A heating process of Kuchi-erabu-jima volcano, Japan, as inferred from geomagnetic field variations and electrical structure

Abstract

Since August 2000, we have recorded the total intensity of the geomagnetic field at the summit area of Kuchi-erabu-jima volcano, where phreatic eruptions have repeatedly occurred. A time series analysis has shown that the variations in the geomagnetic field since 2001 have a strong relationship to an increase in volcanic activity. These variations indicate thermal demagnetization of the subsurface around the presently active crater. The demagnetization source for the early variations, until summer 2002, was estimated at about 200 m below sea level. For the variations since 2003, the source was modeled on the basis of the expansion of a uniformly magnetized ellipsoid. The modeling result showed that the source is located at 300 m above sea level beneath the crater. We carried out an audio-frequency magnetotelluric survey with the aim of obtaining a relation between the demagnetization source and the shallow structure of the volcano. A two-dimensional inversion applied to the data detected two good conductors, a shallow thin one which is restricted to a region around the summit area, while the other extends over the edifice at depths between 200 and 800 m. These conductors are regarded as clay-rich layers with low permeability, which were assumed to be generated through hydrothermal alteration. The demagnetization source for the early variations was possibly located at the lower part of the deep conductor and the source after 2003 lies between the two conductors, where groundwater is considered to be abundant. Based on these results, as well as on seismological, geodetic, and geochemical information, we propose a heating process of the Kuchierabu-jima volcano. In the initial stage, high-temperature volcanic gases supplied from the deep-seated magma remained temporarily at the level around the lower part of the less permeable deep conductor since the ascent path had not yet been established. Then, when the pathway developed as a result of repeated earthquakes, it became possible for a massive flux of volcanic gases to ascend through the conductor. The high-temperature gases reached the aquifer located above the conductor and the heat was efficiently transported to the surrounding rocks through the groundwater. As a consequence, an abrupt increase of the gas flux and diffusion of the heat through the aquifer occurred and the high-temperature zone expanded. Since the high-temperature zone is located beneath another conductor, which acts as caprock, we assume that the energy of the phreatic explosion is accumulated there.The proper evaluation of volcanic activity is an essential problem related to the forecasting of eruptions 2 and the mitigation of volcanic hazards. It can be relatively simple for volcanoes where magmatic eruptions 3 are expected based on historical experience since anomalous phenomena such as swarms of volcano-tectonic 4 earthquakes or notable inflation of the ground are observed in the process of ascent of magma and are iden-5 tified as precursors of eruptions (e.g. Ishihara, 1990; Voight et al., 19...

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“…However, their model is a conceptual one and does not account for the localized release of latent heat at a certain depth, nor does it provide mechanism for reducing heat discharge from the crater. A fully viable explanation might be common to other volcanoes where remarkable magnetic field changes during quiescent periods have been reported (e.g., Kanda et al 2010;Kuchi-no-Erabujima Volcano).…”

Aeromagnetic survey using an unmanned autonomous helicopter over Tarumae Volcano, northern Japan

“…Tanaka (1993) monitored the magnetic field near the active crater of Aso volcano, Japan to determine that gradual demagnetisation (due to heating) occurred beneath the crater when the surface activity appeared calm; however, rapid remagnetisation (on cooling) followed explosive events. 6 6 Kanda et al (2010), on the basis of continuous monitoring, detected a gradual build-up of thermal demagnetisation that coincided with localised ground inflation at Kuchi-erabu-jima volcano, Japan. Further, Nakatsuka et al (2009) and Hashimoto et al (2011) both reported temporal changes in the magnetic field between separate airborne surveys by using a crewed helicopter at Asama and Usu volcanoes, respectively.…”

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“…In particular, we have already used the MT method to evaluate the large-scale subsurface resistivity structure present within the seismogenic zone beneath the Japanese Archipelago Island Arc system (Ogawa et al, 2001;Uyeshima et al, 2005;Yoshimura et al, 2009), and to evaluate the smaller-scale (shallow) resistivity structure beneath a number of historically active Japanese volcanoes (Ogawa et al, 1998;Hase et al, 2005;Aizawa et al, 2009b;Kanda et al, 2010).…”

“…Conductive clay layers usually exhibit relatively low permeability (e.g., Jones and Dumas, 1993;Nurhasan et al, 2006;Aizawa et al, 2009a;Yamaya et al, 2009;Kanda et al, 2010). In particular, the top of such a clay layer can be sealed (i.e., impermeable), such that in the subsurface, any shallow, cold groundwater do not become wellmixed with deep, hot hydrothermal waters (Aizawa et al, 2009a).…”