Variation Of Seismic Velocities Under The Northeast Japan

To investigate the correlations of the seismic velocities and the seismicarchitecture in the source area of the2011Tohoku earthquake (Mw9.0), we carriedout seismic tomographies before and after the main-shock using a large number ofP-wave and S-wave arrival-time pairs from the local earthquakes occurred in bothonshore and offshore regions. We conducted several seismic inversions using the P-and S-phase pairs from the earthquakes in five different periods.The three-dimensional (3-D) seismic structures indicate strong velocity variationsunder the source area in the five different periods as well as the rupture zone. Before themain-shock, the hypocenter of2011Tohoku earthquake is located at the edge portion ofa low-velocity area in the thrust zone. The rupture zone is imaged as laterallow-velocity belt on the upper boundary of the subducting Pacific slab. The3-D imagesshow that the low-velocity areas revealed before the main-shock gradually became alow-velocity belt after the main-shock along the upper boundary of thesubductingPacific plate.We consider that such a variation of architecture is contributedto the migration of the plate compressive stress, which could have the influence on the2011Tohoku earthquake generation.