Hydrothermal System Beneath Aso Volcano As Inferred From Self-Potential Mapping And Resistivity Structure

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Journal Article: Hydrothermal System Beneath Aso Volcano As Inferred From Self-Potential Mapping And Resistivity Structure

Abstract
We conducted self-potential (SP) surveys sequentially from part to part over the central cones of Aso volcano since August 1998 by December 2001. The compiled SP map revealed large SP anomalies on the central cones. The main feature of the SP map is a 'W-shaped' profile along the NS-transect over the central cones. It is probable that this characteristic SP profile is produced by the combination of hydrothermal upwelling in the middle and topographic effect. A positive anomaly showing a large concentric pattern has appeared after correcting the topographic effect. To evaluate this SP anomaly, we implemented a numerical code that calculates electric potential produced by arbitrarily positioned current sources and sinks in any three-dimensional resistivity structure. A layered structure obtained from a time-domain electromagnetic (TDEM) field experiment was used for the resistivity model. The estimated current source is 300 A, being located in a conductive layer around the sea level. Meanwhile, sinks were estimated to sit on a circular area corresponding to the marginal part of the conductive layer. Water and heat budget study gives a lower limit of water mass transfer from depth to the bottom of the crater lake of Nakadake. This value was used to estimate the equivalent current in either case of electro-kinetic (EK) (Mizutani, H., Ishido, T., 1976. A new interpretation of magnetic field variation associated with the Matsushiro earthquakes, J. Geomag. Geoelectr., 28, 179-188.) or rapid fluid disruption (RFD) process (Johnston, M.J.S., Byerlee, J.D., Lockner, D., 2001. Rapid fluid disruption: A source for self-potential anomalies on volcanoes, J. Geophys. Res. 106(B3), 4327-4335.). This comparison suggests that the former process is preferable to explain the observed SP anomaly. From these results we infer a large-scale hydrothermal system beneath the central cones of Aso volcano, in which the fluid flow initiates from the surrounding area, converging to the central vent to transport the heat and materials up to the crater lake of Nakadake through a vapor-filled conduit.

Authors 
Hideaki Hase, Takeshi Hashimoto, Shin'ya Sakanaka, Wataru Kanda and Yoshikazu Tanaka








Published Journal 
Journal of Volcanology and Geothermal Research, 2005





DOI 
Not Provided
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Citation

Hideaki Hase,Takeshi Hashimoto,Shin'ya Sakanaka,Wataru Kanda,Yoshikazu Tanaka. 2005. Hydrothermal System Beneath Aso Volcano As Inferred From Self-Potential Mapping And Resistivity Structure. Journal of Volcanology and Geothermal Research. (!) .