[S-EM12] Electric, magnetic and electromagnetic survey technologies and scientific achievements

[SEM12-P14] On long-baseline self-potential monitoring on the Izu-Oshima Island

*Makoto Uyeshima¹, Nobuo Matsushima², Marceau Gresse², Takao Koyama¹ (1.Earthquake Research Institute, The University of Tokyo, 2.Geological Survey of Japan)

Keywords:Izu-Oshima volcano, monitoring volcanic activity, self-potential monitoring, Network-MT

The Izu-Oshima Island is an active volcanic island on the Izu-Bonin Arc, in the south of the Japan mainland about 120km from Tokyo. Since the last eruption in 1986, geodetic surveys have indicated long-term inflation of the volcanic body. From eruption history of the volcano, it is thought that we will soon have the next eruption. In preparation and eruption stages, we can expect several EM phenomena due to Streaming potential and spatiotemporal variation of the resistivity structure from enhancement of hydrothermal activity and ascent of the magmatic body. In order to monitor the spatial distribution of the self-potential (SP), we have continued the Network-MT survey with the aid of the commercial telephone network of Nippon Telegraph and Telephone Corp. (NTT). Since the network-MT survey can only be done in the area, where telephone service is available, there existed large observation free area in the central caldera area. On the other hand, in order to mainly monitor the spatiotemporal variation of the SP field, we have maintained SP monitoring network in the vicinity of the central summit cone. But the network was not connected to outside of the summit caldera. Therefore, in addition to the Network-MT survey, we recently have combined both surveys by setting one common electrode, which is located near the northern rim of the caldera. The electrode is connected both to the inner SP survey and to the outer Network-MT survey. The number of total electrodes is 21 and 14, respectively for the inner and the outer networks. In this presentation, we will show basic design of the whole observation system and characteristics of the spatial distribution of the SP field and frequency response functions between respective voltage differences and the horizontal magnetic field.