[M-IS16] Tsunami deposit

[MIS16-P05] Tsunami deposit survey in the Yatagano marsh along the coast of Kii Penisula, Southwestern Japan

*Takuto Sumi¹, Masanobu Shishikura², Hideaki Maemoku¹, Takaharu TAWARA³ (1.HOSEI University, 2. Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 3.Nanki high school)

Keywords:Nankai Trough, Kii Peninsula, tsunami deposit, paleo-tsunami, tephra

Tsunami damage has repeatedly occurred in the coastal areas of the Nankai Trough following huge earthquakes. Historical records, studies of crustal deformation and other factors indicate that huge earthquakes along the Nankai Trough have occurred at intervals of approximately 100 to 200 years. The previous major earthquakes were the 1944 “Tonankai earthquake” and the 1946 “Nankai earthquake”, and 76 to 78 years have already passed since then. We can say that it is in imminent danger of the next major earthquake. In this study, several core samples were analyzed to investigate paleo-tsunami history in the coastal area of the Kii Peninsula.

The study area is located in a wetland of Yatagano, Nachikatsuura-town in Kii peninsula along the Nankai trough ,pacific coast of southwestern Japan. At an elevation of approximately 1.9 m above mean sea level. Several sand layers were observed between peaty deposits in the sampled cores. The thickness of these sand layers become thinner and change grain size finer to the inland direction, showing grading or reverse grading in sedimentary structure. We can assume that they were transported and deposited by sudden and strong water flow such as storm surges or tsunamis.

The sand layers are dated 7200-5200 cal B.P. for E1.1, 5400-5200 cal B.P. for E1.2, 4500-4200 cal B.P. for E1.3, 4000-3800 cal B.P. for E2.1, and 2700-2300 cal B.P. for E2.2 respectively.