Characteristic seismic activity in the subducting plate boundary zone off Kamaishi, northeastern Japan, revealed by precise hypocenter distribution analysis using ocean-bottom seismometers

High seismic activity prevails along the plate boundary to the east of northeastern Japan. To understand how this seismic activity is related to subduction process, hypocenter locations are re-determined using data obtained over 6 years by fiber-cabled permanent ocean bottom seismometers off Kamaishi. The double-difference method is adopted to obtain the relative location in more detail. As a result of this analysis, a number of seismic clusters including small repeating earthquakes are identified along the plate boundary. This observation supports the hypothesis that seismic coupling is very low in this region and there only small asperities are distributed along the plate boundary off Kamaishi. A number of events with down-dip compressional focal mechanisms are identified, corresponding to the upper plane seismicity of the double seismic zone in this area. Although most of these events are located in the crust of the subducting Pacific Plate, some are located in the mantle of the slab.