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In 1997, a seismic experiment using an airgun array and ocean bottom seismographs (OBSs) was performed in the forearc region of the northeastern Japan (NEJ) arc. The objectives of this experiment were to clarify seismic structures of the forearc region off Sanriku, Japan using airgun–OBS data and to understand the nature of the Japan trench seismogenic zone. Fundamental features of the structure are as follows:(i)The subduction angle becomes steeper from east to west, from 3 to 8 to 11°, with the top of the plate located at depths of 11 km (40 km west from the trench), 21 km (120 km west from the trench) and 28 km (140 km west from the trench), respectively.(ii)The NEJ Moho beneath the forearc is defined at a depth of approximately 20 km.(iii)P-wave velocity of the island arc upper mantle is approximately 8 km s−1.(iv)TheP-wave velocity of the Cretaceous sedimentary layer exhibits significant lateral variability (4.0–5.5 km s−1) in the E–W direction.(v)The upper crust of NEJ forearc also has the significant lateral heterogeneity (5.0–6.3 km s−1) in the E–W direction, but not in the N–S direction.(vi)The middle crust defined on land extends in some locations 40 km east of the coastline.(vii)Lateral variability in theP-wave velocity of the lower crust (6.7–7.0 km s−1) is small compared with that of the upper crust.According to these results, it is suggested that the rupture areas of large earthquakes (>M7) in this region are limited within the contact zone between the NEJ arc crust and the subducting oceanic crust. Although a thin layer with low velocity is probably present within this contact zone, the serpentinization of uppermost mantle wedge of the NEJ arc by reaction between the mantle materials and the water from the thin layer beneath forearc region is limited. This might indicate that a significant volume of water and sediments subduct with the slab to depths of over 20 km. [ABSTRACT FROM AUTHOR]