IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3W_3C-P10

Date/Time: July 23 Poster

Near-vent morphology and dispersion timing of the climactic PDC in 7300 BP marine
caldera formation of Kikai caldera in southern-off Kyushu Island, through seismicreflection survey
Fumihiko Ikegami1 , Shoichi Kiyokawa1 , Hisashi Oiwane2 , Yasuyuki Nakamura3 , Katsura Kameo4 , Yuto
Minowa1 , Takashi Kuratomi11Department of Earth and Planetary Sciences, Kyushu University, Japan, 2 National Insitute for Polar
Research, Japan, 3 Japan Agency for Marine Earth Sciences and Technology, Japan, 4 Atmosphere andOcean Research Institute, Tokyo University, Japan
E-mail: f.ikegami@gmail.com

Kikai caldera (Matsumoto, 1943) is a mostly submerged highly active caldera complex located in the southern
Japan 40 km off Kyushu Island. The formation of the Kikai is believed to be responsible for dispersion of Akahoya
tephra (Machida and Arai, 1978) in 7300 cal. BP (Fukusawa, 1995). The eruption is characterized by intense
earthquakes (Naruo and Kobayashi, 2002), low-aspect ratio Koya-Takeshima PDC (pyroclastic density current;
Maeno and Taniguchi, 2007) and tsunami (Geshi, 2009) which were taken place at the climax of the eruption.
We conducted seismic reflection observations in two survey cruises (KT-10-18 and KT-11-11) in 2010 and 2011using a research vessel Tansei-maru of JAMSTEC (Japan Agency for Marine-Earth Science and Technology). The
sound source was a 105+45 cubic inches G-I gun with 10 seconds of shot interval, and a 48-channled 1.2 km-length
streamer cable was used for acquisition. Totally 24 profiles were obtained with the speed of 4 knots.
1. Possible climactic PDC deposits
Facies in Kikai area show clear distinction between caldera outskirts, caldera basin, and central rise of the caldera.
At the caldera outskirts, there is a thick (around 100 m) acoustically chaotic layer named A3 in our interpretation thatalike to other presumed PDC-originated deposits (e.g. Lebas et al., 2011). The layer is covered by some intensively
stratified deposits except on the southern steep slopes. In contrast with outskirts however, equivalent consideredlayer in the caldera basin shows acoustic transparency with slight stratification within it, and the bottom terrain of itmarks major unconformity. Neither of such characteristic layers was observed at the center of the caldera wherelarge topographic rise exists.
2. Intrusive structure along the caldera-rim
Volcanic bodies are distributed along the caldera-rim. The largest one at the southeastern end has 2.5 km wide,
and the rolling-up horizons beneath of it indicate it occurred just before the climactic dispersion. Presence of suchbodies make difficult to evaluate caldera displacement however, it reaches 400 m in maximum at the rim of east tosoutheast.
3. The timing of the climactic dispersion and caldera collapses
Both caldera displacements and inner fractures seem to cut every deposit at every direction, therefore the caldera
collapse should be occurred enough after the climax of the eruption. Enigmatic shortage of the talus deposits may
reflect the subsidence and opening of the fractures happened gradually.

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