HIGH HEAT-FLOW ANOMALY AROUND HATSUSHIMA BIOLOGICAL COMMUNITY IN THE WESTERN SAGAMI BAY, JAPAN

Heat flow values have been determined at sites that are densely distributed around biological communities located along a topographic inflection line to the southeast of Hatsushima Island in the western Sagami Bay through four research cruises since 1988. Anomalously high heat flow values up to 2,000 mW/m2 were observed at the largest community, where a high methane content anomaly had been found indicating fluid venting activity. Within 1 km of the inflection line, which is presumed to be a surface expression of an active fault, heat flow values are high and variable (about 400 mW/m2), whereas they are low and uniform (50 mW/m2) in a flat area more than 1 km away from the inflection line. Since this area is located near a convergent plate boundary and also near the volcanic front of the Izu-Bonin arc, either a seepage of the pore fluid at the inflection line or a conductive heating by the shallow magma intrusion below the community is thought to be a possible cause of the high heat flow anomaly. From simple estimations, however, neither of them was found to be adequate to explain the observation. Probably a pore fluid circulation is occurring in the fault system beneath the community that is possibly driven by a shallow magma below the fault and/or the compressional stress field.