Study on the possible sources of the tsunami boulders in Lanyu by TRTM

Abstract:

Recently, six coral boulders were founded on Lanyu, Taiwan (Yoko et al., 2014). The highest elevation is higher than 5 meters above the sea level. Four out of six boulders were considered as transported by extremely high energy wave, such as tsunamis or storm surges. Boulders were found only on the northern coastal line which is facing the Ryukyu trench. According to the radiometric dating result, the ages of the boulders are between ca.6000 years BP and ca.200 years BP. The youngest boulder was suggested to relate with the 1771 Yaeyama earthquake event (Yoko et al., 2014). In this study, we consider the boulders were transported by tsunami waves and look for the possible sources. On the other hand, if the possible sources are geologically unreasonable, we might have a chance to rule out the tsunami mechanism and focus the future studies on the storm surges. We implemented the Tsunami Reverse Tracing Method (TRTM) to find the possible tsunami sources, and perform the scenario analysis. In the TRTM, we place reverse tracing sources offshore the study area, and solve the linear shallow water equation by COMCOT (Cornell Multi-grid Coupled Tsunami Model) tsunami model. The result shows that four possible tsunami sources are identified. They are trench-type tsunami sources and coming from Ryukyu trench, Manila trench, Philippines trench and Yap trench. Detailed results of the scenario studies will be presented in the full paper.