TSUNAMI INUNDATION HISTORY IN SENDAI PLAIN, INFERRED FROM TSUNAMI DEPOSITS

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.