Modeling Releasing Steps of Strike-Slip Fault Systems: Implications for Conflicting Estimates of Long-Term Slip Rates

Abstract:

Long-term fault slip rates typically are assumed to sum to total plate velocity in kinematic models of strike-slip plate boundaries. However, in several locations in southern California, slip rates determined from geodetic data differ from geologic estimates. That sites of geologic investigation are not uniformly distributed along fault systems may compound these discrepancies. For example, releasing steps where sediment accumulates and leaves a continuous record of slip are favored trenching sites, but may not yield representative rates. We investigate the influence of site location on slip rate with: 1) a parametric study of step geometry using 2D mechanical models and 2) case studies of active southern California faults using 3D models. The suite of 2D models reveals how fault length, friction and step geometry (fault overlap and perpendicular separation) affect fault slip and off-fault strain over geologic time scales. We determine the probability that a site along the fault will reveal a representative rate (i.e. mean ± 1 mm/yr) and calculate each system’s kinematic efficiency. We find that the probability of sampling a representative slip rate is greater along short faults, due to the low overall slip magnitudes relative to typical errors. For longer faults the probability of sampling a representative rate is much less and more strongly controlled by step geometry. Faults with longer segments are most efficient, accommodating ~56-86% of plate displacement. For fault segments with no overlap, steps with the smallest separation are most efficient, whereas for overlapping segments, maximum efficiency occurs at larger separations. Systems with short segments, large overlap and small spacing are the least efficient, accommodating as little as 24% of plate displacement. Geologic slip rates from within the releasing steps along the San Jacinto Fault in southern California show reduced rates relative to other sites. This is consistent with the results of 3D models of the Elsinore and San Jacinto faults undertaken for this study. We find that rates within the step are within mean values for the faults, while those along the segments are not. The results of this study suggest that sites located in steps between long fault segments are most likely to yield representative geologic slip rate estimates.

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