Structural Controls On The 1998 Volcanic Unrest At Iwate Volcano- Relationship Between A Shallow, Electrically Resistive Body And The Possible Ascent Route Of Magmatic Fluid

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Journal Article: Structural Controls On The 1998 Volcanic Unrest At Iwate Volcano- Relationship Between A Shallow, Electrically Resistive Body And The Possible Ascent Route Of Magmatic Fluid

Abstract
Magnetotelluric (MT) measurements were conducted at Iwate volcano, across the entirety of the mountain, in 1997, 1999, 2003, 2006, and 2007. The survey line was 18 km in length and oriented E-W, comprising 38 measurements sites. Following 2D inversion, we obtained the resistivity structure to a depth of 4 km. The surface resistive layer (~ several hundreds of meters thick) is underlain by extensive highly conductive zones. Based on drilling data, the bottom of the highly conductive zone is interpreted to represent the 200°C isotherm, below which (i.e., at higher temperatures) conductive clay minerals (smectite) are rare. The high conductivity is therefore mainly attributed to the presence of hydrothermally altered clay. The focus of this study is a resistive body beneath the Onigajo (West-Iwate) caldera at depths of 0.5-3 km. This body appears to have impeded magmatic fluid ascent during the 1998 volcanic unrest, as inferred from geodetic data. Both tectonic and low-frequency earthquakes are sparsely distributed throughout this resistive body. We interpret this resistive body as a zone of old, solidified intrusive magma with temperatures in excess of 200°C. Given that a similar relationship between a resistive body and subsurface volcanic activity has been suggested for Asama volcano, structural controls on subsurface magmatic fluid movement may be a common phenomenon at shallow levels beneath volcanoes.

Authors 
Koki Aizawa, Yasuo Ogawa, Masaaki Mishina, Kosuke Takahashi, Shintaro Nagaoka, Nobumasa Takagi, Shin'ya Sakanaka and Takuya Miura








Published Journal 
Journal of Volcanology and Geothermal Research, 2009





DOI 
Not Provided
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Citation

Koki Aizawa,Yasuo Ogawa,Masaaki Mishina,Kosuke Takahashi,Shintaro Nagaoka,Nobumasa Takagi,Shin'ya Sakanaka,Takuya Miura. 2009. Structural Controls On The 1998 Volcanic Unrest At Iwate Volcano- Relationship Between A Shallow, Electrically Resistive Body And The Possible Ascent Route Of Magmatic Fluid. Journal of Volcanology and Geothermal Research. (!) .