Prevalence of updip rupture propagation in interplate earthquakes along the Japan trench

Abstract

The development of seafloor seismic observations facilitates the reliable estimation of rupture directivities from offshore earthquakes. We used seismic waveforms obtained by a new seafloor seismic network (S-net) and stations on land to systematically examine the rupture directivities of interplate earthquakes along the Japan trench. We estimated the rupture directions of 206 Mw 3.5–5 events, most of which occurred near the base of the seismogenic zone. We found that most earthquake ruptures (>80%) were directional, primarily propagating in the updip direction. This tendency favoring updip rupture cannot be explained by the effect of the bimaterial interface. The prevalence of updip rupture in the data suggests that deep aseismic slip and upward fluid migration along the plate interface affected earthquake ruptures in the subduction zone. The updip ruptures redistributed the accumulated shear stress from the base of the seismogenic zone to shallow large seismic patches. Furthermore, the updip ruptures may open paths for deeper fluids to migrate further upward along the plate interface. Both the stress redistribution and upward fluid migration may facilitate the occurrence of shallow megathrust earthquakes.