[S25-1] Development and application of geothermal databases

[S25-1-02] Thermal data beneath in and around Japan: What we know and do not yet know

^○Akiko Tanaka (Geological Survey of Japan, AIST, Tsukuba, Japan)

Surface heat flow provides important constraint on the thermal and chemical evolution of the Earth. General pattern of heat flow distribution in and around Japan had been revealed by the early 1970s, and heat flow data have been continuously updated and improved by adding data. However, the number of heat flow data is limited and spatially very inhomogeneous. To provide an attempt at a higher resolution map of heat flow, thermal conductivities are measured using sampled cores and used from existing and newly collected data. Also since we have published a CD-ROM “Geothermal Gradient and Heat Flow Data in and around Japan" [Tanaka et al., 2004], new data are provided. We plan to update the database including thermal properties.

Meanwhile, there exist many indicators that are proxies for quantifying the thermal structure. One of the promising indicators is the cut-off depth of shallow seismicity. Several studies have been conducted to assess the inverse correlation between the cut-off depth and heat flow, since it has attributed primarily to the temperature. Another indicator is the depth of magnetic sources based on spectrum analysis of magnetic anomaly data. This analysis is still controversial, however, good correlation between estimated depths of crustal magnetic sources and heat flow suggests that this depth may reflect the broad average temperature. We address the advantages and limitations of each data and method.