Sedimentary differences between the 1993 Hokkaido-nansei-oki tsunami and the 1959 Miyakojima typhoon at Taisei, southwestern Hokkaido, northern Japan

Abstract

Abstract Tsunami deposits differ distinctly from storm deposits as seen in a trench in the southwestern Hokkaido town of Taisei, which was struck by a typhoon in 1959 and a tsunami in 1993. The typhoon generated a storm surge that crested about 6 m above ordinary high-tide levels for 1 h. The tsunami, caused by a nearby earthquake with a magnitude of 7.8, contained two main waves and was 1–3 m higher than the typhoon surge. We found deposits from both the typhoon and the tsunami in a 3 by 9 m trench located 60 m inland from the beach. The deposits are similar in thickness, which in both cases decrease landward from a maximum of about 50 cm. The tsunami deposits can be divided into four layers probably correlative with landward and seaward flows from the two main tsunami waves; the flow directions are shown by gravel fabrics, and remains of knocked-down plants. The landward tsunami flow deposited marine sand and rounded gravel, whereas the return flow deposited a poorly sorted mixture of soil, non-marine sand, and stream gravel with plant fragments. Only the storm deposit shows foreset bedding. This deposit is mostly marine sand that is better sorted than any of the tsunami layers.