The variability of recurrence intervals is represented by the coefficient of variation, COV, the standard deviation divided by the mean of recurrence intervals, and is determined using the events that occurred before the 2004 M6 Parkfield earthquake (filled circles in Figure 2.7). A COV of 0 implies perfect periodicity, while COV=1 implies Poissonian recurrence and COV of greater than 1 indicates temporal clustering. The locations and COVs of repeating earthquake sequences are shown by circles in Figure 2.7, where large circles indicate the REQSs from NCSN data. We find that 50% of the NCSN REQSs are characterized by a COV of less than 0.3, and 67% of these small COV REQSs correspond to zones of low background seismicity, suggesting that these quasi-periodic repeaters are more isolated in space. The aperiodicities of REQSs do not reveal a systematic dependence on depth or magnitude. Following the 29 September 2004, M6.0 Parkfield, California earthquake, a large number of postseismic repeats are observed in many of the 25 updated repeating sequences (1987 - Sept. 15, 2006, which are shown by crosses in Figure 2.7, Nadeau, 2007, unpublished data). The event chronologies and time evolution of inter-event time spans (recurrence intervals) for two clusters of REQS are shown in Figure 2.8a and 2.8b, respectively. The events in the cluster in the upper panel (see location in Figure 2.7) reveal a similar pattern, whereas the events in the cluster in the lower panel appear more randomly distributed. We note that the range of separation distances among the REQSs in these two clusters are similar, but the magnitude differences in the upper cluster in Figure 2.8a range from M 0.24 - 0.95 (dM = 0.71), whereas the lower cluster includes events from M 0.58 - 1.70 (dM = 1.12). The larger size difference may play a role in the temporal interaction between the sequence events and requires further examination. We next evaluate how the appearance of temporal interaction correlates with separation distance and magnitude difference among all REQS pairs.
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