沖縄トラフの拡大と九州地方の地殻変動 (2)

抄録

The N-S extensional strain field in the Kyushu is understood as the result of the crustal deformation caused by the rifting and the spreading of the Okinawa Trough which crosses the Central Kyushu in the E-W direction. The spreading velocity and the spreading direction of the Okinawa Trough in the Kyushu have been estimated as 1.4cm/a and N-S direction, respectively, by the analysis of the distance survey. The horizontal and the vertical crustal movements, the normal active faults, the gravity anomaly, the volcanism, and the fault plane solutions of the earthquakes in the Shimabara Peninsula suggest that the Unzen Volcanic Graben is an active rift of the Okinawa Trough. The beginning of the spreading of the Okinawa Trough in the Kyushu has been estimated to be during the period from 0.8Ma to 0.5Ma B. P. by the spreading velocity and the width of the Unzen rift. The distributions of the active faults, the low gravity anomaly belts, the hypocenters of the shallow earthquakes and their fault plane solutions suggest that Okinawa Trough in the Kyushu consists of the three rifts whose strikes are E-W direction and the three transform faults like shear fault zones whose strikes are the NE-SW direction.