2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 18
Presentation Time: 6:00 PM-8:00 PM

TSUNAMI INUNDATION HISTORY IN SENDAI PLAIN, INFERRED FROM TSUNAMI DEPOSITS


SAWAI, Yuki, SHISHIKURA, Masanobu, OKAMURA, Yukinobu, MATSU'URA, Tabito, KOMATSUBARA, Junko and AUNG, Than Tin, Geological Survey of Japan, AIST, Higashi 1-1-1 C7, Tsukuba, 305-8567, Japan, yuki.sawai@aist.go.jp

We reconstructed history of unusually large tsunami inundations during the past 3000 years, on the basis of distribution of tsunami deposits. The tsunami deposits were revealed by over 240 core- and sliced-samples from rice paddies (swales) and beach ridges. These samples show that tsunami deposits in Sendai consist mainly of massive, poorly-sorted and normaly-graded (sometimes, inversely-graded) sand sheets interbedded with swamp/marsh peat or brackish mud above littoral well-sorted coarse sand. The sand deposits include many marine and brackish diatoms. They can be traced continuously over a few kilometers from the present coast to landward using historical volcanic ash (To-a, AD 915), and the distributions are extensively larger than the recent tsunamis (e.g. 1933 Showa Sanriku tsunami, 1960 Chile tsunami, and other 20th-century Miyagi-oki tsunamis) and historical and recent storms. These marine-originated, continuous and widespread sand sheets were regarded as tsunami deposits associated with unusually large tsunamis. Radiocarbon ages and tephrochronology permit correlation of a sand sheet with a tsunami in AD869 that reportedly devastated at least 100 km of coast approximately centered on Sendai. A later well-known tsunami, in AD1611, may account for another sand sheet above ash To-a. Further deeper part of the cores record that usually large tsunamis repeatedly inundated Sendai Plain during the past 3000 years, and the recurrence interval is about 1000 years.