著者

松野 哲男 巽 好幸 島 伸和 鈴木 桂子 市原 寛 清杉 孝司 中岡 礼奈 清水 賢 佐野 守 井和丸 光 両角 春寿 杉岡 裕子 中東 和夫 山本 揚二朗 林 和輝 西村 公宏 古川 優和 堀内 美咲 仲田 大地 中村 崚登 廣瀬 時 瀬戸 康友 大重 厚博 滝沢 秀明 千葉 達朗 小平 秀一

出版者

日本地球惑星科学連合

雑誌

日本地球惑星科学連合2018年大会

巻号頁・発行日

2018-03-14

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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.

著者

清杉 孝司

出版者

日本地球惑星科学連合

雑誌

日本地球惑星科学連合2016年大会

巻号頁・発行日

2016-03-10

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噴火記録から噴火の発生率を見積もる際には,噴火記録の数え落しを考慮する必要がある.世界の大規模爆発的噴火(LaMEVE)データベース(Crosweller et al., 2012, Brown et al., 2014)の内,日本の噴火は39%を占める(Kiyosugi et al., 2015).一方,これまでの分析の結果,噴火の規模が大きくなると数え落しの程度は減少するものの,第四紀以降の大規模噴火にも数え落しがあることがわかっている.例えば,89 %のVEI 4噴火が10万年以内に,65–66 %のVEI 5噴火が20万年以内に,46–49 %のVEI 6噴火が30万年以内に,36–39 %のVEI 7噴火が50万年以内に失われていることがわかった(Kiyosugi et al., 2015).また,日本と世界の噴火頻度の比較から,世界の噴火記録の数え落しは日本の7.9倍から8.7倍であると示唆される(Kiyosugi et al., 2015). 噴火の数え落しをモデル化するためには,こうした日本全体のデータの分析に加え,地域ごとや時代ごとに見たときのデータの不均質性を検討することが必要である.数え落しの主要な原因は,歴史記録がないことや,火砕堆積物の浸食・変質,新しい堆積物による被覆,浸食や被覆による給源火山自体の消失などであると考えられる.そのため噴火の数え落しは地質学的・歴史学的事情を反映して時空間的な不均質性を持つ.例えば,伊豆‐ボニン弧は大規模噴火の火砕堆積物が保存されにくい小規模の火山島からなっているため,大規模な噴火の地質記録が多く失われていることが示唆される.こうした異なる地質条件による噴火の数え落しを理解することは,噴火の再発生率を推定する際に重要である.また,小山(1999)は日本の歴史地震記録が政治的・社会的状況に応じて二つの時期(7世紀末から西暦887年までの時期と17世紀初めから現在までの時期)に増加することを指摘した.このような歴史記録を含む最近の噴火記録は,噴火の数え落しをモデル化する際の重要なファクターであるため,記録の時間的不均質性の詳細な分析が必要である. 本研究では日本の噴火記録の時空間的不均質性について議論する.日本の噴火記録は世界の噴火記録の約39%を占めるため,日本の噴火記録の分析は世界の噴火記録の数え落しと噴火再発生率の推定に有用である. 引用文献:Brown et al (2014) Journal of Applied Volcanology, 3:5.Crosweller et al (2012) Journal of Applied Volcanology, 1:4.Kiyosugi et al (2015) Journal of Applied Volcanology, 4:17.小山 (1999) 地学雑誌, 108(4), 346-369.

著者

清杉 孝司 巽 好幸 鈴木 桂子 金子 克哉 中岡 礼奈 山本 由弦 羽生 毅 清水 賢 島 伸和 松野 哲男 菊池 瞭平 山口 寛登

雑誌

JpGU-AGU Joint Meeting 2020

巻号頁・発行日

2020-03-13

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Catastrophic caldera-forming eruptions that discharge more than 40 km3 of Si–rich magma as pyroclastics are rare but extremely hazardous events (eruption magnitude >7). Estimating the eruption volume of pyroclastics and the magma discharge rate in caldera–cycle is essential in evaluating the risk and cause of catastrophic caldera–forming eruptions. For this reason, we took sediment cores with Hydraulic Piston Coring System (HPCS) and Short HPCS (S-HPCS) of D/V Chikyu at Kikai volcano in January 11–14, 2020. Kikai volcano (Kikai caldera) is located about 45 km off southern Kyushu Island, Japan. Except two islands (Satsuma Iwo-Jima Island and Take-Shima Island) on the northern part of the caldera rim, most of the caldera structure is under the sea. At Kikai volcano, three ignimbrites are known; the 140 ka Koabi ignimbrite, the 95 ka Nagase ignimbrite, and the latest 7.3 ka Koya ignimbrite.Sediments were recovered from 5 sites about 4.3 km off the northeastern side of Take-Shima Island. Each drilling site was separated by 10–20 m from any other site. The sediment was not consolidated. Bioturbation was not observed. The sediment sequence, from the top of the cores, consists of gravel unit, ill-sorted lapilli unit, reddish tephra unit, sandy silt unit, and white tephra unit. The sedimentary facies of these sediments is as follows.Gravel unit: The presence of this unit in the upper part of the sequence is suggested by gravels which fell in the drilling holes and recovered with the sediments of the lower sequence. The gravels are consist of white tuffaceous rock, obsidian, gray volcanic rock, reddish altered volcanic rock, gray pumice and altered pumice. They are angular to sub-angular in shape and varying in size up to 5 cm in diameter.Ill-sorted lapilli unit: This deposit consists of ill-sorted lapilli size light yellow colored pumices and lithics of dark volcanic rock, gray volcanic rock, and obsidian. The maximum grain size of the pumice is more than 5 cm, whereas the maximum grain size of the lithic is about 4 cm. The abundance of the pumice component varies with depth. The thickness of the unit is more than 7 m at the drilling sites. The color of the pumice suggests that this unit may be a secondary deposit of underlying Koya ignimbrite deposit.Reddish tephra unit: It consists of layers (maximum thickness at least 40 cm) of slightly reddish to orange ill-sorted pumice lapilli and thin layers (~1 cm thick) of relatively well-sorted ash. The thickness of the deposit is more than 5 m at the drilling sites. The characteristic color of pumice suggests that this unit is the deposit of Koya ignimbrite. Formation of relatively thin layers of lapilli and ash may be due to the deposition under the sea.Sandy silt unit: It consists of very fine fragments of black volcanic rock. The sediment contains small fragments (~5 mm) of sea shells and other organic materials. Foraminifars were also contained in this deposit. The thickness of this unit is at least 20.36 m.White tephra unit: This deposit mainly consists of ill-sorted white pumice lapilli and relatively well-sorted ash. The maximum pumice size is at least 11 cm. The thickness of the deposit is at least 30 m. The deposit is characterized by the presence of crystals of quartz, which is known as a remarkable feature of the Nagase ignimbrite deposit to distinguish it from the other tephra at Kikai volcano. Especially, the middle part of the recovered Nagase ignimbrite deposit (63–64 m below the seafloor) shows unique sedimentary face: it consists of only crystals of quartz (<2 mm in size), orthopyroxene and clinopyroxene (<1 mm in size), and magnetite (<2 mm in size). Formation of the sedimentary face may be due to the deposition of hot ignimbrite under the sea.Description of these sedimentary units is essential to distinguish the ignimbrite deposits and understand their flow behavior in the sea. We will show the detail of these sedimentary facies in the presentation.