1992年西表島群発地震

Abstract

An earthquake swarm which started on January 1991 in Iriomote Island, Southwest Ryukyu Arc, Japan, was reactivated in August, 1992.Five temporary seismic stations were set on the island to determine the precise hypocenters and focal mechanism of the swarm activity.Hypocenters were located in the region adjacent to the region of the 1991 earthquake swarm, and were distributed on larger area than the 1991 swarm.In southeastern part of the swarm area, the plane distribution trending NSOｰW with a dip of 500 is also defined for the 1992 swarm as well as for the 1991 swarm, but different area of hypocenter concentration is seen in northwestern part of the 1992 swarm area.Focal depths are less than 9km. Mechanism solutions determinated from composite plotting of P-wave first motions were roughly strike-slip type. However, the solutions are not reliable enough to deduce precise tectonic stress direction, because of the ambiguity of the velocity structure and shallow focal depth of the events.Many events showed clear later arrivals, which are reflected waves from the middle crust. The waves are useful to determine the crustal structure and also to reveal the nature of the lower crust.