Philippine Sea Plate
The Kantō Region lies at the complex triple junction, where the convergent boundaries between the subducting Pacific and Philippine Sea Plates and the overriding North American Plate meet. Earthquakes with epicenters in the Kanto region may occur within the Eurasian Plate, at the Eurasian Plate/Philippine Sea Plate interface, within the Philippine Sea Plate, at the Philippine Sea Plate/Pacific Plate interface or within the Pacific Plate. In addition to this set of major plates it has been suggested that there is also a separate 25 km thick, 100 km wide body, a fragment of Pacific Plate lithosphere. The 1703 earthquake is thought to have involved rupture of the interface between the Eurasian Plate and the Philippine Sea Plate.