[S-SS07] Fault Rheology and Earthquake Physics

[SSS07-28] Prevalent aseismic slip: Implications from repeating earthquakes

*Junichi Nakajima¹, Akira Hasegawa² (1.Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2.Graduate School of Science, Tohoku University)

Earthquakes are considered as one of the mechanisms that release the strain accumulated in the Earth, but it remains elusive how an earthquake initiates. Repeating earthquakes (repeaters), which are most observed in an identical location of plate-boundary faults, are considered to represent recurring seismic energy release of a frictionally locked, isolated fault patch triggered by loading of surrounding aseismic slip, and thus have often used as a proxy for monitoring aseismic slip at depth. We show that repeaters occur prevalently in the continental crust and subducting slabs and their recurrence intervals are similar to those of the plate-boundary repeaters. Crustal and slab repeaters occur on a well-defined fault plane delineated by other earthquakes and are not distinct from non-repeaters in their locations, suggesting that repeaters and non-repeaters are both triggered by aseismic slip, as interpreted for plate-boundary repeaters. Extremely enhanced pore-fluid pressures invaded to any fault first promote aseismic slip by increasing both the friction parameter and critical fault distance, and aseismic slip subsequently trigger seismic slip (earthquakes) of patches. Whether we recognize earthquakes as repeaters depends on how large aseismic slip is because small aseismic slip causes a single or no rupture of an isolated patch, thereby generating no repeaters in a limited observation period. Our observations suggest that aseismic slip occurs more frequently than previously considered and silent deformation dominantly undergoes in the brittle part of the Earth.