Annual Meeting - SCEC.org

Report | SCEC Research Accomplishments
San Andreas Fault System Modeling
Figure 9. Interseismic stressing rate and seismicity on San Andreas fault derived from estimated slip rates for all southern
California faults. Seismicity correlates with sections of fault for which neighboring faults make substantial contribution to
interseismic stressing. (Meade et al., 2010)
Platt and Becker used GPS data to investigate strain partitioning around the Garlock fault as a pilot study designed to
understand better the mechanics of sinistral faults that occur in the dominantly dextral San Andreas Fault system. They found
that the Garlock Fault slips left-laterally at ~7.5 mm/yr and rotates counterclockwise at 5.2°/m.y.
Figure 9. Interseismic stressing rate and seismicity on San
Andreas fault derived from estimated slip rates for all southern
California faults. Seismicity correlates with sections of fault for
which neighboring faults make substantial contribution to
interseismic stressing. (Meade et al., 2010)
Modeling Fault Zone Behavior
46 | Southern California Earthquake Center
Hooks et al. developed a new 3D finite difference model for
the southern San Andreas Fault system in order to
investigate vertical deformation. The SCEC geological
vertical motion database provided long-term vertical
deformation rates to refine the model. The resulting model
successfully reproduces the general patterns of horizontal
velocity (from the PBO data) and strain rate. Current work
is focused on assessing the model resolution and influence
of boundary conditions. They plan to integrate the results
from the dynamic model with the results from their semianalytic
crustal deformation models.
Bennett et al. developed geodetically-constrained block
models to investigate four possible fault-block geometries
in the Eastern Transverse Ranges in an effort to constrain
better slip-rate estimates for the southernmost San Andreas
Fault (SSAF). While no model provided a statistically better
fit to the data than the others, for all geometries considered,
the results suggest that the SSAF has a strong along-strike
slip rate gradient with rates decreasing from ~23 mm/yr to
< 10 mm/yr as one moves northwest while the estimated
San Jacinto slip rate is ~12 mm/yr.
In another application of block modeling, Meade et al. used
southern California fault slip rates estimated from a
geodetically constrained block model that includes all
major regional fault structures to calculate fault stressing
rates resulting from interseismic fault system interactions
(Figure 9). Their results suggest that interseismic stressing
over the course of the earthquake cycle is as significant as
co-seismic and postseismic stress changes and that seismic
hazard assessment should consider stressing due to
interseismic fault interactions rather than simply the slip
rates on individual faults.
Efforts at modeling more realistic fault zone behaviors, such as damage regions, progressed significantly this year, with codes
including fault stepovers and dipping faults instead of the simple straight, vertical strike slip faults examined previously.
Hearn &Vaghri explore the effects of lateral rheological contrasts in suites of strike-slip faults as well as deep, viscous shear
zones and their effects on modeling of interseismic geodetic data. Duan et al., 2011, find that triggered elastic and inelastic
deformation on nearby fault zones can occur coseismically when the pre-seismic stress level is comparable to the strength of
the fault zones, and that the sense of inelastic deformation can be the opposite from the elastic deformation that would be
predicted based on the coseismic Coulomb stress change (Figure 10). Fialko et al. explore 2D numerical models that
investigate the origin of shallow slip deficits that appear in many inversions of coseismic data associated with large strike-slip