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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.

SUBMITTER: Okamura Y

PROVIDER: S-EPMC6134009 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Large lacustrine mass movements and delta collapses are increasingly being considered as potential tsunamigenic sources and therefore hazardous for the population and infrastructure along lakeshores. Although historical reports document tsunami events in several lakes in Switzerland, and although the propagation of lake tsunamis has been studied by numerical wave modeling, only little is known about on- and offshore lacustrine tsunami deposits. In Lake Sils, Switzerland, a large prehistoric mass-movement deposit originating from the Isola Delta with a minimum estimated volume of 6.5 × 106 m3 and a basinal thickness of > 6 m in the seismic record has been identified by previous studies and radiocarbon dated to around 700 Common Era. Here, we combine (i) comprehensive sedimentological investigation of sediment cores recovered from the on- and offshore settings, (ii) mineralogical fingerprinting of the inflows from key catchments to characterize sediment provenance, and (iii) numerical tsunami modeling, to test the hypothesis of a tsunamigenic delta collapse in Lake Sils. We observe a clastic event deposit consisting of coarse-grained, fining-upward sand overlying an organic-rich peat deposit in the shallow water. This layer thins and fines landward on the coastal plain. Toward the deeper water (20-40 m), the deposit transforms into a thicker and more heterogeneous sediment package with multiple sequences of fining-upward sand and a well-pronounced clay cap at the top. Radiocarbon dating of the peat underlying the event deposit yields a maximum age of 225-419 calibrated Common Era. The tsunami models, which indicate wave heights reaching up to 5 m, simulate areas of inundation that coincide with the location of event deposits. Based on our results, we propose that the historically undocumented Isola Delta collapse generated a basin-wide tsunami that inundated the lakeshore, transporting large amounts of unconsolidated sediment along the lakeshore toward the coastal plain and into the deeper lake basin.Supplementary informationThe online version of this article (10.1007/s11069-021-04533-y) contains supplementary material, which is available to authorized users.

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

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