The Philippine Sea slab is in contact with the crust at the source region of M 8.0 Nobi earthquake, central Japan

Abstract:

Little is known about the cause of inland earthquakes, as such earthquakes have long recurrence cycles (from several hundred to several thousand years) that hamper our understanding of the processes that give rise to such events. However, these earthquakes can cause major disasters, primarily as their sources are closes to population centers. As such, understanding the mechanisms that cause inland earthquakes is an important step in the mitigation of disasters, and it requires knowledge of the mechanisms of strain accumulation and stress concentration along the source faults of such events.

The Nobi earthquake which occurred in 1891 was one of the largest inland earthquakes in Japan. The magnitude of the 1891 Nobi earthquake was 8. The magnitude is much larger than those of other inland earthquakes in Japan. To know cause of the Nobi earthquake and crustal structure of the area, the temporary seismic observation was conducted. We researched receiver function analysis to obtain seismic image around the 1891 Nobi earthquake area. The earthquakes which occurred from August, 2002 to March 10, 2011 are used. The seismic stations of Hi-net and temporary seismic network were used for the receiver function analysis.

A clear image of the seismic structure was obtained in this area. The configuration of the subducting Philippine Sea slab is distorted at the western part of the Japan. The Philippine Pea plate is horizontally layered from Ise Bay to Wakasa Bay. In this area, a seismic experiment with artificial sources was done in November, 2012. We compared the results of the receiver function analysis and that of refraction analysis. The Philippine Sea slab is distorted up-ward at the area of the 1891 Nobi earthquake. The up-ward distorted Philippine Sea slab is in contact with the crust. The shallowest point of the subducting Philippine Sea slab is shallower than Moho boundaries at the both sides. The fault plane of the 1891 Nobi earthquake is located in parallel to the iso-depth lines of the horizontally layered Philippine Sea slab. The stress concentration might be caused by the structure, resulting in 1891 Nobi earthquake.