Ongoing persistent slope failures at the toe of a giant submarine slide in the Ryukyu trench that generated the AD 1771 Meiwa tsunami

This study investigates sedimentation processes and associated submarine sliding on the floor of the Ryukyu Trench. Three dive surveys were conducted using the manned submersible Shinkai 6500 (hereafter 6K) in the trench. The AD 1771 Meiwa tsunami was generated by a giant submarine slide on the landward trench slope in this region. Dive survey site 6K#1469 was located on a mouth of the submarine canyon in the forearc basin. Dive survey site 6K#1468 was located on the uppermost part of the submarine slide, where steps and fissures were observed. Dive survey site 6K#1467 was located at the toe of the submarine slide, which is covered by a muddy sediment layer. Core sample 6K#1467 measured 36.3 cm long and comprised muddy sediments interlayered with silt and/or sand laminations. The paleocurrent direction of the laminated sediments is mostly from north to south, indicating that they were not transported along with the trench but were from the landward slope, probably via repeated small submarine slides. The sedimentation rate for sample 6K#1467 was estimated at 2.42 mm/year using a depth profile of 210Pbex concentration within 10 cm deep and a date for the sediment of ~ 50 year ago. Based on the sedimentation rate and the number of laminae, the typical recurrence interval of the silt/sand layers is calculated to be ~ 6 year. We interpret these results as indicating that repeated small submarine slides have occurred persistently (recurrence interval of ~ 6 year) at the toe of the investigated giant submarine slide in the Ryukyu Trench over the last 50 year.