DISTRIBUTION OF SHORTENING LANDWARD AND OCEANWARD OF THE EASTERN NANKAI TROUGH DUE TO THE IZU-OGASAWARA RIDGE COLLISION

The convergence of the Philippine Sea and Eurasia plates is accommodat ed primarily by plate subduction along the Nankai trough west of about 137.3 degrees E, but additionally by widely distributed onshore and o ffshore deformation to the east. This zone, with a total dimension of 200 km (parallel to the Nankai trough) by 350 km, has developed as one highly coupled region since the collision of the Izu peninsula with m ainland Japan about 1-2 m.y. ago. The onshore faulting in central Japa n is consistent with a WNW-ESE direction of maximum horizontal stress, while the oceanward deformation is driven by diffuse NW-SE shortening of the Izu-Ogasawara volcanic ridge which produces a kinematic discon tinuity with respect to the Philippine Sea oceanic crust to its west. The strain release associated with the 1891 Nobi earthquake, and the o nshore deformation as a whole, suggests that the Philippine Sea-Eurasi a convergence is partially accommodated by onshore shortening between the Tsuruga Bay-Ise Bay Tectonic Line and the Izu peninsula.