Seismicity in the outer rise forearc region and configuration of the subducting lithosphere with special reference to the Japan Trench

Along nine subduction zones where the largest (M(W) or M(S) greater than or equal to 7.6) normal-faulting earthquakes have occurred beneath the outer rise, there is an apparent correlation between a large curvature of the subducting slab and regions of low background seismicity in the forearc. In six cases, a gentle inner trench is; also observed. Taking advantage of the excellent coverage of teleseismic stations and extensive previous studies using local data, we investigate the nature of this spatial correlation between seismicity and bathymetry along the Honshu forearc in same detail. An aseismic region in the forearc lies directly West of the rupture zone of the 1933 Sanriku earthquake (M(W) = 8.4), a great normal-faulting earthquake that most likely have ruptured the entire brittle portion of the lithosphere in the outer rise. Along this particular cross section of the Japan trench; large to moderate-sized interplate earthquakes span a short interval of depths between approximately 14 acid 31 km, about half the interval observed along an adjacent profile to the south (12-45 km).; The aseismic portion of the plate interface (between depths of approximately 30 and 45 km) has persisted for over 100 years. The occurrence of lithospheric normal-faulting earthquakes in the outer rise region indicates that bending moment within the subducting slab, limited by yield strength of the lithosphere, has nearly reached an asymptotic value (moment saturation). At this state, the curvature of the plate can increase with little increase in bending moment. As such, any stresses transmitted across the plate interface will be relaxed by a permanent increase in the plate's curvature and little elastic strain can accumulate across the interplate thrust zone to produce large earthquakes. Thus the saturation of bending moment in the subducting lithosphere offers a straightforward interpretation for the observed correlation between seismicity in the outer rise-forearc region and configuration of the subducting lithosphere.