Bull. Volcanol. Soc. Japan
Vol. 60 (2015) No. 1, pp. 35-46

ArticleInternal Structure of Kuttara Caldera, Hokkaido, JapanYoshihiko GOTO＊ and Akira JOHMORI＊＊(Received August 7, 2014 ; Accepted January 30, 2015)
A controlled-source audio-frequency magnetotelluric (CSAMT) survey was conducted across the caldera ofKuttara volcano, Hokkaido, Japan, to investigate its subsurface structure. The caldera is 3 km in diameter and contains acircular lake (Lake Kuttara) 2.5 km across. The CSAMT survey was conducted along a 12-km-long east‒west-trendingtransect that crossed the volcano and passed over the caldera. A total of 23 receiver stations were distributed along thesurvey line, including 7 stations in the caldera. Unique on-boat measurements were obtained at the surface of Lake
Kuttara. A two-dimensional inversion of the CSAMT data, which revealed the resistivity structure to depths of up to
1000 m beneath the caldera, suggested the existence of a low-resistivity region (<30 Ω･m) beneath the eastern caldera
floor, extending subvertically for >1000 m and with a width of 1300-1500 m. The structure is interpreted to be a region
filled with lava blocks and pyroclasts, which subsided during collapse of the caldera and which has been affected byhydrothermal alteration. The location of the low-resistivity region corresponds to an oval basin in the eastern part of thelake, implying that caldera subsidence occurred mainly beneath this basin. The western part of the caldera floor tilts
gently to the east and has a rugged surface, suggesting that the western caldera rim was significantly enlarged as a resultof landsliding during caldera formation. We thus infer that the Kuttara caldera was produced by asymmetric caldera
collapse, which is attributed to the geological heterogeneity of the Kuttara volcano, with an eastern part comprised
mainly of andesitic lavas and a western part comprised mainly of dacitic pyroclastic deposits.
Key words : resistivity survey, CSAMT method, caldera, internal structure, Kuttara volcano

1．Introduction
The Kuttara caldera (3 km in diameter), Kuttara volcano,southwestern Hokkaido, Japan, is one of the smallest Quaternary calderas in Japan (Fig. 1). The caldera contains aremarkably circular freshwater lake, Lake Kuttara (Fig.
2A). Subsurface geological structures of the caldera havebeen previously studied by a gravity survey (Yokoyamaet al., 1967), but details of the structures remain poorly
constrained. We conducted a controlled-source audiofrequency magnetotelluric (CSAMT) resistivity survey(Milsom, 2003; Sandberg and Hohmann, 1982) to investigate its internal structure. As the Kuttara caldera contains alake, we obtained unique on-boat CSAMT measurementsfrom the lake surface. A two-dimensional inversion of theCSAMT data revealed the resistivity structure at depths ofup to 1000 m beneath the caldera. We here present theresults of the resistivity survey, and discuss the nature of
the subsurface geological structures beneath the caldera.
2．Kuttara caldera
The Kuttara caldera is located at the summit of Kuttara
volcano (Fig. 2). Volcanic activity on Kuttara volcano,which occurred from 80 to 40 ka, included an early period＊

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, Osaka 590-of silicic explosive activity followed by the construction ofan andesitic stratovolcano (Katsui et al., 1988; Moriizumi,1998; Moriya, 2003; Yamagata, 1994). The stratovolcano(elevation, 549 m above sea level; base diameter, 9 km)
consists mainly of andesitic lavas and scoria, and associated dacitic pyroclastic deposits (Katsui et al., 1988).
The andesitic lavas and scoria are distributed on thenorthern, eastern, and southern parts of the stratovolcano,whereas the dacitic pyroclastic deposits are distributed on
the western part (Fig. 2B).
The Kuttara caldera is subcircular (diameter, 3 km) andretains its primary morphological features, such as acaldera rim and inner caldera wall (Fig. 2A). The caldera
formed during violent and explosive silicic eruptions at ca.
40 ka (Moriizumi, 1998; Yamagata, 1994) that resulted ina large-volume dacitic pyroclastic fall and flow deposits
around the caldera (Kt-1 tephra; Yamagata, 1994). Lake
Kuttara is 2.5 km in diameter with a maximum depth of
148 m (water level, 258 m above sea level). The lake wateris clear and transparent to a depth of 22 m, as measured in1991 (Environment Agency of Japan, 1993). The caldera
has never been drilled, and its subsurface geology is poorly
known. Gravity data (Yokoyama et al., 1967) show that
0521, Japan.
Corresponding author : Yoshihiko Gotoe-mail : ygoto@mmm.muroran-it.ac.jp