Structural factors controlling the coseismic rupture zone of the 1973 Nemuro-Oki earthquake, the southern Kuril Trench seismogenic zone

The Kuril Trench is an active seismogenic zone where historic great earthquakes have occurred repeatedly with temporal and spatial regularities. This paper presents the results of a wide-angle ocean bottom seismographic (OBS) survey across the coseismic rupture zone of the 1973 Nemuro-Oki earthquake (Mw = 7.8), together with thermal modeling. The main purpose of this study is to understand the crustal and thermal controls on the Kuril seismogenic zone. Revealing structures that can be controlling factors of a megathrust earthquake helps in estimating earthquake hazards. We focus specifically on the precise structure of the seismogenic interface. Our imaged subduction structure clearly indicates that the coseismic rupture of the 1973 event occurred between the subducting oceanic crust and the Kuril island arc crust at 15-30 km depth. A thermally determined updip limit of the seismic rupture (100degreesC) corresponds to the seismically determined updip limit (15 km depth). An analysis of amplitude variations of wide-angle reflections demonstrated a highly reflective plate interface immediately trenchward of the coseismic rupture zone ( shallower than 15 km depth), while a weakly reflective interface is observed at the coseismic rupture zone. From the observed amplitude variation with offsets, a thin low-velocity layer (4.5 km/s) is observed at the top of the highly reflective interface. This suggests the presence of fluid, which would indicate weak coupling updip of the rupture zone. We also deduced, from these observations, that a weakly reflective interface in the coseismic slip zone represents a strongly coupled zone pending the next megathrust earthquake.