Hideki Hamamoto

Heat flow distribution and thermal structure of the Nankai subduction zone off the Kii Peninsula

Detailed heat flow surveys were carried out in the central part of the Nankai Trough southeast of the Kii Peninsula (off Kumano) for investigation of the thermal structure of the subducting plate interface. At stations in the Kumano Trough (forearc basin) and its vicinity, long-term monitoring of temperature profiles in surface sediments was conducted because bottom water temperature variations (BTV) significantly disturb subbottom sediment temperatures. Heat flow values were successfully determined at seven stations by removing the influence of BTV from temperature records for 300 to 400 days. The surface heat flow data were combined with estimates from depths of methane hydrate bottom simulating reflectors to construct a heat flow profile across the subduction zone. Heat flow decreases from 90–110 mW/m2 on the floor of the Nankai Trough to 50–60 mW/m2 at around 30 km from the deformation front, while it is rather uniform, 40–60 mW/m2, in the Kumano Trough. The values measured on the Nankai Trough floor are concordant with the value estimated from the age of the subducting Philippine Sea plate, about 20 m.y., taking into account the effect of sedimentation. The obtained heat flow profile was used to constrain thermal models of the subduction zone. The subsurface thermal structure was calculated using a two-dimensional, steady state model, in which the frictional heating along the plate interface and the radioactive heat production are treated as unknown parameters. Comparison of the calculated surface heat flow in the Kumano Trough with the observed data indicates that the effective coefficient of friction is small, about 0.1 or less, and thus the shear stress on the plate interface is very low in this subduction zone.

Multiple‐scale hydrothermal circulation in 135 Ma oceanic crust of the Japan Trench outer rise: Numerical models constrained with heat flow observations

Anomalous high heat flow is observed within 150 km seaward of the trench axis at the Japan Trench offshore of Sanriku, where the old Pacific Plate (∼135 Ma) is subducting. Individual heat flow values range between 42 and 114 mW m−2, with an average of ∼70 mW m−2. These values are higher than those expected from the seafloor age based on thermal models of the oceanic plate, i.e., ∼50 mW m−2. The heat flow exhibits spatial variations at multiple scales: regional high average heat flow (∼100 km) and smaller-scale heat flow peaks (∼1 km). We found that hydrothermal mining of heat from depth due to gradual thickening of an aquifer in the oceanic crust toward the trench axis can yield elevated heat flow of the spatial scale of ∼100 km. Topographic effects combined with hydrothermal circulation may account for the observed smaller-scale heat flow variations. Hydrothermal circulation in high-permeability faults may result in heat flow peaks of a subkilometer spatial scale. Volcanic intrusions are unlikely to be a major source of heat flow variations at any scale because of limited occurrence of young volcanoes in the study area. Hydrothermal heat transport may work at various scales on outer rises of other subduction zones as well, since fractures and faults have been well developed due to bending of the incoming plate.

Comparing anthropogenic heat input and heat accumulation in the subsurface of Osaka, Japan

In metropolitan areas, shallow groundwater temperatures are affected by anthropogenic heat sources. The resulting thermal conditions in the subsurface are highly site-specific, and spatial and temporal trends have only been revealed for a few cities. In this study, the anthropogenic heat input is quantified for 15 locations in Osaka, Japan using an analytical, one-dimensional conductive heat transport model. Mean anthropogenic fluxes into the subsurface are determined annually between 2003 and 2011. The model depicts fluxes from buildings and from different land cover types separately. The main objective is to compare the predicted annual mean heat input to heat storage increase, and to identify site-specific factors relevant for the thermal evolution of the underground at each well location. Our results indicate that mean fluxes from asphalt covered areas (0.28 ± 0.07 W/m2) and from buildings (0.32 ± 0.18 W/m2) are significantly higher than fluxes from unpaved (0.06 ± 0.06 W/m2) and grass-covered (-0.04 ± 0.06 W/m2) areas. Furthermore, the temporal variation of mean fluxes from buildings is stable over the studied time period, while annual mean fluxes from asphalt, grass and unpaved areas vary as much as 0.8 MJ/m2. Still, the uncertainty associated with the combined annual heat input of all heat sources is slightly higher than the changes between the years. Overall, the predicted cumulative heat input (2003 to 2011) at the wells ranges from 4 MJ/m2 to 60 MJ/m2. Comparing these results to heat storage increase, additional local heat fluxes, such as from construction work or a sewage treatment plant, have to be considered for about 1/3 of the wells. In addition, it becomes apparent that a significant percentage of determined anthropogenic heat input is not stored in the urban aquifer and heat input is predicted to be considerably higher than heat storage increase.

Evaluation of the Shallow Geothermal Potential for a Ground-Source Heat Exchanger: A Case Study in Obama Plain, Fukui Prefecture, Japan

A ground-source heat exchanger (GHE) is an energy system exploiting shallow geothermal energy that is economical, environmentally friendly, and is rapidly increasing in popularity worldwide. Evaluating the available subsurface heat energy through thermal response tests and/or numerical simulations to design appropriate GHE systems (e.g. deciding the depth and number of boreholes for heat exchange) is important. Geological structures, groundwater properties, and subsurface temperatures are essential input data for such numerical simulations.