Tsunami deposits associated with the 1983 Nihonkai-Chubu earthquake tsunami in coastal forests near Happo Town, Akita Prefecture, Japan

On 26 May 1983 the Nihonkai-Chubu earthquake occurred off the western coast of Noshiro City, Akita Prefecture, Japan. The tsunami associated with this earthquake caused widespread damage to the northeastern coastal region of the Sea of Japan, including Akita Prefecture, and left behind sand and mud deposits. These deposits were first described in the 1990s, but have not been studied further. During December 2019 and January 2020, we conducted geological surveys to investigate post-1948 soil thinning in the pine-based coastal protective forests planted near Happo Town. A sand layer that thinned inland was observed in the soil at depths greater than 10 cm. Because the sand layer contained well-preserved fossil brackish-marine diatoms and exhibited a high bulk density, it is likely that the sand was transported inland from the coast. The sand layer was distributed from the coast to 150-270 m inland, but only within the coastal protective forest. By reference to historical records, we concluded that this sand layer was deposited by the 1983 tsunami, because this region could not have been reached by any event other than the tsunami produced by the Nihonkai-Chubu earthquake. We also observed another sand layer above the tsunami deposits, which may have been formed by Typhoon 9119 on 28 September 1991.