Comparison of Regional Thermal Model With Seismogenic Layer of the Japanese Islands

Abstract

With compilation of thermal gradient and heat flow density data, new geothermal databases of the Japanese Islands have been established. However, owing to local hydrodynamic perturbation and heterogeneity of thermal conductivity, measurements of thermal gradient and heat flow are often scattered and sometimes insufficient to understand a regional thermal regime. Thus we obtain the average model of the Japanese Islands. These values reveal regional thermal structure from the geothermal database compiled.
Seismic structure should be related with the regional thermal structure. Earthquakes mainly occur within the upper crust and there is a lower boundary of the seismogenic layer, called cut-off depth. It is interpreted that the cut-off depth is related to the brittle-ductile transition in the crust. Many studies suggested that the cut-off depth varies over a depth range of 5 - 25 km and that the thermal structure is the major factor governing the cut-off depth. Recent study at the geothermal field in the Japanese Islands expects that the cut-off depth lies at 300 - 350 ?.
We here obtain the average thermal model of the Japanese Islands and compare it with the cut-off depth by the observed seismicity. Then we discuss the relationship between seismogenic layer and thermal structure.