九州-琉球列島における稍深発地震とテクトニクス

Abstract

The minute investigation of the spatial distribution of intermediate-depth earthquakes reveals that the deep earthquake zone beneath the Kyushu-Ryukyu region is dislocated transversely and changes its strike under the Tokara Channel.Left-lateral dislocations of the isobaths of intermediate-depth earthquakes amount to 70-80km and 30-40km at depths of 80km and 150km, respectively. The isobath of the depth of 200km, however, is scarcely dislocated. The change in strike of the deep earthquake zone comes up to about 30° there.The discontinuity of the zone appears to be related closely to the surface geology. The Tokara Channel represents the left-lateral geological fault offsetting by 80-100km the tectonic belts in the pre-Miocene basement rocks running parallel to the island arc. In this region, there are a few left-lateral faults including the Tokara Channel, which traverse the Ryukyu Islands and divide the pre-Miocene tectonic belts into segments of 200 to 400km in strike length. But the other transverse discontinuities of the deep earthquake zone corresponding to the surface geological structure can not be found from the present analyses.