- 著者
- Kiyoji SHIONO
- 出版者
- The Seismological Society of Japan, The Volcanological Society of Japan , The Geodetic Society of Japan
- 雑誌
- Journal of Physics of the Earth (ISSN:00223743)
- 巻号頁・発行日
- vol.25, no.1, pp.1-26, 1977 (Released:2009-04-30)
- 参考文献数
- 53
- 被引用文献数
- 35 46
Focal mechanisms are investigated for 56 major earthquakes with magnitudes around 6.0 and greater that have occurred in southwest Japan since 1920's in order to clarify the relationship between seismic activity and the underthrusting of the Philippine Sea plate along the Nankai trough and the Ryukyu trench.Focal mechanisms of large earthquakes along the Nankai trough and in the Hyuganada are similar to one another, indicating a low-angled thrusting. This feature can be explained in terms of elastic rebound of the continental block which is dragged down by the underthrusting Philippine Sea plate.For very shallow inland earthquakes except those located around the Izu peninsula and in the Kyushu region, the P-axes tend to be oriented in the eastwest direction. This seismological evidence, together with geologic evidence, indicates that the E-W tectonic compression has prevailed in the crust of southwest Japan for one or two million years up to the present. The E-W compression has been interpreted to arise mainly from the subduction of the Pacific plate.Focal mechanisms of subcrustal earthquakes are characterized by systematic alignments of the T-axes. In the region from Iyonada to the southern Chubu district where the Philippine Sea plate does not penetrate into the asthenosphere but remains in the continental plate, the T-axes are parallel to the leading edge of the underthrusting plate (parallel extension). Under the Kyushu island and the northern Ryukyu arc, on the other hand, the T-axes dip westwards and are approximately parallel to the dip of the inclined seismic zone extending down to 150-200 km, suggesting down-dip extension characteristic of the intermediate-depth seismic zones. From these facts, the following tectonic model can be derived: While the leading edge of the underthrusting oceanic plate remains within the continental plate, subcrustal earthquakes with T-axes parallel to the leading edge occur as a result of interaction between the oceanic and the continental plates. As soon as the oceanic plate begins to penetrate down into the asthenosphere, subcrustal earthquakes with down-dip extension occur in the underthrusting plate by the gravitational pull.