中部九州の活断層 : 九州における中央構造線の第四紀活動  [in Japanese] Active faults in Central Kyushu, Southwest Japan Quaternary faulting along the Median Tectonic Line in Kyushu  [in Japanese]

九州の中央構造線は松山-伊万里線, 大分-熊本線, 臼杵-八代線という主要な構造線との関係で論じられてきた。活断層からみるとこれらの構造線はいずれも第四紀に活動しているが, とくに大分-熊本線の活動が顕著である。大分-熊本線を南限として, その北方はに7つの地溝が存在し, 中部九州地溝帯を形成している。これらの地溝のうち最東端の別府湾地溝と最西端の雲仙地溝では地溝形成活動が活発に行われている。大分-熊本線の西部は布田川-日奈久断層系の活動により代表され, 右ずれの活動で特徴づけられる。中部九州地溝帯は大分-熊本線の右ずれにともなう引張割れ目として形成されたもので, 北東-南西方向に延びる大分-熊本線の右ずれを引き起こすための東西の圧縮応力は0.7 Ma以降の局部的な圧縮応力場の形成で説明しうる。これはまた, アムールプレート南東部において, それが東進することで生じたshear zoneとの考え方で説明可能である。

The Median Tectonic Line in Kyushu has been discussed in relation to three major tectonic lines named the Matsuyama-Imari Line, the Oita-Kumamoto Line and the Usuki-Yatsushiro Line from north to south. The Matsuyama-Imari Line, at the northern limit of Central Kyushu called the Nagasaki Triangle, has the Beppu-kita fault, the Minoh fault group and the Todogi-Kawakubo fault group on the view point of active fault. The Minoh fault group displaces the lower alluvial fan, distributed at the northern foot of the Minoh mountains. The amount of fault displacement is in proportion to the height of the Minoh mountains. This fact means that the activity of the Minoh fault group brings about the evolution of the mountains. The Oita-Kumamoto Line is the most active tectonic line in Central Kyushu. Seven grabens formed by normal faulting developat the Quaternary volcanic area to the north of the line. So, the writer names this area as the Mid-Kyushu Graben Zone. The grabens are named the Beppu-wan, the Yufuin, the Kuenohirayama, the Kuju, the Haneyama-Kameishiyama, the Kuradake and the Unzen Graben by the writer from east to west. Among these grabens, the Beppu-wan Graben and the Unzen Graben are particularly active on the graben building. The western part of the Oita-Kumamoto Line is represented by the Futagawa-Hinagu fault system characterized by right-lateral movement. The Usuki-Yatsushiro Line situated at the southern most part of Central Kyushu, is mostly in activein Quaternary time. The faulting of the Median Tectonic Line in Kyushu has excelled in the right-lateral movement since 0.7 Ma, thus the activity of graben building has begun. The reason the right-lateral movement has begun in the N-S extensional stress field is that the E-W compressional stress field has locally taken on. Besides, since 0.1-0.15 Ma the Ryukyu System tectonics with axis of NE-SW direction has added to the area. Therefore, volcanic activity of the Ryukyu System in Kyushu mainland and graben forming activity with the direction of NE-SW in Yatsushiro Bay and Amakusa-nada has started.