Accretionary prism collapse: a new hypothesis on the source of the 1771 giant tsunami in the Ryukyu Arc, SW Japan

Abstract

The giant 1771 Yaeyama tsunami occurred in the southwestern part of the Ryukyu Arc, a region on an obliquely subducting plate boundary, which shows no direct evidence of inter-plate coupling. Studies of tsunami boulders and deposits suggest that the recurrence interval of comparably giant tsunamis is roughly 500 to 1000 years. Tsunami source models, which include either slip on a shallow plate boundary or active faulting plus a landslide on the overriding plate, are controversial because of inconsistencies in the geophysical and geological data. We discovered a seafloor depression that is approximately 30 km wide and 80 km long extending in the ESE-WNW direction. This depression is accompanied by a seaward bulge on the accretionary prism along the Ryukyu Trench, which is based on detailed bathymetric data and interpreted to be the result of accretionary prism collapse and seaward displacement by rotational slide. A simple tsunami simulation shows that the slide is a plausible source of the 1771 tsunami. We propose a collapse model, in which the accretionary prism remained over-steepened as strike-slip faulting removed the prism toe. Our model indicates that some oblique subduction zones are capable of generating giant tsunamis regardless of weak or strong coupling.

Relative Sea‐Level Changes Over the Past Centuries in the Central Ryukyu Arc Inferred From Coral Microatolls

“…With run‐up of waves up to 30‐m high, and more than 17,000 casualties, the 1,771 Meiwa tsunami was the deadliest event reported along the Ryukyu Arc (Nakata & Kawana, ). It occurred in the southern Ryukyu (Figure a), and several sources are proposed for this event, among them are reverse faulting coupled with landslides, an arc‐perpendicular normal fault located to the east of Ishigaki island (Nakamura, , and references therein), a thrust earthquake at the plate interface at shallow depth (Nakamura, ), and a massive collapse of the accretionary wedge prism close to the trench (Okamura et al, ).…”

Large tsunamis reset growth of massive corals

“…Hsu et al (2013) proposed a mega-splay fault as the tsunami source. Okamura et al (2018) proposed tsunami was generated by a large collapse of the accretionary prism along the oblique subduction zone. All these models are tuned parameters to fit the historical description of tsunami run-up heights (e.g., Goto et al 2012).…”

Seafloor morphology and sediment magnetic fabric in a putative 1771 Meiwa tsunami source region in the southern Ryukyu Islands, SW Japan

“…However, since the distribution of the graben structure is sub-parallel to the margin of the forearc basin (Fig. 5g), the formation of the graben might be related to a tectonic event in the forearc, such as a depression event interpreted to have been formed by a seaward sliding of the slope (Okamura et al 2018).…”

“…2). A farther low seafloor displays complicated surface structures in a broad area, which was inferred to be depressed seafloor by Okamura et al (2018).…”

“…Hiyoshi et al 1986;Miyazawa et al 2012). In the trench landward slope, Okamura et al (2018) recognized a large seafloor depression of an accretionary prism. They proposed that a large area of earthquake-induced rotational sliding generated the Meiwa tsunami.…”