3 0 0 0 OA 九重火山54 ka飯田噴火による噴出物の層序および噴火経緯
- 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
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).
2 0 0 0 OA 四国西部の中央構造線断層帯の地形と地質
- 著者
- 池田 倫治 後藤 秀昭 堤 浩之
- 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.123, no.7, pp.445-470, 2017-07-15 (Released:2017-08-03)
- 参考文献数
- 118
- 被引用文献数
- 4 3
西南日本の地体構造を考える上で中央構造線は欠くことのできない地質要素のひとつである(以下では,便宜的に地質境界の中央構造線を表現する場合には「中央構造線」を,活断層としての中央構造線を表現する場合には「中央構造線活断層系」を,また両方の断層を包括して表現する場合には「中央構造線断層帯」を用いる).中央構造線断層帯は長い活動史を持ち,白亜紀に西南日本内帯/外帯の地質境界として形成されてから,現在もその一部が活断層として活動している.しかし中央構造線と中央構造線活断層系の地下深部構造については現在も議論の分かれているところである.一方で,全長400km以上にわたる横ずれ活断層の破壊過程には不明な点が多いため,地震防災上も注目され地質学的のみならず地震学的にも研究が進められている.特に1995年兵庫県南部地震以降,正確な断層分布の把握,最新活動時期,活動間隔あるいは変位量といった断層活動性評価に資する情報が急速に蓄積されてきた.さらには,その様な活動性情報の収集は,長大横ずれ断層である中央構造線活断層系の断層セグメンテーションの検討を促進し,その結果,断層破壊過程あるいは発生する地震の規模予測の議論へと展開されている.本巡検では,四国西部の中央構造線と中央構造線活断層系を時空間的に意識しながら断層露頭を訪れ,地質境界の産状および活断層地形を観察する.また,中央構造線の活動で形成された第二瀬戸内層群である郡中層の産状についても観察し,様々なフェーズにおける中央構造線断層帯の運動像に迫る.
- 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
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.
The Mw 7.1 (GNS Science) earthquake (Darfield earthquake) occurred near Christchurch, New Zealand on 4 September 2010. The earthquake happened in the area where active faults had not recognized previously, produced the ground surface rupture (Greendale fault). We surveyed the mode of occurrences of the Greendale fault on 8 days after the Darfield earthquake. Measured dextral and vertical displacements along the fault are ~ 4.2 and ~ 1.5 m (predominantly southern parts up), respectively. The both slip components are distributed roughly symmetrically along the west fault segment. However, the maximum displacement points are perhaps different between dextral and vertical sense based on the survey results. The fault is roughly E–W strike, characterized by many Riedel fractures that show shapes of mole tracks accompanying main dextral displacements. Few active faults had recognized previously on Canterbury plain near Christchurch, whereas distribution of active faults and folds had cleared in the Pegasus Bay area where locates in the eastern part of the Christchurch. Moreover, there are records of a few paleoearthquakes (M>5) near Christchurch and in Pegasus Bay. These data would have been clues to forecast existence of blind active faults on Canterbury plain.