[M-IS16] Tsunami deposit

[MIS16-P03] Properties of tsunami deposits observed by a high-density excavation survey by geoslicers

*Yuichi Nishimura¹, Yasuhiro Takashimizu², Takashi Ishizawa³, Daisuke Sugawara³ (1.Graduate School of Science, Hokkaido University, 2.Faculty of Education, Niigata University, 3.International Research Institute of Disaster Science, Tohoku University)

Keywords:Tsunami deposit, Geoslicer, Keicho Ohshu Earthquake

Sandy tsunami deposits from the 1611 Keicho Oshu earthquake are distributed in peat at a depth of 50cm-1m in Atsuma, eastern Iburi, Hokkaido (Nishimura et al, presented at JpGU2023). The elevation at the time of the event was 3-4.5m, and it slopes gently inland. The thickness of the sediments is more than 50 cm at the thickest point, and it becomes thinner inland and invisible at about 500 m from the late Edo period coastline (Ino map). Here, we report the results of high-density excavation surveys using multiple geoslicers in part of the study area. The high-density survey was conducted in a water channel with a depth of about 50 cm, which was established for the maintenance of construction roads. Since it is along a channel, the direction of the survey line is limited, but the labor for excavation is greatly reduced. All the cores obtained by the geoslicers were resampled to an acrylic case and taken to our laboratory. After X-ray CT images were obtained, the core was peeled off and took sampled for several analysis. A group of 13 cores drilled at 1m intervals showed that the layer thickness of tsunami deposits rapidly decreased from about 60cm to about 10cm. In the thick deposit, gravels with a maximum diameter of 1 cm were included in the bottom 5 cm, and it was divided into two units. In part of this survey line, quasi-continuous excavation was also carried out by driving 10 geoslicers at the minimum intervals. The interval between adjacent core centers is about 14 cm, and when these are arranged side by side, almost the same information as the outcrop can be obtained. The thickness of the tsunami deposit varies from 10 cm to 20 cm within the 1.2 m range. This is because the lower boundary of the tsunami deposit is not flat, while the upper boundary is flattened. It is speculated that the unevenness of the lower surface was newly formed by erosion caused by the tsunami, or that the original heterogeneous unevenness of the ground surface was washed out in the flow. An X-ray CT image confirmed that the sand had intruded significantly into the interior of the pre-tsunami surface, centering on the small ridges. In this peatland, not only in waterways, the thickness of tsunami deposits often varies by a factor of two at a distance of several tens of centimeters to one meter. High-density excavation surveys are also useful for understanding the occurrence of such tsunami deposits.