Audio frequency magneto-telluric survey of Norikura Volcano in central Japan

Norikura Volcano has not been active during the last 10,000 years in spite of the activity of the surrounding volcanic mountains. To study past volcan ic activities, geological studies were carried out extensively. However, qu ite a few geophysical investigations were conducted to contribute to volcan ology. Our objective is to detect the present subsurface structure of Norik ura Volcano and to define volcanic stratifications. In the vicinity of Nori kura Volcano, geothermal fields are still active. Subsurface volcanic rocks in this area have been exposed to geothermal activity and altered. To comp rehend volcanic stratifications of Norikura and geothermal activity, we con ducted audio frequency magneto-teluric (AMT) surveys around Norikura Volcan o. AMT survey is useful in clearly defining the resistivity structure relat ed to volcanic regions. The AMT data were acquired over a frequency range 1 0 Hz-10 kHz. Decomposition analysis was applied to the tensor impedance dat a. Subsequently, apparent resistivity and phase data were inverted using a two-dimensional magneto-telluric (MT) inversion and a model of Norikura was derived. The final model manifests that the surface resistors are in agree ment with andesite lava or dacite lava. As for the deeper structure, a hori zontal conductor is situated above resistive basements. The alteration of t he conductor was weak, while basement rocks were strongly altered and/or he ated through the thermal activity. The existence of these layers seems to i ndicate the degree of thermal activity of Norikura Volcano. (C) 1999 Elsevi er Science B.V. All rights reserved.