Abstract
After the 2011 Tohoku tsunami, demand increased greatly for geologists with expertise in the geology of tsunami deposits, who could study prehistoric tsunami deposits. This paper reviews, for those new to tsunami geology, how geologists have studied tsunami deposits during the last two decades. Preliminary identification of a paleotsunami deposit is based on the identification of anomalous sand deposits in peat or mud that was deposited in a low-energy environment. Anomalous deposits can then be evaluated using several criteria, such as lateral extent of the deposit, changes in thickness and grain size, sedimentary structures, single or multiple vertical grading, floral and faunal fossils within the deposits, synchroneity of environmental changes, and correlation with historical records of earthquakes. In this process, a multi-proxy approach is used to discount non-tsunami processes such as floods and storms. Identified tsunami deposits are dated using radiocarbon, lead-210, cesium-137, tephrochronology, and optically stimulated luminescence dating methods. For radiocarbon dating, appropriate selection of materials for analysis is especially crucial for constraining the age of a tsunami. Plant macrofossils or fossil insects within samples from sediments immediately over- or underlying a tsunami deposit will constrain its age well, whereas a bulk peat sample will not. The lateral extent of continuous tsunami deposits allows estimation of a minimum area of tsunami inundation. A meaningful reconstruction of the area of inundation, however, also requires adjustments for changes of the shoreline over the last few thousand years. In studies of paleotsunami deposits, we need to be aware of the uncertainties inherent in geological records. For example, geologic dating at a particular site rarely has the precision to distinguish between a single giant tsunami and a series of small ones.
