1991年西表島群発地震と南琉球弧のテクトニクス

Abstract

An earthquake swarm started on January 23, 1991 in Iriomote Island, southwest Ryukyu Arc, Japan. Six temporary seismic observation stations were set on the island and many events were recorded for the purpose of determining the precise distribution of hypocenters. More than 2, 000 earthquakes were recorded from March 7 to May 4 and about 800 hypocenters were located in a small region of northwestern part of Iriomote Island, with focal depths shallower than 7km. The hypocenter distribution was found to define a plane dipping S50ﾟW with a dip angle of 65ﾟ. Composite plotting of P-wave first motions and the plane on which earthquakes are concentrated show that the maximum pressure axes lie in east-west to northeast-southwest direction. The direction is inconsistent with the stress field derived from the northwesterly subducting Philippine Sea plate. This suggests that stress field changes near Iriomote Island due to the opening of Okinawa Basin.