THE EFFECT OF REVERSE FAULT GEOMETRY ON SLIP RATE ESTIMATES

Estimates of fault slip rates are an integral part of assessing seismic hazard because they affect estimates of earthquake renewal and moment release rates. For some faults, however, slip rate estimates vary among geodetic studies or between geodetic and geologic investigations. These differences may reflect time-transient deformation, but they may also reflect differences in spatial scale of observations and the models used to interpret them. This study seeks to characterize the impact of common reverse fault geometries, such as ramp-flat features, bends, and steps, on geodetic and geologic slip rate estimates using boundary element numerical models.

A suite of two-dimensional models is used to explore ramp-flat geometry, commonly observed in reverse fault systems. The models are subjected to far-field loading representative of horizontal surface velocities recorded by GPS, and are designed to account for both interseismic and geologic timescales by coupling creeping portions of faults with either locked or unlocked portions. The models permit exploration of the impact of down-dip geometry on fault slip rates as well as our ability to accurately estimate rates with simple geodetic inverse models. A similar suite of three-dimensional models are used to explore the added complication of along-strike geometry also observed in natural fault systems. These models are compared to areas where slip rate discrepancies exist, such as the Oak Ridge and Palos Verdes faults in Southern California and the Chelungpu Fault in Taiwan.