Millennial paleotsunami history at Minna Island, southern Ryukyu Islands, Japan

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.

Possible Tsunami-Induced Sediment Transport From Coral Reef to Deep Sea Through Submarine Canyons on the Southern Ryukyu Forearc, Japan

“…Recurrence intervals of calciturbidite depositions in cores YK15 PC01, PC05, PC07, and PC08 are calculated as several hundred-a few thousand years (average: 1,388 years) (Figure 4 and Supplementary Table S5). The intervals are likely concordant with those (500-1,000 years) of large tsunamis as reconstructed from coral boulders and onshore tsunami deposits in the southern Ryukyu Islands (Kawana and Nakata, 1994;Ando et al, 2018;Kitamura et al, 2018;Fujita et al, 2020), and slightly larger than those (150-400 years) from coral boulders on Ishigaki Island (Araoka et al, 2013). Remobilization of coral reef sediments and collapse of reef slopes by strong earthquake ground shaking might be another mechanism for the transport of coral reef sediments to the deep-sea.…”

“…Large earthquakes and tsunamis are possible mechanisms for the transport of shallow marine sediments to the deep-sea (e.g., Feldens et al, 2009Feldens et al, , 2012Ikehara and Usami, 2018;Ikehara et al, 2021b). Especially in coral reefs, large tsunamis have enough potential to transport coral reef materials to both onshore (Goto et al, 2010;Kitamura et al, 2018;Fujita et al, 2020) and offshore (Feldens et al, 2009(Feldens et al, , 2012Paris et al, 2010;Sukuna-Schwartz et al, 2015). Recurrence intervals of calciturbidite depositions in cores YK15 PC01, PC05, PC07, and PC08 are calculated as several hundred-a few thousand years (average: 1,388 years) (Figure 4 and Supplementary Table S5).…”

“…The southern Ryukyu arc is located in a subtropical region, and small islands in the arc are surrounded by coral reefs (Hori, 1977;Hori and Kayanne, 2000). Large tsunamis have repeatedly struck these islands with 500-1,000 years intervals (Kawana and Nakata, 1994;Ando et al, 2018;Fujita et al, 2020) from studies of onshore sandy tsunami deposits and tsunami boulders, although Araoka et al (2013) inferred 150-400 years intervals during the last 2,400 years. The latest tsunami was the Meiwa tsunami in 1771, which had run-up heights of 30 m and approximately 12,000 deaths (Goto et al, 2010).…”