ACCOUNTING FOR FAULT GEOMETRY'S IMPACT ON THE SLIP BUDGET

Determining slip rates of active faults is essential to characterizing their behavior. Slip rates can be estimated through geologic means, either from ages and relative displacements of features offset during discrete past earthquakes or over many earthquake cycles, or through modeling of interseismic deformation observed geodetically. In cases where both geologic and geodetic estimates of slip rates exist for the same faults, however, they are not always in agreement. This discrepancy may reflect temporal variations in slip rate, such that geodetic rates measured over decades are different from geologic rates measured over thousands to millions of years. Alternatively, the discrepancies may reflect spatial variations in slip that persist over many earthquake cycles. In this study we use mechanical models to investigate this second hypothesis, specifically evaluating how releasing steps may affect estimates of fault slip over geologic time scales.

A suite of 2D models of idealized fault systems reveals how fault length, friction and step geometry affect kinematic efficiency of the system and the distribution of slip rate along the faults. We find that although systems with longer fault segments are more efficient, accommodating 80-90% of plate displacement, point sampling of these systems during geologic studies is unlikely (p<50%) to yield representative slip rate estimates. Systems with short segments, although much less efficient, are more likely to yield representative slip rate estimates, particularly when slip on overlapping segments is summed.

A releasing stepover along the San Jacinto fault of the southern San Andreas fault system provides a well-documented real-world example to evaluate long-term spatial variability in slip rates. Geologic investigations reveal slip rates ranging from 1.1 to 14.7 mm/yr with slower rates within the stepover and faster rates outside of the releasing stepover. Geodetic slip rate estimates range from 9-16 mm/yr, overlapping the higher geologic rates. Our 3D model of the region, loaded with plate velocities determined from the geodesy, results in spatially varying slip rates that are consistent with most of the geologic measurements, suggesting that models can be used to put point measurements of slip into the context of slip distribution throughout a fault system.