What determines the timing of earthquake recurrences and their regularity is of fundamental importance in understanding the earthquake cycle and has important implications for earthquake probability and risk estimates. This question cannot be answered without statistically significant observations of recurrence properties in natural earthquake populations. Historical or paleoseismic data of recurring large earthquakes have thus provided limited information about the degree to which stress interactions between earthquakes may produce some of the variability in earthquake recurrence intervals. A detailed record of micro-earthquake data from the borehole High Resolution Seismic Network (HRSN) and surface Northern California Seismic Network (NCSN) network sites at Parkfield provides a unique opportunity to examine how fault interaction acts on the observed timing and aperiodicity of the repeating events. Taking advantage of a large number of repeating micro-earthquakes with precisely determined relative locations, we analyze the repeating-event catalog for empirical evidence of asperity interaction and then offer a conceptual model for the mechanics of such interaction. We consider 217 repeating-earthquake sequences (REQSs) ranging from M = -0.4 to M = 3 to study their recurrence behaviors in space and time. In this effort, we separate the effect of changes in recurrence intervals that stem from documented coherent accelerations of fault slip, such as have been observed in the mid-1990s and following the 2004 Parkfield earthquake, from those caused by local interactions.

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