Geothermal and rheological implications of intracontinental earthquakes beneath the Kanto-Tokai region, Central Japan

Abstract

The Kanto-Tokai region, representing a boundary zone of three lithosphere plates, has been the subject of intense geological-geophysical studies for decades. The increased attention can be explained partly by the complicated tectonic situation and even to a greater extent by the permanent threat of earthquakes in the region. In this work, the rheology of the lithosphere in the area is estimated on the basis of frictional failure in the brittle regime, and power-law steady-state creep in the ductile regime. Rheological models are constructed for a three-layered model involving a granitic upper crust, a granulitic or gabbroic lower crust, and a dunitic upper mantle. Results show a marked rheological stratification of the lithosphere controlled primarily by the thermal regime. In zones of very low surface heat flow (~40 mW m −2), the lithosphere shows brittle features down to depths of ~50–60 km at least. In areas of normal to moderately high heat flows (~ 50–90 mW m −2), a soft ductile zone (at the depth levels of the lower crust) sandwiched between two brittle layers appears. At locations with very high heat flows (~ 100–120 mW m −2), only the upper crust remains brittle and plastic flow of rocks may start here from the depths of 10–15 km. This rheological stratification of the lithosphere is compared with the depth distribution of earthquakes in the area, and a generally reasonable confinement of seismicity to the brittle levels is indicated.