Japan Geoscience Union Meeting 2015

Presentation information


Symbol M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

[M-IS25] tsunami deposit

Sun. May 24, 2015 6:15 PM - 7:30 PM Convention Hall (2F)

Convener:*Kazuhisa Goto(International Research Institute of Disaster Science (IRIDeS),Tohoku University), Masanobu Shishikura(Institute of Earthquake and Volcano Geology, National Institute of Advanced Industrial Science and Technology), Yuichi Nishimura(Graduate School of Science, Hokkaido University)

6:15 PM - 7:30 PM

[MIS25-P05] Tsunami sediment immediately after K-Ah ash fall in Miyazaki Plain

*Toshihiko ICHIHARA1, Hiroshi AKAZAKI2, Kiyotaka MATSUDA3, Mari HAMADA3 (1.Fukken Co.Ltd, 2.Miyazaki Prefectural Museum of Nature and History, 3.Miyazaki Prefecture Archaeological Center)

Keywords:tsunami deposits, Kikai-Akahoya-ash (K-Ah), Miyazaki Prefecture, Peels of Outcrops

Miyazaki Prefectural Museum of Nature and History, along with the Ishizaki river renovation of Miyazaki Sadowara Kuroda in 1996, have created peeling specimen of outcrops.The peeling specimens contains Akahoya volcanic ash of mollusk shells and Jomon transgression period.It is possible to observe the formation of up to 7.5 m depth from the surface. Result of study in sedimentological deposition environments of the stripping specimen, it was possible to find a tsunami deposits.

The peeling specimens include Crassostrea gigas, Tegillarca granosa, Thalassinoides isp..From these, Akahoya volcanic ash was deposited under inner bay environment close to the tidal flats or tidal flats.
Akahoya ash, exhibited a white tinged with pale yellow to pale pink in the specimen, the layer thickness is about 1.5m.The boundary between the lower strata exhibits a clear erosion.Bottom is a massive volcanic ash containing a large amount of wood.The top, HCS-mimics, parallel lamina, is observed climbing ripple laminae.The upper is covered with a non-structure of volcanic ash.Paleocurrent which is determined from the form of climbing ripple is a west direction from the east.Akahoya ash facies, such as erosion surface can not be seen until the climbing ripple from the bottom, i is judged to be continuously deposited.HCS-mimics are formed in the inner bay, we observed flow to the landward, there is a possibility of tsunami deposits.

Fujiwara et al. (2010) reported the tsunami deposits with characteristics similar in Oita, and the like be covered with ash fall of Akahoya, and are to be tsunami deposits caused by eruption.On the other hand, the tsunami deposits reported this time, not that it can be determined that volcanic ash.Also, the thickness of the secondary sediment is developed thick and 1.5m, while climbing ripple formed in sediment supply is often on even the naked eye observation, and the like to become only Akahoya ash, at least drop Akahoya ash is considered to be those that occurred after sufficiently deposited.In view of the above, this time the observed tsunami deposits, I believed to be due to the tsunami that occurred immediately after ash. Because it is information from a single stripping specimen, it is enough assurance not be obtained, it is necessary to consider the tsunami occurrence factors other than the Akahoya eruption, information of tsunami research is scarce Miyazaki coast of tsunami than other regions in performing the studies, it is very valuable sample.