Bull. Volcanol. Soc. Japan
Vol. 59 (2014) No. 1, pp. 1-11Article

Resistivity Structure of the Showa-Shinzan Domeat Usu Volcano, Hokkaido, Japan
Yoshihiko GOTO＊ and Akira JOHMORI＊＊(Received October 30, 2013 ; Accepted January 18, 2014)
A controlled source audio frequency magnetotelluric (CSAMT) survey was conducted over Showa-Shinzan Domeat Usu Volcano, Hokkaido, Japan, in order to investigate its internal structure. The Showa-Shinzan Dome (800-1000 macross, 350 m high) is a partly extruded cryptodome that formed in AD 1943-45 due to the uplift of pre-existing rocks
and sediments by the intrusion of dacitic magma. The dome comprises a flat-topped cryptodome called ‘Yaneyama’ anda dacitic lava dome projecting above the Yaneyama cryptodome. The CSAMT survey was carried out on a 1600-mlong line that crosses the Showa-Shinzan Dome in an east-west orientation. Two-dimensional inversion of the CSAMT
data revealed the resistivity structure at depths less than 1000 m beneath the dome. The resistivity structure suggests theexistence of a sub-spherical dacite intrusion (resistivity 50-130 Ω･m; ~400 m across) below the summit of the ShowaShinzan Dome. The dacite intrusion may represent the solidified dacitic magma emplaced in AD 1943-45. TheYaneyama cryptodome only comprises pre-existing rocks and sediments uplifted by the intrusion of dacite magma. The
upper zone of the Yaneyama cryptodome consists of the Usu Somma Lava (>100 Ω･m), whereas the lower zone
consists of Quaternary pyroclastic flow deposits and sedimentary rocks (<30 Ω･m), such as the Toya pyroclastic flow
deposits, the Fukaba Formation, the Takinoue welded tuff, the Sobetsu pumice flow deposits, and the YanagiharaFormation. There is no dacite intrusion beneath the Yaneyama cryptodome. This structural model is consistent with thedistribution of active fumaroles on the Showa-Shinzan Dome, and also with historical records of dome growth. The
geophysical data provide new insights into the formation mechanism of the Showa-Shinzan Dome.
Key words : resistivity survey, CSAMT method, Showa-Shinzan Dome, internal structure, Usu Volcano

1．Introduction
Resistivity surveying provides valuable information on
the underground geological structures of active volcanoes(e.g., Aizawa et al., 2008, 2009; Aizawa, 2010; Fikos etal., 2012; Matsushima et al., 2001; Nishida et al., 1996;
Ogawa et al., 1998; Risk et al., 2003; Srigutomo et al.,2008; Yamaya et al. 2009). We have conducted a controlled source audio frequency magnetotelluric (CSAMT)
survey (Milsom, 2003; Sandberg and Hohmann, 1982) of
the Showa-Shinzan Dome of Usu Volcano, Hokkaido,
Japan, in order to investigate its internal structure. The
Showa-Shinzan Dome is a partly extruded cryptodome
formed in AD 1943-45 due to the uplift of pre-existingrocks and sediments by the intrusion of dacitic magma(Katsui, 1988; Mimatsu, 1962; Minakami et al. 1951; Soyaet al., 2007; Yokoyama et al., 1973). The internal structureof this dome has previously been studied using various
geophysical techniques, including seismology (Kato andShoji, 1949; Hayakawa et al., 1957; Nemoto et al., 1957),
magnetic surveying (Nishida and Miyajima, 1984), and＊College of Environmental Technology, Graduate School ofEngineering, Muroran Institute of Technology, Mizumotocho 27-1, Muroran, Hokkaido 050-8585, Japan＊＊Neo Science Co. Ltd, Tarui 4-2-30, Sennan, Osaka 590-muon radiography (Tanaka et al., 2007; Tanaka andYokoyama, 2008), but still remains poorly constrained.
Herein, we present the results of a CSAMT survey of theShowa-Shinzan Dome and discuss the nature of thesubsurface geology beneath the dome.
2．Showa-Shinzan DomeThe Showa-Shinzan Dome is located at the eastern footof Usu Volcano (Fig. 1). The dome is elliptical in planview with a diameter ranging from 800 m (N-S) to 1000 m(E-W), and it rises 350 m above the surrounding area (Fig.
2). The highest point of the dome is 398 m above sea level.
The dome consists of a flat-topped cryptodome called‘Yaneyama’ and a pyramidal dacitic lava dome projecting
above the Yaneyama cryptodome (Fig. 2A).
The Yaneyama cryptodome is pancake-shaped, 8001000 m across, and 200 m thick. The surface of the cryptodome mainly comprises andesitic lava blocks (2-5 m insize) of the Usu Somma Lava (Yokoyama et al., 1973) andunconsolidated sediments (soil, clay, and volcanic ash) that0521, Japan
Corresponding author: Yoshihiko Gotoe-mail: ygoto@mmm.muroran-it.ac.jp