抄録
A micro-earthquake swarm was observed at the southern end of Satsuma Peninsula, Kagoshima Prefecture; foci were located at a depth of 5 km in the southeast part of Lake Ikeda, the biggest caldera lake in South Kyushu. The largest event was M=2.0. The focal area, which was adjacent to an area of a local high Bouguer anomaly, lay on a tectonic line geologically presumed. Polarity data of P waves gave a focal mechanism solution of a fault plane striking N60°W, which accords with the strike of the above-cited tectonic line. Although the swarm occurred in a so-called volcanic region, not only the seismograms but also the related phenomena themselves were very similar to those of earthquakes occurring in non-volcanic regions. Compression axes in an NE-SW direction were estimated from the focal mechanisms of the swarm, while a stress field of an E-W compression is widely admitted in Kyushu. They were concordant with the minor principal strain axes in the southwestern part of Kyushu. Compression axes in the NE-SW direction were also observed for the 1968 Ebino Earthquakes. There is a clear difference in trend of compression axis between the southwestern and other parts of Kyushu. A probable explanation is possible if the crust in the southwestern part is cut off from the northern part through the Usuki-Yatsushiro Tectonic Line and from the eastern part through a fault along the volcanic front. From the arrival time data of a clear reflected P phase, a reflection surface was estimated at a depth of about 7 to 9 km around the focal area.
