11:00 AM - 11:15 AM
[HDS09-08] The trace of a giant tsunami around mangrove forest at the Miyara river, Ishigaki Island and the utilization for disaster prevention
Keywords:Tsunami-ishi, Meiwa Great Tsunami, Landslide Tsunami,, Ishigaki Island, numerical simulation
The 1771 Great Yaeyama tsunami caused massive damage and loss of life, especially in south and east area of the Ishigaki island. The tsunami washed many large or small coral boulders up on the shore. These boulders are called Tsunami-ishi and some of them are now protected as a natural monument. However, most of the boulders inland had been removed by historical human activity such as a land improvement project. Thus, there is a little evidence how much distance did the tsunami wash boulders inland. In this situation, mangrove forest at the Miyara river could be one of the important places as very low human influence to study characteristic of tsunami boulders. This study investigated tsunami boulder distribution focusing on mangrove area. From field surveys, we found a large coral boulder with a 11 m length and 4 m height. According to the mobile 3D scanner, the volume and weight is estimated to be a 113km3 and 202 ton, respectively. We also found another coral boulder with a 2m length and 2.2m3 volume at 500 meter inland along Miyara river. Radiocarbon dating indicated that these boulders were located around the time of 1771 event. From these results, we confirmed these boulders were washed by the 1771 event.We also simulated behavior of a tsunami boulder around Miyara area by the 1771 event to understand uncertainty of its behavior. We assumed several cases and one of them reproduced characteristic of the behavior described in a historical document, which one move to inland and another move to offshore. These results could be important clues to understand the tsunami source mechanism of the 1771 evet.