Methane seep activity around the Joetsu Gas Hydrate Field of the western Joetsu Basin, eastern margin of the Japan Sea, was investigated in detail using heat flow measurements. Heat flow was obtained by Ewing-type heat flow probe and SAHF probe with five thermistors at 11-cm intervals using a ROV during nine research cruises in 2004-2008. Average heat flow value obtained on a normal muddy seafloor in this area is 98 ± 13 mW/m2, which is consistent with the ambient heat flow in the Japan Sea. Based on the results of three day's monitoring, temperature fluctuations (> 0.02 K) of bottom water influence sub-bottom temperature at around a depth of 20 cm. Heat flow values greater than 300 mW/m2 were measured not only at the methane venting sites but also in the some areas covered by bacterial mats. This high heat flow value (> 150 mW/m2) is confined to certain areas (several meters to a few tens of meters scale) on the mounds in the Umitaka Spur and the Joetsu Knoll. Therefore, methane migration from the deep subsurface to seafloor occurs on a very local scale, although seismic profiles show the presence of many small faults through gas chimneys just below the mounds. Convex temperature profiles around the gas venting sites indicate the presence of fluid discharges with Darcy's flow velocity of 1.3 × 10-6 m/s and 5.0∼8.6 × 10-7 m/s, respectively. On the other hand, concave temperature profiles, obtained in the “collapsed hydrate zone” on the mounds, may indicate the presence of a recharge zone. Some temperature reversal profiles in areas covered by bacterial mats were probably caused by a lateral fluid movement from a fluid conduit or by the presence of a methane fluid pool. Some apparent negative geothermal gradient anomalies were obtained only in the “collapsed hydrate zone”. Most of these apparent negative anomalies are possibly explained by the influence of bottom water temperature fluctuations. There seem to be some different hydrological regimes in the high methane flux area of the Joetsu Gas Hydrate Field.
