Source of high tsunamis along the southernmost Ryukyu trench inferred from tsunami stratigraphy

• Published in: Tectonophysics 2018-01, Vol.722, p.265-276
• Main Authors: Ando, Masataka, Kitamura, Akihisa, Tu, Yoko, Ohashi, Yoko, Imai, Takafumi, Nakamura, Mamoru, Ikuta, Ryoya, Miyairi, Yosuke, Yokoyama, Yusuke, Shishikura, Masanobu
• Format: Article
• Language: English
• Subjects: Corals; Deposits; Fissures (geology); Landslides; Mollusks; Oceanic trenches; Plate boundaries; Plate coupling; Plate tectonics; Radiocarbon dating; Recurrence interval; Shaking; Shells; Soil; Southernmost Ryukyu subduction; Stratigraphy; Studies; Subduction; Subduction (geology); Subduction zones; The 1771 tsunami; Tsunami boulder; Tsunami deposit; Tsunamis
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/j.tecto.2017.11.007

Four paleotsunamis deposits are exposed in a trench on the coastal lowland north of the southern Ryukyu subduction zone trench. Radiocarbon ages on coral and bivalve shells show that the four deposits record tsunamis date from the last 2000yrs., including a historical tsunami with a maximum run-up of 30m in 1771, for an average recurrence interval of approximately 600yrs. Ground fissures in a soil beneath the 1771 tsunami deposit may have been generated by stronger shaking than recorded by historical documents. The repeated occurrence of the paleotsunami deposits supports a tectonic source model on the plate boundary rather than a nontectonic source model, such as submarine landslides. Assuming a thrust model at the subduction zone, the seismic coupling ratio may be as low as 20%. •Four tsunami deposits were excavated on an island along the southern Ryukyu trench.•These tsunamis occurred at an average interval of 630 y during the last 2000 y.•Among them, the 1771 large tsunami with the max height 30m is included.•The finding of ground fissures supports this event was not a slow earthquake.•The weakly coupled southern Ryukyu zone can generate ordinary great earthquakes.