Palaeo-tsunami inundation distances deduced from roundness of gravel particles in tsunami deposits

Daisuke Ishimura,Keitaro Yamada

doi:10.1038/s41598-019-46584-z

Daisuke Ishimura, Keitaro Yamada

Open Access

https://doi.org/10.1038/s41598-019-46584-z

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Abstract

Information on palaeo-tsunami magnitude is scientifically and socially essential to mitigate tsunami risk. However, estimating palaeo-tsunami parameters (e.g., inundation distance) from sediments is not simple because tsunami deposits reflect complex transport processes. Here, we show a new approach to estimate tsunami inundation distance based on the mixture ratio of gravels from several sources in tsunami deposits. We measured the roundness of source gravels in modern beach and fluvial deposits in a coastal valley in Japan through image analysis and then calculated the mixture ratio of both sediment types in tsunami deposits. Normalising the mixture ratios by inundation distances revealed an abrupt change in the mixture ratio at a constant percentile, regardless of tsunami magnitude. This relation allowed estimation of the inundation distance of palaeo-tsunamis during the last 4000 years.

Highlights

Tsunamis are one of the most devastating natural hazards on Earth, triggered by earthquakes, submarine or onshore landslides, volcanic eruptions and meteorites
We estimated the palaeo-tsunami inundation distance (ID), which is an important parameter for quantifying the magnitude of a tsunami, using typical tsunami deposits from a small coastal valley in Japan
We discovered that the mixture ratios show abrupt changes at a constant percentile in normalised distance, regardless of tsunami magnitude

Summary

Meiji Sanriku

Return waves would mainly erode and transport fluvial sediments (river, talus, and fan deposits), thereby resulting in high fluvial gravel contents in inland tsunami deposits This relation indicates a correlation between changes in the mixture ratio away from the coast and tsunami sediment transport. Distances of the E1–E3 tsunami deposit sample locations from the coast were normalised by each event’s ID (Fig. 4A), and abrupt changes in the mixture ratio were identified at the same percentile for the E1–E3 events These steps mean that the distribution pattern of the composition of source materials in tsunami deposits from the coast to each inundation limit is consistent, regardless of the tsunami magnitudes. By using the ID and TGIP estimated by our method, it is possible to estimate palaeo-tsunami run-up height and/or inundation area, allowing establishment or revision of tsunami generation models and tsunami risk assessments

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