Do earthquakes talk to each other? Triggering and interaction of repeating sequences at Parkfield

Knowledge of what governs the timing of earthquakes is essential to understanding the nature of the earthquake cycle and to determining earthquake hazard, yet the variability and controls of earthquake recurrences are not well established. The large population of small, characteristically repeating earthquakes at Parkfield provides a unique opportunity to study how the interaction of earthquakes affects their recurrence properties. We analyze 112 M -0.4~3.0 repeating earthquake sequences (RESs) to examine the triggering effect from nearby microseismicity. We find that the repeating events with a smaller number of neighboring earthquakes in their immediate vicinity tend to recur in a more periodic manner (i.e., the coefficient of variation in recurrence intervals is less than 0.3). The total static stress perturbation from close-by earthquakes, however, does not seem to strongly influence RES regularity. The uneven distribution of stress changes in time has a modest but significant impact on recurrence intervals. A significant reduction of recurrence intervals occurs in the case of very high-stress changes from neighboring events. Close-by events influence RES timing in a matter of several days or less by short-term triggering. Events that occurred within less than 1 day of an RES often imposed or experienced high-stress changes. A static stress increment of ~30 kPa can be enough to produce such short-term triggering. We find that the triggered repeating events are often near the end of their average earthquake cycle, but some events occur following a substantially shortened interval. When comparing the accelerated occurrence at the time of RES events following neighboring events with varying magnitudes, we find that the distance of short-term triggering increases from <1 km to 4 km for M1 to M4 events. Key Points Earthquakes interaction affects their recurrence properties Short-term triggering in a matter of several days Triggered repeating events are often near the end of their cycle