Fracturing Associated With The 1977-1978 Eruption Of Usu Volcano, North Japan, As Revealed By Geophysical Measurements

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Journal Article: Fracturing Associated With The 1977-1978 Eruption Of Usu Volcano, North Japan, As Revealed By Geophysical Measurements

Abstract
None of the ground temperature measurements inside the somma caldera of Usu volcano in southwestern Hokkaido, Japan, show patterns that correlate with the development of surface faults and the hypocenter distribution of earthquake swarms observed during the 1977-1978 eruptions. Audiomagnetotelluric (A.M.T.) soundings on the surface of the somma caldera display well-defined high-resistivity zones (H.R.Z.) within the low-resistivity edifice. H.R.Z. are explained as being due to rising dry steam and low-resistivity zones are attributed to ionized water in the volcanic edifice. Inversion of data shows that elongated H.R.Z. have a shallow roof and are some hundred meters wide. Steam rising along narrow vertical fractures generates widely distributed vapor-dominated geothermal systems, because rising steam condenses near the surface. The condensed steam creates a water-filled zone which inhibits the free escape of succeeding steam. Distribution of elongated H.R.Z. correlates well with the high ground-temperature zones. A self-potential survey reveals a positive anomaly developed over the Usu edifice, about 350 mV in magnitude. Electrokinetic coupling effect due to intensive hydrothermal convection is a probable interpretation of the anomaly because the high-potential zone is closely related to a high ground-temperature zone. Hydraulic fracturing of the upper crust at the time of eruption allowed release of elastic strain energy due to magma pressure at depths with the spectacular consequence that one side of the major fracture was uplifted about 180 m. The geometry of such a process may be drawn from seismological results; the roof of the edifice is pushed up by expanding magma accompanying intensive seismic activities. We suggest that earthquake-free zones are proof of magma-filled fractures and that eruption took place when one of these fractures opened to the surface.

Authors 
R. Ballestracci and Y. Nishida








Published Journal 
Journal of Volcanology and Geothermal Research, 1987





DOI 
Not Provided
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Citation

R. Ballestracci,Y. Nishida. 1987. Fracturing Associated With The 1977-1978 Eruption Of Usu Volcano, North Japan, As Revealed By Geophysical Measurements. Journal of Volcanology and Geothermal Research. (!) .