著者

活断層電磁気研究グル-プ

出版者

東京大学地震研究所

雑誌

東京大学地震研究所彙報 (ISSN:00408972)

巻号頁・発行日

vol.58, no.1, pp.p265-286, 1983

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1979年10月~11月に,伊豆半島北部の丹那・浮橋断層およびその周辺地域において,地殻表層部の電気抵抗分布の調査を行なった.活断層の構造の特徴を明らかにするのが目的である.調査には,大きく分けて(a)人工電位法と(b)電磁誘導法とを用いた.人工電位法ではSchlumberger法による垂直探査,および双極子法や傾度法による水平探査を実施した.電磁誘導法では,主として17.4kHzの人工電磁波を用いた水平探査(VLF探査)を行なった.人工電位法では,電流電極の間隔を2kmまで拡大して地中に電流を流し,電気抵抗分布を調べた.この方法で深さ1kmまでの比抵抗構造が明らかになった.これに対して,VLF探査では,地下15~50mの範囲での電気抵抗水平分布が得られた.この地域の地下構造の大きな特徴は,断層を境にして東と西とで電気抵抗分布が顕著に異なるということである.人工電流による傾度法の結果を見ても,VLF探査の結果を見ても,得られた見かけ比抵抗は断層の東側で大きく,西側で小さい.Schlumberger法による垂直探査結果をも考慮すると,断層の東側では約10mの厚さの高抵抗の表層に続いて,深さ数10mまでは抵抗が低いが,それより深部では1km以上の深さまで抵抗が高いと推定される.これに対して断層の西側では,表層数mは比較的抵抗が低く,それより数10mの深さまでは抵抗が高い.さらに深部では断層の東側とは対称的に抵抗が低くなっていると考えられる.断層の直下では,丹那断層でも浮橋断層でも1km以上の深さにわたって抵抗が低い.浮橋断層を横切る東西線上のVLF探査では,断層を含む約200mの範囲で顕著な見かけ比抵抗の減少が見られた.In order to investigate the geoelectric structure of active faults, measurements of the earth resistivity were conducted in an area including the Tanna and the Ukihashi faults in 1979. Two types of techniques were employed in the measurement. One was the direct current method, in which artificially controlled electric currents are driven into the ground. The other was the electromagnetic induction method, in which the resistivity is estimated from simultaneous observations of electric and magnetic variations at the ground surface. The direct current methods were used both for the vertical soundings and for the horizontal profilings. For the vertical soundings, Schlumberger arrays were employed for the electrode arrangements. For the horizontal profilings the bipole method and the gradient method were used. As an induction method, electromagnetic waves of 17.4kHz of an artificial source were measured. This method (VLF survey) was used for the horizontal profilings. Marked difference were obtained in the restivity structure across the faults. Lateral surveys by the gradient method as well as the induction method gave a higher apparent resistivity to the east of the faults and a lower one to the west. Vertical soundings revealed that at greater depths the resistivity was high in the eastern area of the fault and low in the west, whereas at shallow depths it was low in the east and high in the west. Measurements conducted right above the fault indicated that the resistivity was low to the depth of at least 1km.