Triggering Effect of M 4–5 Earthquakes on the Earthquake Cycle of Repeating Events at Parkfield, California

Stress perturbations influence earthquake recurrence and are of fundamental importance to understanding the earthquake cycle and determining earthquake hazard. The large population of repeating earthquakes on the San Andreas fault at Parkfield, California, provides a unique opportunity to examine the response of the repeating events to the occurrence of moderate earthquakes. Using 187 M 0:4 to ∼1:7 repeating earthquake sequences from the High Resolution Seismic Network catalog, we find that the time to recurrence of repeating events subsequent to nearby M 4–5 earthquakes is shortened, suggesting triggering by major events. The triggering effect is found to be most evident within a distance of ∼5 km, corresponding to static coseismic stress changes of >0:6 26:6 kPa, and decays with distance. We also find coherently reduced recurrence intervals from 1993 to 1998. This enduring recurrence acceleration over several years reflects accelerated fault slip and thus loading rates during the early 1990s. Online Material: Figures of event chronologies of RES, estimates of dt , and coseismic static stress changes.