大分平野と別府湾(深部)  [in Japanese] Oita Plain and Beppu Bay : Subsurface structure and geological development around Beppu Bay  [in Japanese]

マルチ・チャンネル地震探査と海底重力調査により, 別府湾の地下構造が明らかにされた。別府湾横断構造線が主要構造線で, 別府湾を東西の2地域に区分する。東地域は舟底状基盤構造が特徴的で, 最深部は3,500 mに達する。西地域は主要部分の大きな負の重力異常(-58 mgal)を示す地域と北側のリストリック断層群が発達する部分にわけられる。また, 別府湾において現在も活動的な3地域がみられる。リストリック断層が発達する地域, 別府湾横断構造線に沿う地域, 東地域北部の垂直に近い断層が発達する地域である。別府湾の堆積物の年代は不明であるが, 周辺に分布する新生代層と同じ地層であるとすれば, そのもっとも古いフィッション・トラック年代(約3 Ma)から, 堆積速度は1.2 mm/年よりも大きいことになる。

Subsurface structure in Beppu Bay is introduced on the basis of the airgun multichannel seismic reflection survey and gravity measurement. The Beppu Bay Crossing Tectonic Line running in the north-south direction is the main tectonic line in Beppu Bay, and divided into the eastern and the western areas. A graben-like structure is characteristic in the eastern area, and the deepest part reaches to 3,500 m below sea level. In the western area, two distinct areas are recognized. The main part indicates an extremely low Bouger gravity anomaly, and a basin structure. The north part is characterized by listric faulting. Three active tectonic areas are recognized in the Beppu Bay. First is the north-western area where active listric faulting is developed. Second is the area along the Beppu Bay Crossing Tectonic Line. Third is the northern part of the eastern area where more or less vertical fractures or faults reaching to basement rocks like wrench faults are developed. These are very important for considering the recent tectonic stress condition of this area. The oldest age of the sediments on the basement rocks distributed to the south of Oita Plain is about 3.O Ma from the fission-track age determination. If the sedimentation had started in the Beppu Bay region at about 3.O Ma, sedimentation rateis calculated about 1.2 mm/year.