西南日本前弧域及び瀬戸内区のネオテクトニクス  [in Japanese] Neotectonics of the forearc zone and the Setouchi Province in southwest Japan  [in Japanese]

西南日本前弧域は逆L字形の隆起帯とその北西側の前弧海盆を一揃いとする5つの地質構造単元に区分される。同単元は南海トラフ沿いの巨大地震の震源領域に対応し, その地質構造の特徴は巨大地震に伴う地殻変動の特徴と一致する。このことから, 地質構造単元は巨大地震時の地殻変動が累積して形成されてきたと考えられる。第二瀬戸内海はNE-SW方向の長軸をもつ3つの右雁行沈降盆地から構成され, 現在の瀬戸内海と同様にMTLを南縁とする右横ずれ剪断帯として捉えられる。この考え方では, 従来の知多変動はMTL活動期の右横ずれ剪断テクトニクスに, また, 六甲変動はMTLの活動衰退・停止期における瀬戸内区南部の前弧側への組入れに起因する衝突型テクトニクスに対応する。地質構造単元の形成及びMTLの右横ずれ運動は, フィリピン海プレートの右斜め沈み込みの開始に伴って4〜5 Maに東部地域で始まり, 時代と共に西へ拡大していったと考えられる。

The forearc zone of southwest Japan is divided into five equivalent regions (structural units). Each unit is 120 to 150 kilometers long and composed of a forearc basin fringed by an inverted L-shaped anticline stretching from each of five promontories on the Pacific coast to the outer ridge zone along the Nankai Trough. The structural units coincide, in both dimension and location, with the source regions of so-called plate-boundary earthquakes. Furthermore, the topographic and structural features of the units also correspond to the pattern of crustal movements associated with the earthquakes. From these facts, it is concluded that the structural units have been formed by accumulation of coseismic crustal movements. The formation of the structural units began about 4 Ma in the Sagara-Kakegawa region along the eastern most part of the Nankai Trough, and has extended westward to the Shikoku region by the beginning of the Quaternary. The 2nd Paleo-Seto Inland Sea, which occupied the Kinki Triangle area on the east of the present Seto Inland Sea in Pliocene to Middle Pleistocene time, was composed of three basins with a NE-SW major axis, arranged in right-handed echdon along the Median Tectonic Line (MTL). The evidence shows that the 2nd Paleo-Seto Inland Sea was formed as a right-lateral shear zone along the MTL in the same manner as the present Seto Inland Sea. The formation age of the basins becomes younger westward, being estimated to be 4〜5 Ma for the easternmost Tokai Basin and 3〜3.5 Ma for the westernmost Osaka Basin. This fact indicates that an active domain of the MTL has extended from east to west. The suspension of the faulting along the MTL is also considered to have propagated from east to west, because the northward shifting of subsidence center and the formation of E-W compressive structures also began earliest in the Tokai Basin, and have extended westwards. Both the structural units in the forearc zone and the 2nd Paleo-Seto Inland Sea are considered to have begun to form in relation to the initiation of the right oblique subduction of the Philippine Sea Plate beneath southwest Japan about 5 Ma. The westward propagation of neotectonic movements in both the outer and the inner zones of southwest Japan may reflect a westward-increasing distance between the Nankai Trough and the MTL.