide estimates of quantities that feed directly into probabilistic seismic hazard estimates such as UCERF3. Though several GPS-constrained models of deformation kinematics have been developed for the region, my kinematic models improve on most of them by accounting for spatially varying deformation within fault-bounded blocks. Models of deformation dynamics for the entire southern California lithosphere that take into account physically reasonable representations of major faults are important for understanding stresses (for a truly integrated, physics-based understanding of fault systems and potential hazard). Stresses are also required as input for sophisticated new models of rupture and strong motion that take nonlinear materials into account.
Broader Impacts An understanding of present-day fault slip and stress accumulation rates, and the proportion of accumulating strain energy that is either inelastic or unrelated to large earthquakes on known, major faults (“off-fault deformation” ) is central to assessing seismic hazard in southern California. Contributions of modeled stressing rates and stresses to the SCEC CSM help build a robust community resource for SCEC and other scientists.
Exemplary Figure Figure 1. Results from two kinematic model suites, for slip rates on the Mojave section of the San Andreas Fault. Panels a and d show the fit to GPS velocities as a function of slip rate on this fault segment, for models calibrated to the SCEC CMM4 GPS velocity field without (a) and with (c) a correction for viscoelastic seismic cycle effects (see text). The fit is represented as the weighted residual sum of squares (WRSS) normalized to the lowest WRSS on panel a. Model faults are locked to depths based on values from the UCERF3 report. Panels c and f show slip rate distributions for models with WRSS and strain energy rate below thresholds indicated with the colored lines on panels a and b. Panels e and f show distributions of slip rates for accepted models, for the Metropolis-Hastings MCMC method. 10,000 models were run in each case and the acceptance rate was about 35%. Gray shading shows the UCERF3 slip rate range.