Seismic Swarm Activity in Iriomote Island, Southwest Japan

西表島における群発地震は1991年1月に始まり, 4月には一時終息した.しかし, 1992年8月に再び活発化し, 約半年間活発な活動が続いた.その後は震源域が拡大し, 島の周辺における海域での断続的な活動が1994年末まで継続した.これらの活動を島内における強震計及び高感度地震計によって観測し, 震源の移動を詳細に調査した.その結果, 当初島内の北東部に発生した地震は, 面上の分布を示し, 陸域ではその傾きは南西方向, 海域では北東方向と異なっていることがわかった.地震はこの面の方向に内陸に向けて最初に拡大し, 後に広範囲に広がった.地震の深さは13kmまで, 6ー11kmに集中している.地震の波形には地殻中部からの反射面と思われる反射波が観測されたが, 火山性微動など火山活動を示すものはなかった.
Earthquake swarm activity occurred in and around hiomote Island during January to April in 1991. The activity once subsided in April 1991 but reactivated in August 1992 and it continued until the end of 1994. The activity was monitored with strong motion seismographs and high-gain seismographs after the 1992 earthquake swarm activity on the island. Eight to ten stations were opemted on the island and many hypocenters were located. Heavy swarm activity in 1992 ended after a half year. The activity, however, continued intermittently in broad areas in and around Iriomote Island.The swarm, in the beginning. Concentruted on a plane trending toward the northwest-southeast direction, which is well coincided with the active faults and lineaments on the island. The plane dips towards southwest in the land area, while towards northeast in the sea. The swarm first extended towards the inland direction and after that to broad areas of the island. The depths of the earthquakes are shallow, less than 15km deep, concentmting at the depths of 6-11km. Clear wave form in the later phases, supposed to be reflected waves from the middle crust, were recorded, but no direct evidence of volcanic activity such as tremors was observed during the swann activity.