- 著者
-
小林 哲夫
- 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.67, no.3, pp.335-350, 2022-09-30 (Released:2022-10-27)
- 参考文献数
- 90
In the Kikai caldera, a major caldera-forming eruption, the Akahoya eruption (Ah eruption), occurred at 7.3 cal ka BP. It started with a plinian eruption (K-KyP), accompanied by a small intra-plinian Funakura pyroclastic flow (K-Fn). In the second eruptive stage, large Koya pyroclastic flow eruption (K-Ky) occurred, which covered the southern part of Kyushu with widespread co-ignimbrite ash (K-Ah (c)). These series of pyroclastic materials are collectively called Kikai-Akahoya tephra (K-Ah (T)). It has been thought that the Akahoya tsunami (Ah tsunami), occurred in connection with the Ah eruption. However, in outcrops below 50 m elevation in the proximal area of the caldera (~60 km), the K-Ah (T) was either replaced by Ah tsunami deposits of various sedimentary facies or completely eroded away by the same tsunami. The largest tsunami was therefore estimated to be due to the collapse of the caldera rim, which occurred some time after the end of the Ah eruption. On the other hand, in the Yokoo midden at Oita city, approximately 300 km from the caldera, it was considered that the K-Ah (c) was deposited immediately above the sandy tsunami deposit. However, the parent material of these distal Ah tsunami deposit is presumed to be K-Ah (r), which was transported and deposited from hinterland to the estuary, and was then incorporated and redeposited by the subsequent striking Ah tsunami. That is, the particles in the tsunami can be interpreted as separating and settling into two different layers, i.e. the basal sand layer and the upper K-Ah (r) set as the same tsunami deposit, due to differences in density. This interpretation is also supported by the chemical analyses of volcanic glass. Thus, the erosion and deposition either proximal or distal area of the caldera indicate that the largest Ah tsunami occurred some time after the Ah eruption. The caldera rim shows a double depression structure which was formed during the Ah eruption, and there are many channel structures on the caldera rim that suggest intense seawater movement. It is therefore highly probable that the sudden collapse of caldera wall after the Ah eruption is the cause of the tsunami, together with the run-up height near the caldera. However, it is not possible to estimate the time until the collapse that caused the Ah tsunami.