Aseismic slip in earthquake nucleation and self-similarity: evidence from Parkfield, California

Aseismic slip, microearthquakes, and moderate earthquakes all take place along the San Andreas fault at Parkfield, California. In this letter we discuss the relationship of these slip mechanisms by establishing and modelling the local microearthquake size–frequency distribution. By examining the effect of background noise on earthquake event detection, we show that the distribution is complete for M>0.3. Unlike the Gutenberg–Richter law, the distribution begins with a relatively small number of very small events. Further, the scaling relation of the distribution changes continuously with event size. Noting that many Parkfield aseismic slip (or `creepʹ) events have moments equivalent to the smaller microearthquakes, we show that the nucleation of microearthquakes by this type of slip can account for the size–frequency distribution. Such a model does not require a lower limit to either earthquake or creep event size. Rather it suggests that at Parkfield a transition from creep-dominated slip to earthquake-dominated slip takes place in the range of moments equivalent to magnitudes close to M=0.9. This is well above the M=0.3 detection limit of the seismic network, but right at the limit of current near-surface strain instruments.