Precise earthquake relocation of double seismic zone beneath Tohoku region in Japan

Japan is located in western Pacific， where the intermediate-deep earthquakes occur frequently. Early researches indicate that the intermediate-deep earthquakes beneath Tohoku region in Japan constitute a double seismic zone. In order to analyze the spatial distribution characteristics of the double seismic zone in Tohoku region， we use a method test to show the precision and validity of the double-difference location algorithm， which has been improved by 3-D ray tracing method in spherical coordinate system. We analyse the errors before and after relocation along different directions, and determine the most suitable parameters for relocation. Then we obtain the precise relocated earthquakes in the double seismic zone beneath Tohoku region, and fit the relocated hypocenters in three-dimensional space. The relocation results of intermediate-deep earthquakes in Tohoku region indicate that the earthquakes in the seismic zone are distributed obviously in double layers， which are approximately parallel to the subducting western Pacific Plate. The results of this paper are significant to reveal the mechanisms of earthquakes in double seismic zone and the fine structures in the deep subduction zones.