Triggering effect of M$>$4 earthquakes on the observed occurrence of repeating events at Parkfield

Kate Huihsuan Chen, Roland Bürgmann, and Robert M. Nadeau


A characteristic repeating earthquake sequence (RES) is defined as a group of events with nearly identical waveforms, locations, and magnitudes and thus represents a repeated rupture of the same patch of fault. The recurrence intervals of repeating earthquake ruptures are found to be highly variable, where the irregular recurrence of observed repeating events may reflect a response to nearby earthquakes, change in the strain rate, time-dependent or spatial variation in the frictional strength of the fault, or other effects such as fluid pressure variations (e.g., Lay and Kanamori, 1980; Sleep and Blanpied, 1994; Vidale et al., 1994; Nadeau et al., 1994; Ellsworth, 1995). The questions of interest regarding the recurrence properties of natural earthquake sequences are: How do the RESs respond to stress perturbation associated with larger earthquakes and to what range (both in space and time) are the triggering effects effective? To answer these questions, we need a statistically sufficient observational documentation of recurrence properties in natural earthquake populations.

Data and results

The detailed record of micro-earthquake data from the borehole High Resolution Seismic Network (HRSN) sites at Parkfield provides a unique opportunity to examine how larger events act on the observed occurrence of the repeating events. With the high level of detection of micro-earthquakes, the HRSN has revealed a larger number of repeating earthquakes ranging in magnitude from -0.4 to +1.7 (Nadeau et al., 1995; Nadeau and McEvilly, 1997). During the period 1987-1998, 187 repeating sequences were identified, with a total of 1123 events. Using these data, we seek to illustrate how the larger earthquakes influence RESs' timing. We are interested in any potential variation of recurrence interval associated with the time of major events. The analysis has the following recurrence elements (Figure 2.57a): (1) "dt+": the time difference between a major earthquake and the recurrence of a repeating event subsequent to the major event; (2) "dt-": the time difference between a major earthquake and the repeating event prior to the major event; (3) "Tr-post": the duration of the first full recurrence interval following the major event; and (4) "Tr-cos": the recurrence interval spanning the major event, that is, the sum of dt- and dt+. These elements are later divided by the average recurrence interval of a given RES (Tr). The percentage of normalized interval estimates at a distance of less and greater than 5 km from the major events are shown in Figure 2.57b. Extremely short times between the main event and subsequent RES events (dt+/Tr $<$0.1) reflect triggering by the major events. In Figure 2.57b, the peak at dt+/Tr $<$ 0.1) only appears in the near-field (e.g., in the plot for distances less than 5 km from the major event). This may suggest triggering by static stress as a cause of the short recurrence intervals immediately after the larger event.

Figure 2.57: (a) Illustration of the four recurrence elements in time window. Star is the time of major event. Circles are the repeating events in a RES. (b) Percentage of normalized interval measurements (recurrence element divided by average recurrence interval) at a distance of less (left panel) and greater (right panel) than 5 km from M 4 hypocenters.
\epsfig{file=katechen08_1_1.eps, width=7cm}\end{center}\end{figure}

To further explore the region (both in space and time) where interaction between RESs and larger events is most evident, we have selected the 1993 M 4.5 earthquake, with a large number of RESs in the vicinity, for the following analysis. In Figure 2.58a the event chronologies of the RESs adjacent to the 1993 event reveal coherently reduced recurrence times over years (1993 to 1998) following the M$>$4 event. To quantify the enduring acceleration of recurrence, we determine the ratio of averaged post-1993 to pre-1993 recurrence intervals as a function of distance from the 1993 M 4.5 hypocenter in Figure 2.58b. Within a distance of 5 km from the M 4.5 event, most of the RESs (84%) have shorter post-1993 recurrence intervals compared to the pre-1993 intervals. We find that the enduring recurrence acceleration, defined by 'post-1993-Tr/pre-1993-Tr' of less than 1 in Figure 2.58b, can be documented 5 km from the major event.

Figure 2.58: (a) Event chronologies of RESs as a function of distance from the 1993 M 4.5 hypocenter. Note that the distance is confined to be less than 4 km due to abundant RESs beyond 4 km. Red and blue arrows indicate the post-1993 and pre-1993 recurrence interval, respectively. (b) Ratio of the averaged post-1993 recurrence intervals and the average values of the pre-1993 recurrence intervals as a function of distance from 1993 M 4.5 event.
\epsfig{file=katechen08_1_2.eps, width=7cm}\end{center}\end{figure}


The triggering effect of moderate events (M 4$\sim$5) is observable in the temporal variation of recurrences from repeating earthquake data as shown by the unusually short dt+/tr in Figure 2.57b. The immediate acceleration of recurrence following the M 4$\sim$5 events is only found to occur in the near field, which is possibly explained by static stress triggering. Different from the decaying recurrence rate that reflects afterslip following a large event, many of the Parkfield RESs exhibit a steady, accelerated recurrence since 1993. We argue that the long-lasting, reduced recurrence interval is not likely explained by the afterslip process. This acceleration may be part of a more broadly distributed increase in creep rates along this fault segment that is associated with a remarkable aseismic transient following several M $>$ 4 events during the early 1990s (Nadeau and McEvilly, 1999; Niu et al., 2003; Murray and Segall, 2005).


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