[M-IS15] Tsunami deposit: research progress after the 2011 Tohoku-oki earthquake and prospects

[MIS15-P06] Event deposits in a coastal lake at Minami-ise Town, Mie Prefecture during the last 3000 years

*Yumi Shimada¹, Yuki Sawai¹, DAN MATSUMOTO¹, Koichiro Tanigawa¹, Yuichi Namegaya¹, Masanobu Shishikura¹, Kazumi Ito¹, Toru Tamura¹, Shigehiro Fujino² (1.Geological Survey of Japan, AIST, 2.Faculty of Life and Environmental Sciences, University of Tsukuba)

Keywords:event deposits, tsunami, storm, lacustrine sediments, fossil diatom assemblages, Nankai Trough

Sediment cores taken at Kogare-ike, a coastal lake in Minami-ise town facing the Nankai Trough, contain the sixteen event deposits likely representing tsunamis or storm surges. Nine of the event deposits were invisible but identified by computed tomography (CT) images. Depositional ages of the event deposits were estimated by combination of ¹³⁷Cs concentration and radiocarbon ages from pollens and plant macrofossils. An age-depth model was then established from these radiometric analyses was constructed using Bchron (Haslett & Parnell, 2008). This model revealed that the event deposits in the study site had settled during the last 3000 years. In order to detect environmental changes during the period, fossil diatom assemblages from the core taken at the center of the lake were identified and counted. Until the deposition of the youngest event deposit (E1), diatom assemblages were dominated by freshwater species such as Tabellaria fenestrata and Staurosira spp. After the deposition of the E1, however, assemblages were dominated by brackish species such as Amphora coffeiformis, Bacillaria paxillifer, Navicula gregaria.

There are some causes of event deposits in coastal lakes: tsunamis, storm surges and floods. However, the event deposits are likely to represent seawater inundation associated with tsunamis and/or storm surges, not river floods, because Kogare-ike has no significant fluvial input. The youngest event deposit (E1) exists above the lowest horizon occurrence of ¹³⁷Cs. Because the first occurrence of the cesium shows a timeline of 1940's, E1is likely to be correlated by the recent documents with the 1959 Isewan typhoon. The change in diatom assemblages before and after the E1 suggested breaching of the barrier generated with the storm surge that enabled more seawater to flow into the study site. As for the other event deposits at Kogare-ike, some of them may represent tsunami because there are the written documents and the tsunami monuments at Minami-ise town inferring the repetitive inundation by tsunamis associated with the earthquakes along the Nankai Trough (e.g. Nakata, 1991; Namegaya & Tsuji, 2005).