Variation in recurrence intervals

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of the 25 updated repeating sequences (1987 - Sept. 15, 2006, which are shown by crosses in Figure 2.7, Nadeau, 2007, unpublished data).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.

Figure 2.7: Along fault depth section showing the distribution of HRSN (1987 - 1998) and NCSN (1984 - 2004) repeating sequences (filled circles), background seismicity (1987-2005, open circles color coded for pre- and post-2004 earthquake), 1966 M6 hypocenter (red star), and their relationship to the slip distribution of the 2004 Parkfield mainshock (Kim and Dreger, 2007). Fill color/shades are keyed to the COV in recurrence interval. White crosses and numbers indicate the updated 25 HRSN repeating sequences.

Figure 2.8: (a) Event chronology for two of the updated HRSN REQSs clusters (see white crosses with numbers in Figure 1 for locations). Note that in the upper panel, events rapidly recurred after the 2004 September Parkfield earthquake, which suggests a characteristically decaying afterslip pattern, whereas in the lower panel, the afterslip pattern is not clear. (b) Inter-event time spans (recurrence interval) as a function of time for each REQS in the two clusters. (c) Similarity in recurrence history (cross-correlation coefficient between the curves in (b)) between the 25 updated HRSN REQSs as a function of separation distance using different interpolation intervals. (d) Similarity in recurrence history curve as a function of magnitude difference (in Mw unit) by different interpolation rates. Filled symbols indicate the cross-correlation coefficients calculated by 0.5-yr and 0.1-yr interpolation intervals before and after the 2004 Parkfield earthquake respectively. Open symbols indicate the cross-correlation coefficients from 1-yr and 0.1-yr interpolation intervals before and after the 2004 Parkfield earthquake respectively.