九州中部地方の地殻変動とテクトニクス : 別府-島原地溝の分裂拡大  [in Japanese] Crustal deformationin central Kyusyu, Japan and its tectonic implication : Rifting and spreading of the Beppu-Shimabara Graben  [in Japanese]

測地測量の繰り返しから, 九州中部地域に於て南北方向の張力場が卓越すること, これが別府-島原地溝の南北方向への1.4 cm/年の拡大, 2.5 mm/年の沈降によることが明らかになった。九州に起きる大部分の浅い地震は別府-島原地溝で発生しており, それらの発震機構は南北張力軸の正断層型または四象限型で, 地殻変動と一致する。重力異常から推定される別府-島原地溝の地殻構造は地殻の陥没, モホ面の上昇を示している。これらのことから別府-島原地溝は大陸地殻に出来た活動的なリフトバレー, コンチネンタルリフトバレであり, 沖縄トラフの北東延長部である可能性がある, と結論される。

Repeated triangulation and trilateration surveys have revealed the conspicuous horizontal crustal deformation such that the crust of central Kyusyu, Japan, is spreading gradually in the N-S direction as the Beppu-Shimabara Graben is a spreading axis. Most of shallow earthquakes in Kyusyu have occurred in the Beppu-Shimabara Graben and their fault plane solutions are normal fault type or strike slip fault type a N-S extensional axis. The crustal structure of the Beppu-Shimabara Graben deduced from the gravity anomalies shows crustal depression and uplift of the Moho discontinuity, that is, the crustal thinning. It is concluded that the Beppu-Shimabara Grabenisan active rift valley formed in the continental crust; i.e., a continental rift vally, and is a unique active continental rift valley in Japan. Double side spreading rate and subsidence rate at the rift during the past 90 years are estimated by the repeated geodetic surveys to be 1.4 cm/year and 2.5 mm/year, respectively. The spreadingat the Beppu-Shimabara Graben are considered to be the cause of the extension tectonics inCentral Kyusyu. Geophysical and geomorhological date suggest that the Beppu-Shimabara Graben consists of three E-W trending rift zone and three NE-SW trending shear fault zones regarded as the transform faults. Although the origin of the Beppu-Shimabara Graben is still unknown, the distribution of rift zones suggests a possibility that the Beppu-Shimabara Graben is the north-eastern extension of the Okinawa Trough which is an active back-arc basin. This should be forther studied in detail.