Seismic velocity structure of Japan Trench off Miyagi fore arc region, Northeastern Japan using airgun-OBS data

The Japan Trench subduction zone is located at eastern side of the NE Japan, and many earthquakes occurred around the Japan Trench associated with the Pacific Plate subduction. Magnitudes (M) of these earthquakes and hypocenters are spatially heterogeneous along the Japan Trench. Large earthquakes (M > 7) are more frequently occurred in the northern Japan Trench than in the southern Japan Trench. On the other hand, background seismicity is more active in the southern Japan Trench than in the northern Japan Trench. To investigate detailed structure of Japan Trench subduction zone and to understand the spatial heterogeneity of seismic activity, intensive seismic surveys were conducted by Japan Marine Science and Technology Center (JAMSTEC) since 1996. Off Miyagi is located in the middle of the Japan Trench fore arc region and where the trench axis changes its direction. Seismic activity off Miyagi is similar to that of northern Japan Trench, but not that of southern Japan Trench: (1) Large earthquakes (M > 7) have occurred frequently, e.g., 1897 (M7.7), 1933 (M7.1), 1936 (M7.5), and 1978 (M7.4); and (2) background seismicity is not so active as that of southern Japan Trench. In 1999, a seismic survey was conducted off Miyagi fore arc region to investigate detailed crustal structure by R/V Kaiyo of JAMSTEC. We used a 12000 cu. in. airgun array for controlled source and shooting interval was 50 m. Thirty-six ocean bottom seismometers (OBS) were deployed to record both refraction and wide-angle reflections. A twelve-channel hydrophone streamer was also towed simultaneously to reveal shallow structures. The seismic line of the survey was across the rapture zone of 1978 off Miyagi earthquake. According to the results in progress, crustal thickness of fore arc region is about 20 km, and the dip angle of the Pacific Plate is 4-5 degree between the trench axis and 100-km landward of the trench axis.