南九州の地震活動の予察

Abstract

Studing on the seismicity around Sakurajima volcano, following preliminary results areobtained.(1) By the projection of focus on the vertical section perpendicular to the volcanic front, clear W-B zone can be perceived. This W-B zone is characterized by the sharp change ofinclination at the depth of 70km 80km. The inclination of Shallower zone is 40-50 degreeswheras about 70 degrees in deeper zone. The thickness of deeper zone is nearly 20km.(2) The depth of seismic surface directly beneth the volcanic front is 100km. Excludingthe shallow earthquakes whose focal depth are less than 20km, no earthquake is observedin the zone between the volcanic front and the seismic surface.(3) Comparing to other island arcs, seismic activity in South Kyushu is low in the depthof 8Okm-120 km whereas high in l4Okm-l6Okm.(4) Preceding to the increasement in number of explosion at Sakurajima volcano, high seis-mic activity in the depth more than 80km was observed.