Three‐dimensional resistivity structure and magma plumbing system of the Kirishima Volcanoes as inferred from broadband magnetotelluric data

Abstract

Broadband magnetotelluric (MT) measurements were conducted in 2010 and 2011 in the vicinity of Shinmoe-dake Volcano in the Kirishima volcano group, Japan, where sub-Plinian eruptions took place 3 times during 26-27 January 2011. By combining the new observations with previous MT data, it is found that an anomalous phase in excess of 90°is commonly observed in the northern sector of the Kirishima volcano group. Because the anomalous phase is not explained by 1-D or 2-D structure with isotropic resistivity media, 3-D inversions were performed. By applying small errors to the anomalous phase, we successfully estimated a 3-D resistivity structure that explains not only the normal data but also the anomalous phase data. The final model shows a vertical conductor that is located between a deep-seated conductive body (at a depth greater than 10 km) and a shallow conductive layer. By applying the findings of geophysical and petrological studies of the 2011 sub-Plinian eruptions, we infer that the subvertical conductor represents a zone of hydrothermal aqueous fluids at temperatures over 400°C, in which a magma pathway (interconnected melt) is partially and occasionally formed before magmatic eruptions. To the north of the deep conductor, earthquake swarms occurred from 1968 to 1969, suggesting that these earthquakes were caused by volcanic fluids.

Three-dimensional resistivity structure of Asama Volcano revealed by data-space magnetotelluric inversion using unstructured tetrahedral elements

“…As with Aizawa et al (2008), we interpret that the resistive body consists of old and solidified magma and it forces the ascending magma to move eastward to the present conduit. Such structural controls were also suggested for Iwate Volcano, Japan (Aizawa et al 2009b), Aso Volcano, Japan , Kirishima Volcanoes, Japan (Aizawa et al 2014) and El Hierro Island (Canary Islands), Spain (Garcia-Yeguas et al 2014). Because the resistive body is located beneath the centre of the old stratovolcano, repeated intrusions of magma under the old eruption centre might have reduced the connected porosity of the area, causing it to become highly resistive.…”

Electrical image of subduction zone beneath northeastern Japan

“…Aizawa et al . [] also indicated that there was a minimal RMS misfit as a function of model covariance parameters in their data set.…”

Lateral Magma Flow in Mafic Sill‐complexes

“…1A; e.g., Curtis, 1968;Cartwright and Hansen, 2006;Gudmundsson, 2006;Aoki et al, 2013;Aizawa et al, 2014;Muirhead et al, 2014;Tibaldi, 2015) or giant radiating dike swarms, which can facilitate lateral magma flow over hundreds to thousands of kilometers (e.g., Ernst and Baragar, 1992;Ernst et al, 1995;Macdonald et al, 2010). Recent field-, seismic reflection-, and modeling-based research further indicate that plumbing systems at shallow levels (typically <3 km depth), particularly those hosted in sedimentary basins, may instead be characterized by extensive sill complexes (e.g., Chevallier and Woodford, 1999;Smallwood and Maresh, 2002;Thomson and Hutton, 2004;Planke et al, 2005;Cartwright and Hansen, 2006;Kavanagh et al, 2006Kavanagh et al, , 2015Lee et al, 2006;Leuthold et al, 2012;Leat, 2008;Menand, 2008;Polteau et al, 2008a;Cukur et al, 2010;Bunger and Cruden, 2011;Gudmundsson and Løtveit, 2012;Muirhead et al, 2012;Svensen et al, 2012Svensen et al, , 2015Jackson et al, 2013;Magee et al, 2014;Zhao et al, 2014;Button and Cawthorn, 2015;Schofield et al, 2015).…”