Bull. Volcanol. Soc. Japan
Vol. 60 (2015) No. 2, pp. 241-249

LetterMultiple Vent-forming Phreatic Eruptions after AD 1663 in the Noboribetsu
Geothermal Field, Kuttara Volcano, Hokkaido, Japan
Yoshihiko GOTO＊, Yoshimasa TORIGUCHI＊, Hirotaka SASAKI＊ and Akira HATAKEYAMA＊(Received July 8, 2014 ; Accepted February 2, 2015)
The Noboribetsu geothermal field (area 1×1.5 km) is located in the western part of Kuttara volcano, southwestern
Hokkaido, Japan. Seventy-one shallow trenches (<90 cm deep) were systematically dug within the geothermal field, inorder to study the stratigraphy and distribution of the most recent phreatic fall deposit (the Nb-a deposit) erupted fromthe field. The Nb-a deposit (<68 cm thick) consists of altered dacitic lithic clasts in a fine-grained clay-rich matrix. Thedeposit overlies the Us-b tephra that was emplaced in AD 1663. Stratigraphic sections constructed from the 71 trenches
indicate that the deposit is distributed within an elliptical area measuring 850×1250 m across and extending NNWSSE. Isopach and maximum-grain-size isopleth maps indicate that the deposit increases in thickness and maximumgrain-size toward several explosion craters. These data suggest that phreatic eruptions, through multiple vents, occurredin the Noboribetsu geothermal field after AD 1663.
Key words : phreatic eruption, deposit, trench survey, Noboribetsu geothermal field, Kuttara volcano

1．Introduction
The Noboribetsu geothermal field at Kuttara volcano,southwestern Hokkaido is one of major geothermal fields
in Japan (Fig. 1). Geological and geochronological studies(Goto et al., 2011a, b, 2013 ; Katsui et al., 1988 ; Yamazaki,1986) indicate that at least 12 episodes of phreatic eruptions (here, the term ‘phreatic eruptions’ is used for steamdriven explosions that do not involve fresh magma,following Barberi et al., 1992 and McPhie et al., 1993)
have occurred in this field within the past 8500 years.
However, the details of each episode, such as the mode andmagnitude of eruptions, remain unclear. We performed astratigraphic survey of the most recent phreatic fall deposit
erupted in this field (the Nb-a deposit ; Goto et al., 2013)
in order to clarify the style of the most recent phreaticeruptions in this field. As this field is generally covered
with thick vegetation, we excavated and examined 71
shallow trenches throughout the field. This paper reports
the results of the stratigraphic survey and discusses thestyle of the most recent eruptions.
2．Noboribetsu geothermal field
The Noboribetsu geothermal field lies in the westernpart of Kuttara volcano (Fig. 1) and is approximately 1 km
wide (ENE-WSW) and 1.5 km long (NNW-SSE) (Katsuiet al., 1988 ; Moriizumi, 1998). This field is inferred tohave formed at ca. 15 ka (Goto and Danhara, 2011 ; Gotoet al., 2013), and is characterized by a dacitic cryptodome＊College of Environmental Technology, Graduate Schoolof Engineering, Muroran Institute of Technology, Mizumotocho 27-1, Muroran, Hokkaido 050-8585, Japan.

(Hiyoriyama cryptodome), a volcanic lake (Oyunuma Lake),
and a fumarolic valley (Jigokudani Valley) (Figs. 2 and 3).
The Hiyoriyama cryptodome, in the northern part of thegeothermal field, is 350-550 m in diameter and rises 130 m
above the surrounding areas (Fig. 2A ; Goto and Johmori,2013). Oyunuma Lake (115×210 m in area), located in thecentral part of the geothermal field, is the largest explosion
crater in the field and is filled with hot acidic water (Fig.
2A). The Jigokudani Valley, in the southern part of thegeothermal field, extends for 500 m in an ENE-WSW orientation and hosts active fumaroles (Fig. 2B). The geothermal
manifestations of the geothermal field (active fumaroles,hot springs, and hydrothermal alteration zones) are distributed in a zone extending NNW-SSE from the Hiyoriyama
cryptodome to the Jigokudani Valley (Goto and Johmori,2011). A number of small explosion craters (diameter25-210 m) occur in this zone (Goto et al., 2011a).
3．Trench survey
The Noboribetsu geothermal field is generally coveredby forests consisting of tall trees and short bamboo, except
in areas where geothermal manifestations occur (Fig. 2A).
Stratigraphic and descriptive studies of the Nb-a deposit
are thus hampered by limited exposures caused by a thickcover of vegetation. To address this issue, 71 shallow trenches
were systematically dug throughout the Noboribetsu
geothermal field (Fig. 3). The trenches were excavatedusing a hand shovel. Each trench had a horizontal area ofCorresponding author : Yoshihiko Gotoe-mail : ygoto@mmm.muroran-it.ac.jp