[M-IS20] Tsunami deposit

[MIS20-06] Study on the diversity of sedimentary structures in event deposits and methods for their identification

*Rina Okada¹, Koji Umeda¹, Takanobu Kamataki² (1.Hirosaki University, 2.Okayama University of Science)

Keywords:Japan Sea, Tsunami, Diatom

The Japan Sea coast of northeastern Japan is in the eastern Japan Sea marginal tectonic zone, and over the past 300 years, M7-class earthquakes has occurred once every 50 years. The Japan Sea coast is a different Pacific coast, the coast dune is developed by the influence of monsoon (10 to 40m above sea level). Therefore, tsunamis run up small rivers and flow irregularly from the levees, indicating that the thickness and distribution (inundation range) of tsunami deposits are limited. In the case of rivers, tsunamis and floods need to be clearly distinguished because rivers can be inundated by floods caused by heavy rainfall. The results show that tsunami sediments can be identified by the following criteria: goodness of the sorting, high roundness of detrital clasts, and not included volcanic glass, compared to flood sediments. On the other hand, a case can be assumed where a tsunami instead of a river enters a lagoon connected to the open sea and forms tsunami deposits. However, the deposition process in this case is considered to be very different from that in the case of river run-up and inundation. We collected data on tsunami deposits found in Lake Jusan at the Iwaki River estuary and Lake Paleo-Kisakata and those formed by river run-up and flooding (alluvial lowlands around Narusawa River in Ajigasawa Town and Gakko River in Yuza Town), and compiled their sedimentological, petrographic, mineralogical, and paleontological characteristics. The results clarify that two types of tsunami deposits exist along the Japan Sea coast of northeastern Japan: (1) river run-up type tsunami deposits and (2) lagoon-invasion type tsunami deposits.