Self-Potential Studies in Volcanic Areas(1) : Usu Volcano

Self-potential fields are well mapped over a major part of Usu volcano. A positive anomaly developed over the mountain, 350-400 m V in magnitude, is the most significant feature of the present survey. Electrokinetic coupling effect due to inten· sive hydrothermal convection is a convincing interpretation of the anomaly because the high potential zone is closely related to a high ground temperature zone. High potential gradients observed near the crater rim are attributed to the effect of lateral resistivity variation between the somma volcano and the crater infill. Positive and negative anomalies of short wavelength are superimposed over the major anomaly. The former, distributed on the fumarolic spots, is also interpreted by electrokinetic effect. The latter is commonly observed on the lithic topographic highs and is possibly interpreted by local atmospheric electric potential variations at relatively poor conducting ridge. A repeated survey after two years reveals the decrease of nearly 50 mV in amplitude of the major anomaly. This may be an indication of secular variation of a hydrothermal system in Usu volcano.

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