Abstract
AbstractThe Nankai Trough of Japan is one of the most active seismic zones in the world. In order to understand the mechanisms of great interplate earthquakes and the tectonics, the seismic activity, the focal mechanism solutions were analyzed in detail in this research. Based on the geometry of slabs and status of stress within the slabs, the Nankai Trough can be divided into two parts, the eastern (Shikoku and the Kii peninsula) and the western (Kyushu) part. The seismic slab of the Philippine Sea plate subducts with shallow dip angels between 10~ 22° and its leading edge reaches down only to about 60~85km depth beneath Shikoku and the Kii peninsula. However, the seismic slab with a dip angle of 40° subducts down to about 160km depth beneath Kyushu. Two types of seismogenic stress field exist within the slabs of the Philippine Sea plate in the eastern and western parts of the Nankai Trough. The stress field of down‐dip compression type can be seen in the region beneath the most of Shikoku and the Kii peninsula. On the other hand, the stress field of down‐dip extension type within the slab is dominant beneath Kyushu. The analysis results of gravity, GPS, heat flow and other geophysical data suggest that the tectonic properties in the eastern part differ from the western part. Two types of characteristics of seismotectonics and intraslab stress field in the same trough might be related to the differences of slab age in the eastern and western parts.
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