Repeating Earthquakes as Low-Stress-Drop Events at a Border between Locked and Creeping Fault Patches

Abstract

The source of repeating earthquakes on creeping faults is modeled as aweak asperity at a border between much larger locked and creeping patches on thefault plane. The x^(-1/2) decrease in stress concentration with distance x from the boundaryis shown to lead directly to the observed scaling <T>~<M0>^(1/6) between the average repeat time and average scalar moment for a repeating sequence. The stress drop in such small events at the border depends on the size of the large locked patch. For a circular patch of radius R and representative fault parameters, Dr 7.6(m/R)3/5MPa, which yields stress drops between 0.08 and 0.5 MPa (0.8–5 bars) for R between2 km and 100 m. These low stress drops are consistent with estimates of stress dropfor small earthquakes based on their seismic spectra. However, they are orders ofmagnitude smaller than stress drops calculated under the assumption that repeatingsources are isolated stuck asperities on an otherwise creeping fault plane, whoseseismic slips keep pace with the surrounding creep rate. Linear streaks of microearthquakesobserved on creeping fault planes are trivially explained by the present modelas alignments on the boundaries between locked and creeping patches.