There are many tsunami boulders in the islands of the Ryukyu Archipelago. Among them, tsunami boulders in Okinawa (Kasakanja), Miyako (Higashi-Hennasaki), Shimojiri (Obiwa) and Ishigaki (Ohama) Islands are famous. These tsunami boulders are definitely due to the past mega-tsunamis. An experimental study has been undertaken on the mechanism of tsunami boulders by using a specially developed tsunami generation device. Various single blocks having different densities and cliff models with different geometry and materials are prepared and tested. The experiments shown that tsunami boulders found on terraces are likely to be associated with cliffs having a toe erosion and failing like passive flexural toppling failure. The authors describe these experimental studies and discuss their implications in the tsunami boulder formation as well as the estimation of mega-earthquakes.
Tsunami induced boulders are found worldwide and the mechanism involved in the formation of such tsunami boulders is still controversial. There is no doubt that such huge tsunami boulders can only be caused by huge tsunamis resulting from mega-earthquakes. There are many tsunami boulders in the islands of the Ryukyu Archipelago. Among them, tsunami boulders in Okinawa (Kasakanja), Miyako (Higashi-Hennasaki), Shimojiri (Obiwa) and Ishigaki (Ohama) Islands are famous and they are definitely due to the past mega-tsunamis (Aydan and Tokashiki, 2018). The authors have initiated an investigation program to quantify the geometry and position of tsunami boulders and topographical conditions in their close vicinity including the erosion state along the shoreline. However, the formation of tsunami boulders and associated conditions are quite important for both understanding the past mega-earthquakes and future earthquakes in relation to the preparation of disaster mitigation plans.
The authors have developed an experimental device to investigate the formation mechanism of tsunami boulders. The device is capable of inducing both tsunami waves due to thrust faulting and normal faulting. The model cliffs with toe erosion were prepared. The cliffs were either continuum type or blocky type. The continuum type cliffs were prepared using plaster while the blocky cliffs were made of Ryukyu limestone blocks. The experiments were carried under different velocities of faulting and the geometry of the model cliffs. The experiments shown that tsunami boulders found on terraces are likely to be associated with cliffs having a toe erosion and failing like passive flexural toppling failure. The authors will describe the findings from these experimental studies and discuss their implications in the tsunami boulder formation as well as the estimation of mega-earthquakes.
