著者

辻 智大 岸本 博志 藤田 浩司 中村 千怜 長田 朋大 木村 一成 古澤 明 大西 耕造 西坂 直樹 池田 倫治 太田 岳洋 福岡 仁至

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.68, no.3, pp.129-160, 2023-09-30 (Released:2023-11-02)

参考文献数

49

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Kuju volcano, located within Beppu-Shimabara graben central Kyushu, Southwest Japan, has been active in the recent 200,000 years. The 54 ka Handa eruption, as large as VEI 5 or 6 and the largest one of the volcano, released large-scale pyroclastic flow deposits (Handa pfd; Kj-Hd) and a wide-spread tephra (Kj-D ash and Kj-P1 pumice fall deposits) that has been reported at more than 500 km from the source. The stratigraphic relationships among the deposits from the Handa eruption are important for volcanology and disaster prevention, and have been studied in various studies, but there is no consensus on the stratigraphy. In this study, we examined the stratigraphic relationships and the eruption history based on the stratigraphic and petrographic studies around Kuju volcano, as well as on Shikoku and Honshu Islands. As the results, the stratigraphic relationships were revealed as follows. 1) The pumice fall deposit, that has been named Kj-Yu, was previously included in Kj-D ash layers, but is revealed to be a much older ejecta than Kj-D ash, along with the tephras newly named Kj-Tb1 and 2. 2) The clay-rich layer just below Kj-D was previously considered to be soil, but it contains a large number of volcanic ash particles so that it is defined as Kj-Y ash layer. 3) Three light brown fine ash layers, newly named Kj-D-U2, 4 and 6, sandwich between the blue grey sandy ash layers i.e. Kj-D-U1, 3, 5 and 7, are revealed to be the co-ignimbrite ash derived from Kj-Hd 1, 2 and 3 pfd, respectively. It suggests that the Kj-Hd1, 2 and 3 pfd are interbedded with Kj-D-U ash layers. 4) Kj-P1 overlies on Kj-D-U7 ash layer that mantled the reworked deposit of Kj-Hd3. 5) Kj-P1 is divided into lower and upper units based on the grain-size analysis, petrography, the chemical composition of glass shards and the isopach maps. Kj-S pfd was formed in the same time as the upper unit. Based on the results, the eruption history is assumed as follows. Pre-Handa eruption: the activity was low and the small-scale explosive eruptions that had released the pumice and volcanic fragments in loam (Kj-Y), followed by a relatively large explosive eruption that had formed Kj-AL. Early phase: the eruption started with phreatic eruption, sub-plinian eruption that deposited the lower unit of Kj-D ash. Subsequently, the eruption changed to vulcanian eruptions that ejected Kj-D-U. This eruption continued for a long period time. During the time, three large-scale pyroclastic flow eruptions happened and has formed Kj-Hd1, 2 and 3. Their co-ignimbrite ashes generated from the Kj-Hd pfds were deposited as Kj-D-U2, 4 and 6. Lahar were generated after Kj-Hd2 and 3 deposition. This phase was terminated by the deposition of Kj-D-U7 ash. Late phase: the plinian plumes occurred twice and deposited lower and upper lalyers of Kj-P1. The second one is the largest plinian eruption in the whole volcano history, with a large umbrella plume producing a wide-spread tephra at more than 500 km from the source and an intraplinian pyroclastic flow (Kj-S).

著者

七山 太 山口 龍彦 中西 利典 辻 智大 池田 倫治 近藤 康生 三輪 美智子 杉山 真二 木村 一成

出版者

一般社団法人 日本地質学会

雑誌

地質学雑誌 (ISSN:00167630)

巻号頁・発行日

vol.126, no.9, pp.493-517, 2020-09-15 (Released:2020-12-15)

参考文献数

93

被引用文献数

3

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南海トラフ巨大地震によって沈降が予測されている宿毛臨海低地において沖積コアを採取し,LGMの開析谷を埋積する沖積層の特徴と堆積シークエンスの検討をした.松田川開析谷はLGMに形成され,その後の後氷期海進により,9.8kaに標高-30mに海水が到達し,エスチュアリー環境へと変化した.その後も海水準は上昇し続けて内湾泥底環境となり,7.5kaに最高水深時となった.7.3kaに起こった南九州の鬼界カルデラ噴火により,給源に近い宿毛湾周辺においてもK-Ah火山灰が厚く降灰し,その直後に大規模なラハールが発生した.その結果,水中二次堆積物が急激に堆積した.7.0ka以降にデルタの成長が他の地域に先行して活発化したが,これは大規模なK-Ah火山灰の影響と考えられる.SKMコアから得られた過去1万年間の海面変動情報に基づくならば,宿毛湾地域は南海トラフ巨大地震によって一時的に沈降するものの,長期的に見るとそれらの沈降量は相殺されると理解される.

著者

佐藤 隆春 岡本 朋子 上武 治己 木村 一成 諏訪 斎 田崎 正和 都築 宏 南場 敏郎 丸山 正 三上 映子

出版者

地学団体研究会

雑誌

地球科學 (ISSN:03666611)

巻号頁・発行日

vol.55, no.4, pp.201-216, 2001-07-25

被引用文献数

1

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阪神地域では1596年の伏見地震,5世紀末〜6世紀中頃の古墳時代中期,および約6,300年前に地層の液状化を引き起こす強い地震動があった.これらの地震の直後に土石流・洪水堆積物が増加しているかどうか検討した.土石流・洪水堆積物の堆積相はおもに埋蔵文化財調査の露頭で観察し,考古学による年代とアカホヤ火山灰層準の対比でその堆積年代を求めた.その結果,土石流・洪水堆積物は古墳時代中期の地震直後に扇頂〜扇央で16地点,伏見地震直後に扇央〜扇端および低地で10地点確認された.土石流・洪水堆積物は地震以後に増加する傾向と,時代とともにその堆積場が扇状地から低地に移動させている傾向が明らかになった.六甲山地では1995年の兵庫県南部地震による斜面崩壊が多発し,崩壊土砂が増加している.これが豪雨で流出し土石流を発生させる自然条件は高くなっていると考えられる.