Off Sanriku region is a well known plate subduction zone, which lies on the west side of the Japan Trench. High seismic activities have been observed, but there are also low seismic activity regions. We conducted an OBS-controlled source seismic experiment across the seismic-aseismic boundary in 1996 to examine the relationship between seismic activity variation and crustal structure. A traveltime analysis using refraction and reflection waves was applied to the observed data to determine the 2D crustal structure. An amplitude analysis of reflection waves from the subducting plate boundary revealed a good correlation between acoustic-impedance (seismic reflectivity) and seismic activity; that is, high-impedance region is a low seismic region and vice versa. We propose the hypothesis that high-impedance is caused by fluid or hydrate rocks around the plate boundary, and this hypothesis explains the crustal velocity structure and the variations of observed reflection waves and seismic activities.
