Assessing Tsunami Hazard from the Geologic Record

The 11 March 2011 Tohoku-Oki tsunami dramatically demonstrated the vulnerability of the world's coastlines to the impact of tsunamis. Although northeast Japan had experienced large tsunamis in the past, there was no historical precedent for the March 11 tsunami. Most areas of the world capable of receiving such catastrophic tsunamis have not experienced them during the short period of written history. Sedimentary deposits left by tsunamis are being used to extend the record of tsunamis back through time. The state of the science for tsunami deposits has now evolved to a point where false positives (e.g. misinterpreting a storm deposit as a tsunami deposit) are less frequent. Tsunami hazard assessment is beginning to incorporate the spatial distribution of tsunami deposits and the record of tsunami recurrence. A recent development in the use of tsunami deposits for tsunami risk assessment is to obtain tsunami magnitude estimates by applying sediment transport models to replicate the observed deposits. Models have focused on estimating two parameters, tsunami height and flow speed. These models are developed and tested using data sets collected from recent tsunamis (Papua New Guinea 1998, Peru 2001, Indian Ocean 2004, and Samoa 2009, and most recently, Tohoku-Oki 2011). The extent of tsunami deposits were less than the maximum inundation, but typically were within 10% on gently sloping coastal plains. However, recent field investigations on the coastal plain of Sendai, Japan after the 2011 tsunami bring into question whether the extent of tsunami deposits are a good proxy for maximum inundation distance. There, because of sediment source limitations, an easily recognizable deposit (sand thickness >0.5 cm) only extended about 2/3 of the way to the limit of inundation. This new data highlights the need to incorporate other proxies such as geochemical signatures and approaches such as sediment transport modeling in tsunami hazard assessment.