To illustrate the temporal association between the neighboring REQSs, we have selected the 25 updated HRSN REQSs with more event repeats to calculate the similarity of recurrence interval curves (example shown in Figure 2.8b). We group the REQSs into pairs when the separation distance is less than 1.5 km and then calculate the cross-correlation coefficient between their recurrence curves by different interpolation intervals. Note that the different number of repeats of the REQSs lead to a different sampling rate of recurrence interval curves in Figure 2.8b, which requires interpolation into the same number of data points. High cross-correlation coefficient indicates a similar recurrence-interval history and implies stronger interaction in time and/or correlated slip-rate changes. Taking account of all 25 REQSs, in Figure 2.8c we show that when the separation distance is small, their recurrence histories appear to be similar. The relationship between recurrence history and magnitude difference, however, is unlikely to follow a linear pattern.
Following the 29 September 2004, M6.0 Parkfield, California earthquake, a large number of postseismic event repeats occurred, where the sequences show extremely shortened recurrence intervals that gradually increase with time (upper panels in Figure 2.8a and 2.8b). This behavior is consistent with rapid afterslip adjacent to the coseismic rupture that is also evident in geodetic measurements (*Johanson et al., 2006*; *Johnson et al.*, 2006; *Murray and Langbein*, 2006). However, this accelerated recurrence behavior is not obvious for some of the deeper repeating sequences we analyzed (e.g., seqs. 25, 10, 13, 2). Note that events in the seqs. 10 and 2 have not recurred since 2004 (lower panel in Figure 2.8a and 2.8b), suggesting that the Parkfield rupture somehow shut off these REQSs near the NW end of the rupture front, at least for the time being. A less pronounced, but widespread acceleration was also associated with a series of earthquakes with M greater than 4 and accelerated fault creep in the mid-1990s(also evident in the upper panels in Figure 2.8a and 2.8b).

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