What controls interplate coupling?: Evidence for abrupt change in coupling across a border between two overlying plates in the NE Japan subduction zone

Interplate coupling in the southernmost extent of the northeast Japan subduction zone (east off-Kanto, Japan) is investigated in detail using a large data set of small repeating earthquakes. In this region of the trench-trench-trench type triple junction, the subducting Pacific plate is in contact with two different overlying plates (the North American and Philippine Sea plates). The border between the two overlying plates extending northwestward from the triple junction is identified along the upper surface of the Pacific plate based on the slip vectors of many interplate events including small repeating earthquakes. The interplate coupling coefficient estimated from the cumulative slip of small repeating earthquakes for the period from 1993 to 2007 reveals a distinct change from south (ca. 0.3) to north (ca. 0.7) across the border. This border also corresponds to the southern limit of the source areas of M>7 interplate earthquakes in the last 80 years along the Japan trench, again indicating the stronger coupling to the north. We also investigate hypocenter distribution and seismic velocity structure along the plate boundary from a large number of travel-time data obtained from the nationwide seismograph network. The results reveal a distinct low-velocity zone just above the Pacific plate and low seismicity along the plate boundary in the region overlain by the Philippine Sea plate, whereas there are no low-velocity zones and the seismicity is high in the region overlain by the North American plate. These observations imply that the geological difference of overlying plate controls large-scale coupling at the plate interface.