Kinematics of southern California crustal deformation: Insights from finite-element models

Elizabeth H. Hearn

Submitted July 19, 2018, SCEC Contribution #8217

Two sets of finite-element model suites were developed to address discrepancies between geodetic and geologic fault slip rates in southern California, as well as the extent of “off-fault” deformation that does not result from elastic strain around known, locked faults. GPS-constrained models, which are elastically locked, represent current deformation, while unlocked, strain energy rate (TSE) minimizing models represent time-averaged deformation over thousands to millions of years. Both classes of models suggest low slip rates (<29 mm/yr) for the San Andreas Fault (SAF) in the Mojave and Big Bend regions. Ensemble slip rates of less than 8 mm/yr are also inferred for thrust faults within the Transverse Ranges. GPS-constrained, locked models suggest high slip rates for the Imperial Fault and the Coachella SAF (>35 mm/yr and >32 mm/yr) but TSE-constrained, unlocked models suggest low slip rates (<41 and <21 mm/yr) for these two faults. All models are consistent with strain transfer from the SAF Coachella segment to the Eastern California Shear Zone, bypassing the SAF, but the GPS-constrained models suggest this far more strongly. This would seem to support variations in rates of fault slip and regional deformation patterns over millennial timescales, but the low inferred rate for the SAF Mojave segments for both model classes argues against this hypothesis. Assuming a “propeller” geometry for the SAF (Fuis et al., 2012) does not alter inferred slip rates. Correcting the GPS velocity field for seismic cycle effects associated with large earthquakes on the 1857 SAF rupture segment increases the inferred slip rate of the Mojave S segment of the San Andreas fault by up to about 5 mm/yr. Off-fault deformation accounts for 38% of the total moment accumulation in the preferred GPS-constrained model, broadly consistent with previous estimates.

Hearn, E. H. (2018). Kinematics of southern California crustal deformation: Insights from finite-element models. Tectonophysics, (submitted).