[M-IS11] Tsunami deposit

[MIS11-01] Sedimentological and micropaleontological characteristics of tsunami deposits observed in Suzu City associated with the 2024 Noto Peninsula earthquake

*Yumi Shimada¹, Dan Matsumoto¹, Koichiro Tanigawa¹, Yuki Sawai¹ (1.Geological Survey of Japan, AIST)

Keywords:The 2024 Noto Peninsula earthquake, Tsunami deposits, Fossil diatom assemblages, Climbing ripples, Vented sediments, Suzu City, Ishikawa Prefecture

This study reports sedimentological and paleontological features of deposits left by the 2024 Noto Peninsula tsunami in Ishikawa Prefecture, Japan. We carried out field surveys for the tsunami deposits in Suzu City on January 18, February 19, and June 5, 2024. On the southern bank of the Ukai River, tsunami deposits were found up to 70 m inland from the post-tsunami shoreline, with a thickness ranging from 0.6 to 11.2 cm. The tsunami deposits were collected at one location on a footpath along the seawall (SZ1) and at four locations on a small cultivated field located further inland (SZ2–SZ5) for observation by eyes, soft X-ray and X-ray CT images, grain-size analysis, and diatom analysis. Grain-size analysis showed that the tsunami deposits were generally composed of fine to very fine sand at all sampled locations. Soft X-ray and CT images identified five stratigraphic units (Units 1–5 from bottom to top) at the most seaward location (SZ1) and deposits with faint laminae at the other locations (SZ2– SZ5). Unit 3 at SZ1 exhibits climbing ripples with their leeside facing seaward, suggesting a substantial influence of a backwash flow. The ripple tops were probably dragged seaward, indicating that overlying Unit 4 was formed by another backwash flow. The eroded upper boundary of Unit 4 implies yet another current at SZ1. Diatom assemblages within the tsunami deposits are dominated by marine and brackish species, except in Unit 4 at SZ1, which contained more than 30% freshwater terrestrial species. Detrended correspondence analysis of diatom assemblages in the tsunami deposits and comparative samples (vented sediments beach sand, cultivated field soil, and paddy soil around the sampled locations) suggests that the SZ1 tsunami deposit was derived from both terrestrial and marine sources, while the main source was the coastal beach at the other locations.