High eat Flow Anomaly around Hatsushima Biological Community in the Western Sagami Bay, Japan

Abstract

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.