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Slip rates and off-fault deformation in southern California inferred from GPS data and models

Kaj M. Johnson

Published October 2013, SCEC Contribution #1805

In southern California, fault slip rate estimates along the San Andreas fault (SAF) and Garlock fault from geodetically-constrained kinematic models are systematically at the low end or lower than geologic slip rate estimates. The sum of geodetic model slip rates across the Eastern California Shear Zone is higher than the geologic sum. However, the ranges of reported model and geologic slip rate estimates in the literature are sufficiently large that it remains unclear whether these apparent discrepancies are real, or attributable to epistemic uncertainties in the two types of estimates. We further examine uncertainties in geodetically-derived slip rate estimates on major faults in southern California by conducting a suite of inversions with four kinematic models. Long-term-rigid elastic block models constrained by the geologic slip rates cannot fit the present-day GPS-derived velocity field. Deforming (permanent off-fault strain) elastic block models and viscoelastic earthquake cycle block models constrained by geologic slip rates can fit the present-day GPS-derived velocity field with 28-33\% of the total geodetic moment rate occurring as distributed deformation off of the major faults. Models incorporating viscoelastic mantle flow predict systematically higher slip rates than purely elastic models on many of the the major southern California faults with ranges of (elastic/viscoelastic) 29-34/30-37 mm/yr for the Carrizo SAF segment, 20-24/20-32 mm/yr for the Mojave SAF segment, 14-17/18-22 mm/yr for the Coachella SAF segment, 13-19/14-22 mm/yr for the San Jacinto fault, and 5-11/5-11 mm/yr for the western Garlock fault.

Johnson, K. M. (2013). Slip rates and off-fault deformation in southern California inferred from GPS data and models. Journal of Geophysical Research, 118(10), 5643–5664. doi: 10.1002/jgrb.50365.