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 sufficient observations of recurrence properties in natural earthquake populations. Following steps are necessary to tend the project towards its successful conclusion:
1)Analyses of Repeating earthquakes sequences (REQSs) using NCSN and HRSN data
- Illustrate the recurrence properties of REQSs in space and time
- Quantify the temporal interaction between neighboring REQSs
- Illustrate change of recurrence rate following 2004 M6 Parkfield event
2)Modeling fault interaction
- Inter-seismic period: How do the separation distance and relative size of the neighboring REQSs act on their interaction in time?
- Post-seismic period: Do the above interaction features change when "time-dependent" in surrounding condition are present?

Our preliminary result shows that 67% of quasi-periodic repeating sequences (i.e., coefficient of variation in recurrence interval less than 0.3) correspond to zones of low seismicity, suggesting that these more regular repeating events are more isolated in space and from perturbing stress changes. We find that closely spaced repeating sequences show evidence of strong interaction in time, reflected in temporally clustered event recurrences. The temporal correspondence appears to be a function of separation distance from nearby earthquakes rather than the relative size of the events. The response of the repeating events to the occurrence of larger earthquakes provides the clearest documentation of the interaction process. Accelerations of repeating sequences are associated with M 4 - 5 events that occurred in the mid-1990s and following the Parkfield earthquake when a large number of sequences exhibit accelerated recurrence behavior consistent with rapid afterslip following the mainshock. However, the characteristically decaying afterslip pattern is not obvious for some of the repeating sequences located close to the 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. Building on the above observations, we will be able to develop mechanical models that test the extent to which fault interaction in the form of static stress changes and transient postseismic fault creep produces the observed aperiodicity in the occurrence of these events, and furthermore, attempt to improve predictions of the times of future event repeats.