Current progress and perspectives of the research on tsunami boulders

Abstract

後藤和久 1＊Current progress and perspectives of the research on tsunami boulders Kazuhisa Goto 1＊After the 11 March 2011 Tohoku-oki tsunami, tsunami geology has been attracted attention from the public in the World. Tsunami boulders are also an important marker of the low-frequency large tsunami and hence they are useful for the tsunami risk assessment. This paper focuses on the current progress and future perspective of the tsunami boulder research based on the recent studies. After 2009, reports of the tsunami or storm wave boulders have increased and the understanding of their natures are well improved. Moreover, number of theoretical and numerical analyses for the identification and transport process of boulders have been performed. However, there have been yet no universal criteria to identify the tsunami boulders sedimentologically and theoretically. The reports of the Japanese tsunami boulders are still very limited except for the studies at the Ryukyu Islands, probably because the field survey intended to find the coastal boulder deposits has not been performed along the coast in other area. Tsunami boulders are useful evidence of the historical and prehistoric tsunamis at the places where rocky platform or coral reefs exist at the shore. Further researches to establish the identification criteria of tsunami boulders and the methodology for the risk assessment are indeed required for future risk assessment around the coast of Japan.

Numerical identification of tsunami boulders and estimation of local tsunami size at Ibaruma reef of Ishigaki Island, Japan

“…Our results suggest that the maximum tsunami wave height possible at the shoreline of the Ibaruma reef was 19.8–21.9 m and that the run‐up height reached 28.2–32.7 m elevation. In the historical document “ Nariyuki‐syo ,” the maximum tsunami run‐up height at Ibaruma is described as 32.7 m, although some measurement error can be inferred for this value (Goto ). Goto et al (2012) conducted field observations based on the historical description and local tradition and estimated that the maximum run‐up height at Ibaruma ranges between 9.4 and 29.3 m. The run‐up height calculated in this study (28.2–32.7 m) is comparable to the historical records.…”

“…Goto et al ). The storm wave period is known to be a few tens of seconds, although it is several minutes to hours in the case of tsunami (Goto ). Therefore, the duration of the tsunami and storm wave forces acting on the boulders differ; consequently, it can be expected that boulders deposited by a tsunami can reach more distant inland areas than those reached by storm waves.…”