Terrestrial heat flow and thermal structure in Tokyo metropolitan area of central Japan

In the Kanto district of central Japan, the Philippine Sea (PHS) plate subducts beneath the Tokyo metropolitan region. The PHS plate overlies the Pacific (PAC) plate, which subducts westward from the Japan trench. In this area, M8-class earthquakes occur repeatedly and the latest event was the 1923 Kanto earthquake (M7.9), which had 105,000 fatalities. Research on thermal structure and plate structure would help to clarify the seismotectonics of the Kanto district and to assessing earthquake hazard and seismic risk. Measurements of heat flow and geothermal gradient using boreholes on land are often concentrated in specific areas such as geothermal region, so the measured heat flow data in non-volcanic area of Japan is poorly mapped. We newly measured thermal gradients and heat flow by using the National Research Institute for Earth Science and Disaster Prevention’s (NIED) high-sensitivity seismograph network (Hi-net) boreholes. In the Kanto district, four deepest (2300-3500 m) borehole stations, 17 very deep (1000-2000 m) borehole stations, and 7 deep (300-1000 m) had been constructed under the four research projects; "Research on Seismic Activities in the Southern Metropolitan Area, NIED" and "Research on Crustal Activities in the Kanto-Tokai Area, NIED" by the early 1990’s, "Fundamental Survey and Observation Plan for Earthquake Research" after the disastrous 1995 Kobe earthquake. Among these, 4 deepest boreholes and 8 deep boreholes reach crystalline basement because of the deep structure of the Pre-Neogene layer in the metropolitan area. We measured temperature profile of very deep boreholes in ten years or more long period time after drilling, so we could get geothermal gradient with low or no heat disturbance at drilling, and estimated accurate terrestrial heat flow data in the deep structure of the Pre-Neogene layer in the metropolitan area. Comprehensive heat flow distribution in Kanto district is as follows: low heat flow in southern region, high heat flow in northern region, and the boundary line along 36N. Low heat flow under 40mW/m2 are observed. The South Kanto area is sedimentary basin, and Philippine Sea (PHS) and. Pacific (PAC) plates subduct beneath there. Heat flow data with the subducted PHS depth range from 20 to 40km in Kanto area is under 40mW/m2. But in the North Kanto area with the subducted PHS depth range from 40km to 80km, heat flow date over 80mW/m2 is observed. We estimated thermal structure by using one-dimensional heat conduction equation, with the variables based on published or measured values for our heat flow data, heat generation, and thermal conductivity. Estimated thermal structure in this study is as follows: Iso-therm 400°C along the iso-depth 30km contour line of PHS plate, the evaluated temperature of seismogenic zone in the upper crust range between 200°C and 400°C.