Repeating earthquakes as low-stress-drop events at a border between locked and creeping fault patches

The source of repeating earthquakes on creeping faults is modeled as a weak asperity at a border between much larger locked and creeping patches on the fault plane. The x(-1/2) decrease in stress concentration with distance x from the boundary is shown to lead directly to the observed scaling [T] proportional to [M-0](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, Delta sigma = 7.6(m/R)(3/5) MPa, which yields stress drops between 0.08 and 0.5 MPa (0.8-5 bars) for R between 2 km and 100 m. These low stress drops are consistent with estimates of stress drop for small earthquakes based on their seismic spectra. However, they are orders of magnitude smaller than stress drops calculated under the assumption that repeating sources are isolated stuck asperities on an otherwise creeping fault plane, whose seismic slips keep pace with the surrounding creep rate. Linear streaks of microearthquakes observed on creeping fault planes are trivially explained by the present model as alignments on the boundaries between locked and creeping patches.