Triggering effect of M>4 earthquakes on the earthquake cycle of repeating events at Parkfield

How the stress perturbation by nearby earthquakes influences earthquake recurrence is of fundamental importance to understanding of the earthquake cycle and to determining earthquake hazard. The large population of characteristically repeating earthquakes at Parkfield provides a unique opportunity to study the degree to which stress interactions between earthquakes may influence earthquake recurrence intervals. Here we examine the response of the repeating events to the occurrence of M > 4 earthquakes. Using 187 M -0.4 ~ 1.7 repeating earthquake sequences (1987-1998) from the High Resolution Seismic Network, we find that the time to recurrence of repeating events subsequent to the M4-5 earthquakes is unusually short, suggesting triggering by major events. The triggering effect is found to be most evident within a distance of 5 km and decays with distance. We also find coherently reduced recurrence intervals from 1993 to 1998. This enduring recurrence acceleration over several years reflects the accelerated slip associated with several M >4 events and aseismic transient during the early 1990s. Using 25 M -0.51 ~ 2.16 updated HRSN repeating sequences (1987-2006), we also evaluate the effect of the 2004 M6 Parkfield event sequence (i.e., M 6 mainshock and three M > 4 aftershocks). The recurrences of the 25 updated HRSN repeating sequences exhibit a strong acceleration pattern associated with 2004 M 6 Parkfield event. The accelerated recurrence patterns of repeating sequences appear to be similar when they are close in space. The characteristically decaying afterslip pattern is not obvious for some of the repeaters adjacent to the largest co-seismic slip area, suggesting either that the stress changes are very heterogeneous, or that the rupture erased or shut off some of the sequence source areas.