Abstract
Large tsunamis are important not only as a serious natural disaster but also a controlling factor in coastal sedimentation processes. Tsunami deposits have been studied in both geological and disaster prevention fields and play an important role in connecting these studies. Large tsunamis cause rapid and large-scale sediment transport in lacustrine, coastal, shallow-marine and deep-marine environments. They play a positive role to form and preserve sand, gravel and shell beds in the geological record. Tsunami deposits probably occupy significant part of coastal geological strata.
Tsunami deposits in the geological records help us to estimate the recurrence interval of subduction-zone earthquakes that cause tsunamis. Histories of such earthquakes inferred from tsunami deposits span 3,000 years for the Nankai and Suruga Troughs, nearly 10,000 years for the Sagami Trough and 6,500 years for the southern Kuril Trench. Tsunami deposits also aid in estimating the relative size of paleo-tsunamis. Studies in eastern Hokkaido, north Japan, revealed the repeated occurrence of unusually large tsunamis with inundation of several kilometers. These mega-tsunamis have occurred at about 500-year-intervals over the last 6,500 years along the southern Kuril Trench.
Although some problems and difficulties remain, tsunami deposits are useful for reconstructing tsunami history and evaluating risk of future tsunami events. With the development of methods for differentiating tsunami deposits from other event deposits, and evaluating the hydraulic force and inundation depths, we will be able to learn much about past tsunamis from the deposits and to make use of such information for disaster prevention. Collaboration of a broad range of scientists and engineers, such as geologists, geomorphologists, seismologists, geophysicists and tsunami engineers, would enable to solve existing problems in paleo-tsunami research.
