Physics of the Earth and Planetary Interiors 152 (2005) 277–291 Statistical analysis for thermal data in the Japanese Islands Yasukuni Okubo a,∗ , Youhei Uchida a,1 , Makoto Taniguchi b,2 , Akinobu Miyakoshi a,1 , Jan Safanda c,3 a National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan b Nara University of Education, Japan c Geophysical Institute of Czech Republic, Czech Republic Received 29 September 2003; received in revised form 13 July 2004; accepted 10 April 2005 Abstract We calculated statistical average of thermal data to speculate regional thermal structure of the forearc area of the Japanese Islands. The three thermal statistical averages show a difference of a high thermal regime in the western part of forearc inner zone and a low in the Kanto forearc outer zone. The Kanto zone marks 18 K km -1 for mean geothermal gradient, 44 mW m -2 for mean heat flow, while the western inner zone shows 27 K km -1 for mean geothermal gradient, 63 mW m -2 for mean heat flow. The geothermal gradients of the Nobi Plain and the Osaka Plain in the western inner zone are 29 and 36 K km -1 , respectively, while the value of the Kanto Plain in the Kanto zone is 21 K km -1 . Taking account of the effect of accumulation of sediments, we see the difference in the thermal regime between the plains and conclude that the difference is significant. Heat flux in the crust depends on the volume of granite rich in radioactive elements. There are few granitic rocks in the Kanto zone, while granitic rocks are dominant in the western inner zone. The heat flow of 20 mW m -2 is attributed to the granitic rocks of about 8 km in thickness. There are two oceanic plate subductions of the Pacific plate and the Philippine Sea plate under the Kanto zone, while only the Philippine Sea plate has been subducting under the western inner zone. The model simulation based on thermal and subduction model shows a heat flow ranging 50–60 mW m -2 in the southwest Japan forarc area and a low value of about 20 mW m -2 in the northeast Japan forearc area. The heat flux from the cooling oceanic lithosphere depends on the age of plate. The Shikoku Basin, a part of the Philippine Sea plate, off the western inner zone is 15–30 Ma, while the Pacific plate off the Kanto zone is 122–132 Ma. Theoretically, heat flux values of 15 and 50 Ma oceanic plates range 60–120 mW m -2 and those of 122–132 Ma could be about 10 mW m -2 . If the heat flux contribution from the Philippine Sea plate under the Kanto zone is smaller than the plate under the western inner zone, there could be a thermal regime difference in order of several tens of mW m -2 . Conclusively, the cause of the difference of heat flux could be the uneven granitic rocks distribution and/or the difference of heat flux between the two subducting plate. © 2005 Published by Elsevier B.V. Keywords: Drillhole temperature; Heat flow; Regional thermal regime; Radioactive elements; Cooling oceanic lithosphere; Sedimentation effect ∗ Corresponding author. Fax: +81 298 61 3702. E-mail addresses: yasu-okubo@aist.go.jp (Y. Okubo), uchida-y@aist.go.jp (Y. Uchida), makoto@chikyu.ac.jp (M. Taniguchi), miyakoshi-a@aist.go.jp (A. Miyakoshi), jsa@ig.cas.cz (J. Safanda). 1 Fax: +81 298 61 3684. 2 Fax: +81 75 229 6150. 3 Fax: +420 2 72761549. 1. Introduction Heat flow measurements across the Japanese Islands, which is located at the margin of continental crust, show patterns of low values from the trench axis to the vol- canic arc, high and variable values over the volcanic zone. Because island arc is located at the margin of conti- nent and oceanic plates have been subducting under the 0031-9201/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.pepi.2005.04.013