Miyazaki, S. and K. Heki, Crustal velocity field of Southwest Japan: subduction and arc-arc collision,

J. Geophys. Res., 106, 4305-4326, 2001.

Heki et al. (1999)でのべたように、西南日本がアムールプレートに属する と仮定し、西南日本の速度場をアムールプレート準拠でプロットすると、東進が 消えてフィリピン海プレートによる弾性変形が浮かび上がってきます。普通はそ れらの速度データを用いて南海トラフ深部でのすべり欠損(backslip)の分布を 最小自乗推定するのが定石ですが、西南日本ではもう一つ東日本(北米プレート) との衝突に伴う未知の地殻変動が重なっており簡単には行きません。そこで本論 文ではAM-PHのEuler vectorをきちんと求め直し、海溝における相対速度ベクトル と熱的モデルから類推されるカップリング強度を掛け合わせて得られたすべり 欠損を既知として与えます。それによって予測される速度場と、観測された速度 場の差がとりもなおさずAM-NAの衝突に伴う long-termな変形場ということになり ます。

We investigate crustal deformation in southwest Japan over a three year period revealed by a permanent dense Global Positioning System (GPS) array. Southwest Japan is a part of the Amurian Plate, a microplate moving about 10 mm/yr toward the east with respect to the Eurasian Plate. It overrides the Philippine Sea plate at the Nankai Trough, and collides with the northeast Japan arc in the central part of Japan. In this paper we first derive GPS site velocities relative to the stable part of the Amurian plate in order to isolate signals of crustal deformation caused by the subduction and/or the collision. The velocity field has a conspicuous feature indicating the interseismic elastic loading by the Philippine Sea Plate slab at the Nankai Trough, characterized by the northwestward movements of points throughout the studied area. Their amplitudes are the largest at the Pacific coast and decay toward the Japan Sea coast with a subtle systematic shift of azimuths. A model assuming an elastic half space, the convergence rate at the Nankai Trough based on a refined Euler vector, and the strength of the coupling inferred from a thermal model, could explain the velocity field in the western part of the studied area to a large extent. Those in the eastern part systematically deviate from them, and the residual components there show east-west shortening and north-south extension. This suggests that the collision between southwest and northeast Japan gives rise to not only crustal thickening but also trenchward extrusion of crustal block. A velocity contrast was found across the Median Tectonic Line, the largest inland active fault in Japan, but the current permanent GPS network is not dense enough for us to discuss its coupling depth.


日置幸介 (email: heki@ep.sci.hokudai.ac.jp)