フィリピン海プレート沈み込みに伴う西南日本のスロー地震群の発見

Abstract

New types of slow earthquakes have been revealed associated with the subduction of the Philippine Sea plate in southwest Japan based on the development of the dense seismic observation network. At the transition zone, which lies at the deeper extension of the seismogenic zone, deep low-frequency tremors, deep very-low-frequency earthquakes, and short-term slow slip events occur along the slab strike. The short-term slow slip event is the main phenomenon as a stick-slip on the plate interface and other seismic phenomena might be triggered events reflecting the source size. Considering the area of migration and activity of the deep low-frequency tremors, which indicate a transient slip, we can divide the deep slow earthquake belt into segments, each of which has a regular recurrence interval of 6 or 3 months. In each episode, the tremor activity migrates according to the propagation of the transient slip within the segment. Sometimes the migration reaches to the neighboring segment, and a connecting fault rupture occurs. These migrating phenomena are very similar to the rupture process of mega-thrust earthquakes. On the other hand, shallow very-low-frequency earthquakes have been detected in the accretionary prism near the Nankai trough. These events are considered seismic slow slip events at the reverse fault system reflecting the deformation of the accretionary prism. Watching these slow earthquakes is important for the monitoring of the plate motion.