九州-琉球弧北部域の稍深発地震

抄録

Seismic activities are investigated in detail for intermediate depth events in a northern part of the Kyushu-Ryukyu arc using seismic data observed at stations of Kagoshima University (KGU) and Fukuoka Meteorological Observatory (FMO): high sensitive KGU stations are effective in reducing errors especially in focal depth and epicenter longitude for events in south Kyushu. On the northeastern side of the Kirishima volcanoes, some relatively large events occurred in a nearly vertical narrow zone at depths from 80km to 120km through the deep seismic plane dipping at a high angle. Possible focal mechanisms suggested thrust faulting with a tension axis dipping steeply but could not always be interpreted as down-dip extension. Considerable activities were also observed at depths from 120km to 180km through the seismic plane under the Kaimon-lake volcano and a chain of events branching off from the cluster continues north-northeastward to a depth of 100km under the Sakurajima volcano. The events in the chain showed repeatedly a tendency to migrate from the cluster zone toward the volcano. It is quite plausible that these seismic activities through the seismic plane are associated with the supply of volcanic energy in Kyushu.
Seismicity gaps traversing obliquely through the deep seismic plane are especially noticeable under the geologically young calderas, namely, Aso and Aira: intermediate earthquakes also rarely occur under the Ata caldera. Considering large discontinuities speculated in the seismic plane in Kyushu as well as prominent recent structures transverse to the arc, the seismicity gap is supposedly formed in a narrow zone of rather high temperature including such a discontinuity segmenting the seismic plane.