Heat Flow in Central Japan and its Relations to Geological and Geophysical Features | 中部日本の地殻熱流量及び他の観測データとの相関性

国立防災科学技術センターの関東-東海地震観測ネットワークのボーリング孔と水資源開発公団,関東地方建設局,中部地方建設局地質調査用のボーリング孔を利用して,18点で地殻熱流量の測定を行った.また,新エネルギー開発機構(NEDO)から3点のデータをいただき,それらのデータと今まで測定された結果と合わせて,中部日本の地殻熱流量コンターマップを作製した.この結果,以下のような特徴が明かになってきた.

An investigation on the distribution of surface heat flow in central Japan was carried out using bore-holes of the Kanto-Tokai seismic network of the National Research Center for Disaster Prevention, and holes drilled for geological survey by the Water Resources Development Public Cooperation, the Kanto Regional Construction Bureau and the Chubu Regional Construction Bureau. Topographic effects on heat flow values measured in areas of high topographic variation were taken into account. Eighteen new heat flow values were obtained and three additional data were provided by NEDO (the New Energy Development Organization). On the basis of the new data combined with previous data, a heat flow contour map of the area was prepared, delineating a very broad zone of low heat flow through the central to eastern part of the Kanto district and a low heat flow zone in the western part of the Tokai district. Heat flow increases up to over 100mw/m2 towards the volcanic front. The low heat flow in these two areas can be interpreted as the cooling effect of subduction of the Pacific Plate and the Philippine Sea Plate. The temperature at the Moho discontinuity and mantle heat flow were estimated by a one-dimensional steady-state conductive model with two layers of granitic and basaltic composition. Mantle heat flow demonstrated a similar distribution to that of the surface heat flow. A comparision was made between the features of the distribution of surface heat flow and those of variations of certain other geophysical characteristics. It was found that the surface heat flow correlates fairly well with the Curie isotherm, the attenuation structure in the layer of 30-60 km depth (HASHIDA, 1987), and seismicity at depths of 40-60 km.