View article

At the Japan Trench convergent margin, many large interplate earthquakes of greater than M7.5 frequently occur. Their epicenters have uneven distribution, mostly located in the northern area. To investigate the relationship between this distribution and tectonic structures, we have conducted multichannel seismic surveys since 1996. Our data show two kinds of interplate sedimentary units: a wedge‐shaped unit and a channel‐like unit. Both units have a lower P wave velocity than the basal part of the overriding island arc crust. The wedge‐shaped unit having a velocity of 2–3 km/s is widely distributed over the forearc region in the northern area. Its thickness decreases with depth, becoming several hundred meters at a depth of ∼12 km. The channel‐like unit having a velocity of 3–4 km/s is observed in the southern area, extending in the downdip direction. Its thickness reaches ∼2 km at a depth of ∼12 km. If the low …