Abstract
Huge tsunami waves have repeatedly bombarded the southern end of the Ryukyu Islands (Miyako and Yaeyama Islands, southwestern Japan) at several-hundred-year intervals. Therefore, clarifying the islands’ paleotsunami history is important for risk assessment. Nevertheless, discrepancies of paleotsunami histories exist among regional studies of tsunami boulders and sandy tsunami deposits. Radiocarbon ages of tsunami boulders indicate that tsunami events of the last 2400 years have occurred every 150–400 years, most recently the historical 1771 Meiwa tsunami. Sandy tsunami deposits at Yaeyama Islands show that four tsunami events of the last 2000 years struck the islands at approximately 600-year intervals. Sandy tsunami deposits of the Miyako Islands have been studied only rarely. Therefore, studying sandy tsunami deposits in the Miyako Islands is crucially important for clarifying the paleotsunami history of this region. We conducted a trench survey on Minna Island, located among the westernmost Miyako Islands, which revealed two sandy tsunami deposits under a coral tsunami boulder transported by the 1771 tsunami. The upper tsunami deposit was likely deposited by the 1771 tsunami, as inferred from stratigraphic correlation to the tsunami boulder. However, the lower tsunami deposit was probably deposited 700–1000 years ago, which is consistent with the age range of the paleotsunami reported for Yaeyama Islands. Because sandy tsunami deposits found in this and earlier studies are thick and deposited at high elevation and far inland, these are useful markers of large tsunami events similar to the 1771 event. However, the reported tsunami boulders of various sizes are deposited along the coast and reefs: they can be formed not only by large tsunami events but also by small ones. It is noteworthy that each tsunami deposit is coarse and thick (40–48 cm) relative to the island elevation (about 12 m maximum, 7 m above the mean sea level at the study site). By assuming that tsunamis have affected this region repeatedly during the past few thousand years at around 600-year intervals, tsunamis might have been important geomorphic agents for building up small reef-surrounded islands such as Minna Island.
Highlights
Understanding paleotsunami history based on the geological record is important for long-term tsunami risk assessment
The tsunami recurrence interval estimated from tsunami boulders is far shorter than that estimated from sandy tsunami deposits
Results reported by Ando et al (2018) and those obtained from the present study indicate that an interval of a tsunami event that is sufficiently powerful to lead to sandy tsunami deposit formation far inland is about 600 years
Summary
Understanding paleotsunami history based on the geological record is important for long-term tsunami risk assessment. Many boulders transported and deposited by paleotsunamis are observed along the shorelines of the Miyako and Yaeyama Islands of the southern Ryukyu Islands in southwestern Japan (Fig. 1, Goto et al 2010a, b, 2019). Some of these boulders, called tsunami boulders, comprise coral rocks (Kawana and Nakata 1994; Omoto 2012; Araoka et al 2013). Numerical analyses (Hisamatsu et al 2014) and residual magnetism measurements of tsunami boulders (Sato et al 2014) further indicated that more than three huge tsunamis as large as the 1771 Meiwa tsunami, Fujita et al Progress in Earth and Planetary Science
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