This article reviews recent studies on the submarine topography and geological structure of Sagami Bay. Sgami Bay straddles a plate boundary that poses seismic hazards to densely populated parts of Japan, including Tokyo just 70km to the northeast. Beneath the bay, the northeastern part of the subducting Philippine Sea plate descends under the conti- nental plate that includes northeastern Japan. This subduction produced devastating earth- quakes in 1703 and 1923, and such earthquakes are likely to occur again in the future. Bathymetric surveys show that Sagami Bay contains many knolls and canyons. The bay also contains a deep basin. Submarine volcanic knolls dot the southwestern part of the bay. Some seismic reflection and refraction surveys have revealed clearly the geological structure of Sagami Bay to a depth of 5 to 10 km from the seabed : the basement reflector in the Sagami Basin inclines toward the northeast from the Izu Peninsula side. The trough-fill layer is more Than 4 km thick. However, these data do not attest the plate subduction distinctly or the Existence of the West Sagami Bay Fracture.
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