Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 4, pp. 529-541

ArticleThe 18-19 ka Andesitic Explosive Eruption at Usu Volcano,Hokkaido, JapanYoshihiko GOTO＊, Yuko SEKIGUCHI＊, Satomi TAKAHASHI＊, Hayuru ITO＊ and Tohru DANHARA＊＊(Received August 8, 2013 ; Accepted October 23, 2013)
Mount Usu is a Quaternary composite volcano located in southwestern Hokkaido, Japan. Here we report on anandesitic pyroclastic fall deposit (the Usu-Kaminagawa [Us-Ka] tephra) erupted during the initial stage of activity atUsu Volcano. The tephra extends from the volcano to the east, and comprises a lower andesitic pumice-fall deposit andan upper ash-fall deposit. The tephra overlies the Nj-Os tephra, which was erupted from the Nakajima Islands, and isoverlain by the Usu Somma Lava, which was extruded during the early stages of activity at Usu Volcano. Radiocarbon
dating of buried soils located immediately beneath the Us-Ka tephra yields ages of 18-19 cal ka BP. The distribution,
stratigraphy, and lithology of the tephra, and the radiocarbon ages of the buried soils beneath the tephra, suggest that anandesitic explosive eruption occurred at Usu Volcano at ca. 18-19 ka. This eruption was probably an early
manifestation of activity at Usu Volcano.
Key words : Usu Volcano, tephra, stratigraphy, radiocarbon dating, early volcanic activity.

1．Introduction
Mount Usu is a Quaternary basaltic to rhyolitic composite
volcano located at the southern rim of Toya Caldera insouthwestern Hokkaido, Japan (Fig. 1). Mount Usu is oneof the most active volcanoes in Japan, and has experienced
at least nine major eruptions since AD 1663 (Katsui et al.,
1985; Katsui, 1988; Matsumoto and Nakagawa, 2010;
Mimatsu, 1962; Minakami et al., 1951; Nakagawa et al.,2005; Oba, 1966; Soya et al., 2007; Tomiya et al., 2010;
Ui et al., 2002; Yokoyama et al., 1973). Understanding theeruptive history of this volcano is essential for mitigationof the volcanic hazards in the region.
This paper presents a study of an andesitic pyroclastic fall
deposit (the Usu-Kaminagawa [Us-Ka] tephra; Yamagataand Machida, 1996), inferred to have been erupted during
the initial stages of activity at Usu Volcano (Kobayashiand Miyabuchi, 2006). Published geological and geochronological data for the tephra are sparse (Kobayashi andMiyabuchi, 2006; Machida and Yamagata, 1996; Yamagataand Machida, 1996), and further detailed study of thetephra may provide invaluable information with which toconstrain the eruptive history of the volcano. Herein, we
describe the distribution, stratigraphy, lithology, and radiocarbon ages of the tephra and discuss the early volcanic
history of Usu Volcano.

＊College of Environmental Technology, Graduate School ofEngineering, Muroran Institute of Technology, Mizumotocho 27-1, Muroran, Hokkaido 050-8585, Japan＊＊
Kyoto Fission-Track Co. Ltd, Minamitajiri-cho, Omiya,

2．Usu Volcano
Usu Volcano rises to an elevation 733 m above sea level,
and is a post-caldera cone of Toya Caldera (Fig. 1). The
caldera is 10 km×11 km in size and formed by violent
explosive eruptions associated with pyroclastic flows at ca.
110 ka (Ganzawa et al., 2007; Machida and Arai, 2003;
Okumura and Sangawa, 1984; Takashima et al., 1992;
Yokoyama et al., 1973). The Nakajima Islands, located inthe central part of the caldera, formed by multiple
extrusions of dacitic lavas at 40-45 ka (Takashima et al.,1992). After formation of the Nakajima Islands, Usu
Volcano became active (Soya et al., 2007).
Usu Volcano is a basaltic stratovolcano with a parasitic
scoria cone, and also includes a number of silicic lava
domes and cryptodomes (Fig. 2). According to Yokoyama
et al. (1973) and Soya et al. (2007), the stratovolcano wasconstructed at ca. 10-20 ka by repeated extrusion ofbasaltic lavas (Usu Somma Lava) and scoria (Fig. 2A). Aparasitic scoria cone (Donkoroyama) formed on thenortheastern foot of the stratovolcano during this stage(Fig. 2B). At ca. 7-8 ka, the summit of the stratovolcano
largely collapsed, resulting in generation of a debris
avalanche that travelled down to the southwestern foot ofthe volcano. As a result of this collapse, an amphitheaterthat is 2 km in diameter formed at the summit of thestratovolcano (Fig. 2B). After the summit collapse, Usu
Volcano was dormant for several thousand years, and theKita-ku, Kyoto 603-8832, Japan
Corresponding author: Yoshihiko Gotoe-mail: ygoto@mmm.muroran-it.ac.jp