Abstract
Offshore of the Hatsushima area, western Sagami Bay, are distributed along the submarine escarpment of eastern rim. Many geological, geophysical, and biological studies have been conducted at this cold seep still present. In May 2005 and January 2006, seismic reflection surveys were carried out in the Sagami Bay by R/V Kaiyo of JAMSTEC. In this paper, we identified the submarine structure of offshore Hatsushima around the chemosynthetic biocommunity area obtained by seismic survey results. These surveys provided very clear seismic images down to 1.5 seconds beneath the seafloor in two-way travel time. We identified three sedimentary units compared with the Ashigara Group and acoustic basement as the Shirahama Group (∼2 Ma) that is commonly absorbed in seismic images of the western Sagami Trough. We identified two reflectors of reverse polarity indicating fluid and/or gas existence of Horizons A and B in the trough-fill sediment layer. Deformed structures due to uplift of Izu Peninsula side are identified in trough-fill sediment layer zone near the submarine escarpment. These structures suggest the existence of concealed fault system or magma intrusion in the deep part of basement layer. In addition, wipe-out structures are recognized in the trough-fill sediment layer. These wipe-out structures are also related to the reflector of reverse polarity (Horizon A) in the trough-fill sediment layer, it was concluded that these structures were the fluid and/or gas based diapirs. The results of this study clarified the relationship between the geologic structures, the fluid and/or gas based diapirs and the high heat flow value area.
