Resistivity Structure of Izu-Oshima Volcano Revealed by the ELF-VLF Magnetotelluric Method

• Published in: Journal of geomagnetism and geoelectricity Vol. 42; no. 3; pp. 169 - 194
• Main Authors: UTADA, Hisashi, SHIMOMURA, Takafumi
• Format: Journal Article
• Language: English
• Published: Tokyo Society of Geomagnetism and Earth, Planetary and Space Sciences 1990; Terra
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Geophysics: general, magnetic, electric and thermic methods and properties; Internal geophysics; Magnetotelluric methods; Japan; Resistivity; Conductive materials; Cinder cones; Inverse problem; O-shima; Craters; Asia; Spatial variations; One-dimensional models; Phreatomagmatism; Ground water
• ISSN: 0022-1392; 2185-5765
• DOI: 10.5636/jgg.42.169

The VLF and ELF magnetotelluric (MT) methods have been applied to study the electrical resistivity distribution in the Izu-Oshima volcano. We utilized a 17.4kHz artificial electromagnetic signal for the VLF method, and three fundamental Schumann resonance frequencies for the ELF method. Measurements were made in 1984 and 1985 at 57 sites including 30 on the central caldera floor, and one-dimensional resistivity models were determined from the results. The modeling results clearly indicate the dominance of a water reservoir beneath the caldera at the depth of a few hundred meters above the sea level. On the crater floor of the central cone Mihara-yama, measurements were carried out at four sites. The result shows a very shallow conductor with a resistivity of less than 10Ω·m. We also found that the edifice of Mihara-yama tends to be more conducting in the southern part than in the northern part, extending in the east-west direction. The important thing is that, within the one-dimensional approximation, this anomalously conducting layer exists only 200-300m below the crater floor. We concluded this conducting layer directly reflects the present thermal activity in the central cone of Mihara-yama. Furthermore, the depth of the conducting layer tends to be shallow at the northern and southern parts of the caldera rim. This shows a peculiar feature of the groundwater distribution beneath the caldera rim.