Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 2, pp. 365-376

ArticleResistivity Structure of the Hiyoriyama Cryptodomeat Kuttara Volcano, Hokkaido, JapanYoshihiko GOTO＊ and Akira JOHMORI＊＊(Received March 1, 2012 ; Accepted March 15, 2013)
Direct current (DC) electrical and controlled source audio-frequency magnetotelluric (CSAMT) surveys wereperformed over the Hiyoriyama Cryptodome in Kuttara Volcano, Hokkaido, Japan. Both surveys were performed on thesame survey line across the cryptodome in a NE‒SW orientation. Two-dimensional joint inversion of the DC electricaland CSAMT data revealed the underground resistivity structure at depths less than 400 m beneath the cryptodome. The
resistivity structure suggests that the cryptodome comprises a dacite intrusion of 150 m wide and 80 m thick (20-50 Ω･m), and overlying pyroclastic deposits that are 10-30 m thick (>100 Ω･m). The dacite intrusion is underlain by aconvex-shaped, low-resistivity layer (<5 Ω･m) that is interpreted to be smectite-rich, altered pyroclastic deposits thathave been subjected to low-temperature (<200℃) hydrothermal alteration. The low-resistivity layer is underlain by aslightly higher-resistivity layer (10-30 Ω･m) that is interpreted to be altered pyroclastic deposits that were subjected tohigher-temperature (> 200℃) hydrothermal alteration in a relatively deep, hot region near the conduit of thecryptodome.
Key words : resistivity structure, DC electrical method, CSAMT method, Hiyoriyama Cryptodome, Kuttara Volcano

1．Introduction
Resistivity surveying provides valuable information on
the underground geological structures of active volcanoes(e.g., Risk et al., 2003; Aizawa et al., 2008; Srigutomo etal., 2008). We performed direct current (DC) electrical (fordetails of the method, see Milsom, 2003) and controlled
source audio-frequency magnetotelluric (CSAMT) surveys(see Sandberg and Hohman, 1982) at the Hiyoriyama
Cryptodome in Kuttara Volcano, southwestern Hokkaido,
Japan, in order to investigate its internal structure. The
cryptodome is inferred to have formed by the uplift of preexisting pyroclastic deposits due to the intrusion of dacitic
magma (Katsui et al., 1988), but its internal structures arepoorly constrained. This paper presents the results of theresistivity surveys and discusses the nature of subsurface
geological structures beneath the cryptodome.
2．Hiyoriyama Cryptodome
The Hiyoriyama Cryptodome is located in the northernarea of the Noboribetsu Geothermal Field, in the westernpart of Kuttara Volcano (Fig. 1). The volcano consists
mainly of an andesitic stratovolcano (summit at 549 m
above sea level) with a small caldera at the summit (LakeKuttara). The volcano evolved over the period 80-45 ka,
involving early silicic explosive activity and subsequent＊

College of Environmental Technology, Graduate Schoolof Engineering, Muroran Institute of Technology, Mizumotocho 27-1, Muroran, Hokkaido 050-8585, Japan.
＊＊Neo Science Co., Ltd., Tarui 4-2-30, Sennan, Osakastratovolcano building associated with caldera collapse at40 ka (Katsui et al., 1988; Yamagata, 1994; Moriizumi,1998; Moriya, 2003).
The Noboribetsu Geothermal Field, which is inferred tohave formed after the collapse of the caldera (Katsui et al.,1988), is approximately 1 km wide (NE‒SW) and 1.5 kmlong (NW‒SE) (Fig. 1). The geology of the Noboribetsu
Geothermal Field consists mainly of pyroclastic deposits
erupted from Kuttara Volcano (mainly the Kt-1 pyroclastic
flow deposit; Moriizumi, 1998). The deposits are >200 mthick (NEDO, 1991) and consist of dacitic pumice clasts
up to tens of centimeters in diameter.
The Hiyoriyama Cryptodome (Fig. 2) is elliptical in planview, ranging in diameter from 350 m (NE‒SW) to 550 m(NW‒SE). In cross-section, it has a pyramidal form withsteeply sloping sides (Fig. 2A). It rises 130 m above thesurrounding area, with the highest point being 377 m above
sea level. The surface of the cryptodome is covered withpyroclastic deposits (<15 m thick) that were uplifted by
growth of the dome (Katsui et al., 1988). The cryptodome
has an explosion crater (Hiyoriyama Summit Crater; Fig.
2B) at its summit. The crater is 40×95 m in area (elongate
along a NW‒SE axis) and 20 m deep, and contains active
fumaroles (Goto et al., 2011a, b) with a maximum temperature of 134℃ (measured on 6 July 2009). The slopes of the590-0521, Japan.
Corresponding author: Yoshihiko Gotoe-mail: ygoto@mmm.muroran-it.ac.jp