Seismicity in the Northeastern Japan Arc and Seismicity Patterns before Large Earthquakes

Abstract

Seismicity and tectonics in the Northeastern Japan Arc are studied by using the seismic network of Tohoku University which covers the northern part of Honshu, Japan. Most of microearthquakes occur along the main thrust zone beneath the Pacific Ocean, the deep thrust zone (the deeper portion of the main thrust zone) being much more active than the shallow thrust zone (the shallower portion of the main thrust zone). Focal area distribution of large earthquakes and focal mechanism study show that the coupling between the descending oceanic plate and the continental plate extends down to 60 km depth. Seismicity and stress distribution in the double-planed deep seismic zone beneath the southwestern end of the Kurile Arc are found to be different from those beneath the Northeastern Japan Arc by the recent cooperative work of Hokkaido, Hirosaki and Tohoku University networks. In the southwestern Kurile Arc the upper seismic plane is less active than the lower plane and is not down dip compression, suggesting relatively strong slab pull force superimposed on the stress system generating the double seismic zone.