Repeating Earthquakes Near Parkfield Offer Insight Into Fault Mechanics

To enhance our understanding of the mechanics underlying stick-slip and creep on faults, we have analyzed the behavior of repeating earthquakes on the San Andreas Fault near Parkfield, California. The repeating earthquakes investigated locate in the transition region between the central creeping segment of the fault and the rupture zone of M 6 Parkfield earthquakes. They are thought to represent the recurrent rupture of individual asperities loaded by creep. We have at our disposal several independently-derived double- difference earthquake catalogs that begin in 1984. As elsewhere on creeping faults, the earthquakes are arranged in horizontal or subhorizontal streaks. The streaks include both relatively isolated repeating sources and clusters of multiple repeating sources. The simple geometry of repeating earthquakes and streaks makes them an ideal medium in which we can examine the phenomenology of earthquakes. Specifically we examine recurrence statistics of events, and are developing a prospective test for future recurrences. With the prospective test, we can effectively gauge how well our earthquake recurrence models are performing, by offering the probability that an individual event is part of a repeating earthquake sequence. More than 80 individual repeating sources can be identified, with the exact number depending on the association criteria and location accuracy of the catalog. Prior to the 2004 M 6 Parkfield earthquake, some sources show remarkably clock-like regularity in their occurrence, while others exhibit greater irregularity in recurrence intervals. Following the Parkfield earthquake, the recurrence interval for many of the repeating earthquake sequences became shorter. We believe that the immediate static stress change caused by the Parkfield earthquake, combined with the post-seismic creep response led to the shortening of these recurrence intervals. We also note that there appears to be a relationship between the proximity of the repeating earthquake to the rupture (none of the repeating earthquakes are within the rupture zone) and whether its recurrence interval was affected.