Structure of the Kuril Trench from seismic reflection records

Abstract

Three multichannel seismic reflection records across the Kuril convergent margin provide the first deep data in this area. The records are located across the southern tip of Kamchatka, the central Kuril arc, and the northern extension of Hokkaido Island platform which show three distinct tectonic regimes. The lower slope contains a wedge‐shaped buttress surrounded by low velocity sediments. Underplating sediment uplifts the buttress, as indicated by faults that displace its upper surface. The middle slope is a block of acoustic basement, which has a rough surface with significant arcward dipping faults. The middle slope is separated from the upper slope along a steep arcward dipping reflection, the “middle‐slope boundary.” The upper slope structure off Kamchatka is different from that off Hokkaido. Off Kamchatka a regular stratified sediment section has been uplifted, tilted, and dips seaward. Off Hokkaido, stronger uplift has tilted regional blocks seaward and arcward. Along the southern line north of Hokkaido, well‐studied nonthrust earthquakes with lateral motions occur beneath the middle slope boundary and on the boundary of the subducting plate. Thrust earthquakes occur under the middle and upper slope, whereas tsunami earthquakes occur under the lower slope. High‐amplitude reflections along the lower boundary of the wedge‐shaped buttress and along the active decollement, indicate high fluid concentrations which reduce the friction along the tectonic units so that through the weak coupling, slow rupture may extend up to the seafloor from a tsunami earthquake.