Seismic reflection profiles recorded east of Honshu show a fairly uniform thickness of acoustically transparent and presumably homogeneous sediment along the outer ridge and seaward slope of the Japan trench. The sediments continue to the bottom of the trench, where they abut the foot of the landward slope. In several localities the transparent sediments of the seaward slope end abruptly as a perched ledge shortly before the bottom of the trench is reached, suggesting post‐depositional subsidence or extension of the sea floor near the trench axis. Seismic refraction measurements indicate that the seaward slope of the trench is a normal ocean floor that has been depressed. A succession of grabens and step faults observed along the entire seaward slope by the reflection technique suggests that the process which formed the trench is still going on. The faults are interpreted to be normal‐antithetic and caused by tensional forces introduced in the convex side of the oceanic crustal plate as it is being further depressed, possibly in response to the load exerted by the weight of the island margin. Refraction profiles recorded along the upper landward slope (continental slope) show that its foundation is composed of material with seismic velocity about 5.8 km/sec; the depth to the Mohorovicic discontinuity is approximately 26 km. A tentative interpretation of one profile recorded along the lower landward slope of the trench indicates that the contact between the rocks of the island arc and the oceanic section lies west of the present topographic axis. A thick wedge of low‐velocity sediment measured near the foot of the landward slope suggests that the topographic axis of the trench has been displaced seaward by outbuilding of the island margin, decreasing the maximum depth of the trench.

In August 1980, observations of seismic waves generated by 35 explosions, one on land and the others at sea, were conducted at about 60 sites on land and four sites at sea bottom in a profile from Hatoyama to off Izu Peninsula to obtain the crustal structure in the northern part of the Philippine Sea plate. The profile, approximately parallel to the Suruga trough, crosses the Kan'nawa fault area, Hakone volcanoes, Izu Peninsula, and the Zenisu ridge.By the time term analysis, a velocity of 5.9 km/s was obtained for the granitic layer and a velocity of 6.8 km/s for the basaltic layer.The Pn velocity of 7.7 km/s was assumed for the time term analysis on the basis of travel time data in this experiment as well as previous results. The crustal structure thus obtained shows an interesting transition from the continental to oceanic type towards the Shikoku basin. It is possible that there are offsets in the upper boundaries of the granitic and the basaltic layers and in the Moho. Since these offsets are located in a narrow zone, this zone might be the boundary between the Philippine Sea plate and the Eurasian plate. Comparison of hypocenter distribution along the profile with the crustal structure supports the above idea since the pattern of hypocenter distribution between the two sides of the offset zone differs. This comparison also shows that earthquakes take place in the granitic layer beneath the Izu Peninsula. The distribution of observed Bouguer gravity anomaly along the profile is consistent with that expected from the crustal structure.

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