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.
Highlights
The 1771 tsunami was high in the eastern part of the Sakishima Islands (Fig. 1)[9,10]
Two seismic profiles (ECr4 and ECr5) obtained in the study area show that the upper slope and forearc basin between 123°E and 125°E consist of a pre-Miocene basement overlain by Neogene sedimentary sequences, which increase in thickness from a few hundred metres under the upper slope to more than 3000 m in the forearc basin[32] (Fig. 2)
No active reverse faults and hanging-wall anticlines are apparent in the upper slope or forearc basin based on these profiles, a linear scarp approximately 70 km long has been mapped in the basin east of 124°55′ E (Fig. 1b)
Summary
The 1771 tsunami was high in the eastern part of the Sakishima Islands (Fig. 1)[9,10]. The distribution of tsunami boulders is restricted to the Sakishaima Islands, and no tsunami boulders during the last 2000–3000 years have been found in the northern and middle parts of the Ryukyu Arc[21,22] Based on these ancient tsunami records, source models that reproduce the 1771 tsunami height have been proposed in the forearc area between 123°45′ E and 125°30′ E, south of Ishigaki-jima and Miyako-jima (Fig. 1b). Some of the models[23,24,25] have attributed the tsunami to active reverse faults plus landslides on the upper slope and forearc basin Another simple model[26] assumed a slip on the plate boundary along the trench under the accretionary prism, which was attributed to a tsunami earthquake. The bathymetric map shows that the width of the indentation east of the ridge is approximately 10 km, which is attributed to westward dislocation of the accretionary prism of approximately 40 km along the strike-slip fault over the last 800,000 years
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