Abstract

Boulders that originated from the Holocene coral terrace and coral reef are distributed on the coral terrace or talus on the shore of Lanyu Island, Taiwan. We employed numerical simulation of storm waves and estimated whether the boulders could be moved by storm waves with a return period of 50 years, by larger storm waves with return periods of hundreds of years, or by tsunamis. The coral boulders are distributed between 36 and 128 m from the shoreline at elevations of 2.7 to 9.3 m. The sizes of the boulders are in the range 0.3 to 6.4 m. The boulder volume and a-axis length versus distance from the shore show weak and moderate correlation (r = 0.21 and 0.48), respectively. We reproduced the runup of waves using the equation of continuity for a two-dimensional non-compressive fluid and the Navier-Stokes formula. In order to handle the free surface of the fluid, the volume of fluid method was applied. For the flow velocity to move the boulder, we employed the theoretical formula for the transport of boulders. We used the wave height for a return period of 50 years and set the height of the input wave to 13.1 m. The results show that all the large boulders on Lanyu Island could be moved by storm waves with a return period of 50 years. Moreover, the computations show that most of the boulders could not have been transported by tsunamis generated by the Mw8.7 earthquakes. The ages of two boulders indicate that they were transported after 340 and 6,330 years ago. If these boulders were moved by tsunamis several hundred years ago, they would have been relocated subsequently by storm waves.

Highlights

  • Analysis of past tsunami sediments is one of the most important tools for past tsunami reconstruction

  • We employed a numerical simulation of storm waves with a 50-year return period on the shore of Lanyu Island and estimated whether the boulders could be moved by the storm waves

  • We surveyed the sizes and distances from the shore of boulders on Lanyu Island, Taiwan, and investigated whether they could have been moved by storm waves

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Summary

Introduction

Analysis of past tsunami sediments is one of the most important tools for past tsunami reconstruction. Boulders of Holocene coral limestone that had been transported by tsunamis were distributed on the beach and land areas of the south Ryukyu Islands (Kawana and Nakata 1994; Goto et al 2010b). As the width of the reef lagoon is 1,000 to 2,000 m in the south Ryukyu Islands, storm waves that are broken at the reef crest cannot reach shore. As the width of the reef lagoon is narrower than the typical tsunami wavelength (which is at least several tens of kilometers), tsunami waves could traverse the lagoon to reach the beach. Using the difference of wave energy distribution between storms and tsunamis, we can distinguish tsunami boulders from boulders in the reef lagoon area (Goto et al 2010a, b)

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