Study of heat flow in the Sengan-Hachimantai region, northeastern Honshu, Japan

Abstract

In the Sengan-Hachimantai region of northeastern Honshu more than 100 shallow heat flow holes with depths ranging from 80 to 250 m have penetrated volcanic lavas and tuffs of andesitic to rhyolitic compositions as well as older sediments. It is generally difficult to determine the heat flow for those holes because their temperature-depth curves seldom show linear conductive patterns. Due to the large irregularities in these curves, it is not easy either, to clearly determine the physical processes responsible for the data obtained at individual heat flow sites. Therefore, the temperature as well as the thermal conductivity of these holes has been discussed collectively, area by area. The Kowase-gawa area shows higher than normal conductive heat flow, whereas four other areas show scattered but extremely high heat flows. Differences in the areal extent of extremely high heat flow (> 400 mW m −2) among the five areas in the Sengan-Hachimantai region may be explained by assuming different structures and sizes of the geothermal systems. Results of subsequent deep drilling to verify the above inference are presented. The background heat flow value is probably about 150 mW m −2, which was estimated at a 2000-m deep well in the Kowase-gawa area. This high regional heat flow value is consistent with other geophysical data: anomalously low P-wave velocity and high electrical conductivity below the region. These lines of evidence indicate the existence of a partially molten zone in the crust.