Characteristic patterns in the space-time distribution of large shallow earthquakes in the Japanese Islands

Abstract

Characteristic patterns of intraplate large shallow earthquake occurrence in the Japanese Islands are investigated. It is shown that in each district there are seismic belts or seismotectonic active zones where large shallow intraplate earthquakes frequently occur. In some cases the activation of seismic belts seems to be related to the occurrence of large shallow earthquakes, such as the 1891 Nobi earthquake with a magnitude of 8.0 and the 1923 Kanto earthquake with a magnitude of 7.9. Also it is probably caused by stress increase due to interplate coupling, such as in the Chugoku district, southwest Japan, before the 1946 Nankaido earthquake with a magnitude of 8.1. In the Kyushu district, other unique tectonics seem to be involved in the intraplate seismic activity. Non-overlapping of active areas and the concentration of earthquake occurrence within a relatively short period are usually observed for each seismotectonic zone. Further, migration of seismic activity and synchronous activity as well as correlation of earthquake occurrences in distant parts along seismotectonic belts are recognized in most tectonic zones. It is pointed out that the same concept of seismic gap of the first kind, originally proposed for interplate large shallow earthquakes along the deep sea trench, may also be applicable to the occurrence of intraplate large earthquakes along seismic belts. These characteristic patterns are considered to give a promising clue to selecting the sites where large earthquakes are expected to occur in the near futue. Seismic belts or seismotectonic active zones probably play an important role in intraplate tectonics. They are a kind of boundary of blocks along which change in stress field can be transferred easily, though their tectonic origin may be different for each seismotectonic zone.