Prevalence of Repeating Earthquakes in the Continental Crust and Subducting Slabs: Triggering of Earthquakes by Aseismic Slip

Repeating earthquakes, or repeaters, which repeatedly rupture the same location and release the strain energy, are interpreted as repeated ruptures of an isolated asperity patch surrounded by a stable sliding regime. While repeaters are frequently observed along plate-boundary faults, the occurrence of repeater sequences are reported in various tectonic settings. Here we systematically investigate whether repeaters occur in the continental crust and subducting slabs beneath the Japanese Islands and show a prevalence of repeaters in both locations. The crustal and intraslab repeaters show the power-law decay of seismicity rates that is identical to that along plate-boundary faults and occur on well-defined fault planes coincidentally with non-repeating earthquakes. These observations suggest that repeaters share a common generation process independent of the tectonics regime and are not distinct from non-repeaters in their source locations and occurrence times. We thus infer that repeaters and non-repeaters are both generated by ruptures of locked asperities in response to stress loading by aseismic slip in the surrounding stable regime. Repeater sequences can be observed when aseismic slip is large enough to rupture an isolated asperity more than twice, while no repeaters are generated when the amount of aseismic slip is not sufficient to cause multiple ruptures at an isolated asperity. Since an earthquake cannot be triggered unless the amount of the aseismic stress loading exceeds the strength of asperities at the fault, hidden aseismic slip probably occurs repeatedly and frequently during an interseismic period.