Geothermal gradient and heat flow data in and around Japan (II): Crustal thermal structure and its relationship to seismogenic layer

The high-quality database of seismicity of Japan (JMA, Japan Meteorological Agency) and an extensive compilation of thermal measurements (Tanaka et al., 2004a) are used to quantify the concept of temperature as a fundamental parameter for determining the thickness of the seismogenic zone. Qualitative comparisons between each data of heat flow and geothermal gradient, and the lower limit of crustal earthquake hypocentral distributions beneath the Japanese Islands show that, as expected, the lower limit of seismicity is inversely related to heat flow and geothermal gradient. However, the density of the data is not uniform but highly clustered and spatially concentrated instead. To fill the data gaps, the available data were interpolated onto regular grids of points with a spacing of 0.25 degrees x 0.25 degrees. Gridded heat flow or geothermal gradient and D-90, the depth above which 90% of earthquakes occur, correlated well with each other. The evaluated temperatures for D-90 range between 250 degrees C and 450 degrees C except for higher heat flow data. The consistency of temperature for D-90 over a large depth interval almost all over the Japanese Islands support the concept that the temperature is the dominant factor governing the focal depth in the crust. A comparison of our results with other tectonic regions could provide evidence for variations in temperatures for D-90.