A Change in the Apparent Electrical Resistivity of Mt. Mihara of Oshima Volcano Observed in Association with the Izu-Hanto-Toho-Oki Earthquake, 1980

Since 1975, we have been making repeated measurements of the earth resistivity of Mt. Mihara on Oshima Volcano by the direct current method with current and potential electrodes fixed in a bipole-bipole configuration (Fig. 1). A remarkable change in the apparent resistivity was observed before and after the Izu-Hanto-Toho-Oki Earthquake of magnitude 6.7 which took place about 30 km northwest of Oshima Volcano on June 29, 1980. Measurements are being made with three pairs of electrodes with different spacings. We shall call the measurement with the shortest spacing between the midpoints of the current and thepotential electrodes measurement A, the measurement with the longest spacing C, and that with intermediate spacing B. The results of measurement A are greatly influenced by a surface feature. At present, unfilled with molten lava, the crater forms a big open hole at the surface. As the lava rises through the volcanic vent and the volume of the hole decreases, the apparent resistivity computed from measurement A is expected to increase. In May, 1980, more than one month before the Izu-Hanto-Toho-Oki Earthquake, an increase of the apparent resistivity for measurement A amounting to 4.4 % was seen. It then gradually decreased. The earthquake took place during this decrease. The increase in the apparent resistivity of 4.4% preceding the earthquake may be interpreted to be due to a 10 m uprise of molten lava. The decrease in the apparent resistivity that followed the rather steep increase seems to suggest that the molten lava subsided at a slower rate over four months. A similar change in the apparent resistivity was also observed at the time of the Oshima-Kinkai Earthquake of magnitude 7.0 that took place about 10 km north-west of the Oshima Volcano on January 14, 1978. It may be said that the volcano has not been immune from the abnormal tectonic activity in this area, characterized by a high level of seismic activity, anomalous land deformation etc., which has been continuing since the time of the Izu-Hanto-Oki Earthquake in 1974