[SIT22-P26] Heat flow determination in the central part of Japan
Keywords:Heat flow, Central part of Japan, Subsurface temperature, Geoneutrino
Heat flow is determined as the product of temperature gradient and thermal conductivity. In boreholes on land, the temperature gradient should be measured in relatively deeper part, where the temperature distribution is little disturbed by ground water flow and/or temporal variation of ground surface temperature (GST). Influence of GST variation is generally not negligible down to a depth of about 200 m. It is therefore difficult to obtain reliable heat flow values in boreholes shallower than 200 m, though the critical depth may vary by region according as GST history varies.
In our target area, the central part of Japan, consisting of Chubu, Kinki and Kanto areas, there are a limited number of over 200 m-deep boreholes, while there exits lots of temperature profile dataobtained in shallow boreholes. As it is important to use densely distributed heat flow data for estimation of subsurface thermal structure, we take the following approach to obtain heat flow values in shallow boreholes. We first reconstruct the past GST history from temperature profiles measured in deep boreholes. Then we correct temperature profiles measured in shallow boreholes for influence of GST variation evaluated in deep borehole(s) in the vicinity. In this presentation, we show results of GST history reconstruction through inversion analysis of temperature data in deep boreholes at seven stations in the target area. In the case of Yokohama site, we could estimate GST variation in the last 300 years, which is characterized by significant surface warming in the last 100 years. The rate of GST increase, 3 K in 100 years, is higher than that of increase in surface air temperature recorded at Yokohama meteorological station, 2 K in 100 years. The difference in temperature increase may be attributed to differences of geographic location and land-use as well as contrast in warming mechanism of air and ground. The amount of GST increase in the last 100 years varies by site and ranges from 1.7 to 4.0 K at the seven stations. These values are similar to those obtained in Tokyo and Osaka metropolitan areas by previous studies.
We have also been working on updating compilation of published heat flow data in the northwestern Pacific area (from 0 to 60 oN and from 120 to 160 oE). The new dataset contains 3759 heat flow values, including 574 data added to the previous version released in 2004. We have compiled geothermal gradient data and thermal conductivity data in and around the Japanese islands as well. These datasets will be published in 2018. They can be used for estimation of crustal temperature structure in Japan and will also contribute to investigation of thermal structure of NW Pacific subduction zones.