Repeating earthquakes as low-stress-drop events at a border between lockedand 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 th e 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] pr oportional 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 t he border depends on the size of the large locked patch. For a circular pat ch 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) f or R between 2 km and 100 m. These low stress drops are consistent with est imates 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 th e surrounding creep rate. Linear streaks of microearthquakes observed on cr eeping fault planes are trivially explained by the present model as alignme nts on the boundaries between locked and creeping patches.