[S-VC41] Active Volcanism

[SVC41-52] Integrated marine investigations of Kikai Caldera, SW Japan

*Tetsuo Matsuno^1,2, Yoshiyuki Tatsumi^1,2, Nobukazu Seama^2,1, Keiko Suzuki-Kamata^1,2, Hiroshi Ichihara^3,1, Koji Kiyosugi^4,1, Reina Nakaoka¹, Satoshi Shimizu¹, Mamoru Sano¹, Hikaru Iwamaru¹, Haruhisa Morozumi¹, Hiroko Sugioka^2,1, Kazuo Nakahigashi^5,1, Yojiro Yamamoto⁶, Kazuki Hayashi², Kimihiro Nishimura², Yuka Furukawa², Misaki Horiuchi², Daichi Nakata⁷, Ryoto Nakamura⁷, Hirose Toki⁸, Yasutomo Seto⁸, Atsuhiro Oshige⁹, Hideaki Takizawa¹⁰, Tatsuro Chiba¹¹, Shuichi Kodaira⁶ (1.Kobe Ocean Bottom Exploration Center, Kobe University, 2.Department of Planetology, Graduate School of Science, Kobe University, 3.Graduate school of environmental studies, Nagoya University, 4.Organization of Advanced Science and Technology, Kobe University, 5.Tokyo University of Marine Science and Technology, 6.Japan Agency for Marine-Earth Science and Technology, 7.Division of Marine Engineering, Graduate School of Maritime Sciences, Kobe University, 8.Department of Planetology, Faculty of Science, Kobe University, 9.Department of Marine Engineering, Faculty of Maritime Sciences, Kobe University, 10.Johnny & Associates, Inc, 11.Asia Air Survey Co., Ltd.)

We started integrated marine investigations of Kikai Caldera with T/S Fukae-maru of Kobe University on October, 2016. Aims of our investigations are to reveal the structure of the caldera, the existence of magma reservoir, and to understand the mechanism of catastrophic caldera-forming eruption at 7.3 ka and a potential for a future catastrophic eruption. We conducted multi-beam echo sounder mapping, multi-channel seismic reflection (MCS) surveys, remotely operated vehicle (ROV) observations, rock sampling by dredging and diving, geophysical sub-seafloor imaging with ocean bottom seismometers, electro-magnetometers (OBEMs), some of which equip absolute pressure gauge, ocean-bottom magnetometers, and surface geomagnetic surveys.

The first finding of our investigations is lines of evidence for creation of a giant rhyolite lava dome (~32 km³) after the caldera collapse. This dome is still active as water column anomalies accompanied by bubbling from its surface are observed by the water column mapping. Chemical characteristics of dome-forming rhyolites akin to those of presently active small volcanic cones are different from those of supereruption. The voluminous post-caldera activity is thus not caused simply by squeezing the remnant of syn-caldera magma but may tap a magma system that has evolved both chemically and physically since the 7.3-ka supereruption.

We have been conducting integrated analyses of our data set, and have planned the fourth research cruise with T/S Fukae-maru on March, 2018, consisting of MCS survey, ROV observation, OBEM with absolute pressure gauge observation, and bathymetric and surface geomagnetic survey. We will introduce results of the data analyses and the upcoming cruise in the presentation.