Study on paleotsunami deposits in geologic stratum

Abstract

After the Tohoku tsunami, demand increased greatly for geol-ogists 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 paleotsu-nami deposit is based on the identification of anomalous sand depos-its 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, flo-ral and faunal fossils within the deposits, synchroneity of environ-mental changes, and correlation with historical records of earth-quakes. 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-, cesium-, teph-rochronology, and optically stimulated luminescence dating methods. For radiocarbon dating, appropriate selection of materials for analy-sis is especially crucial for constraining the age of a tsunami. Plant macrofossils or fossil insects within samples from sediments immedi-ately over-or underlying a tsunami deposit will constrain its age well, whereas a bulk peat sample will not. The lateral extent of con-tinuous tsunami deposits allows estimation of a minimum area of tsunami inundation. A meaningful reconstruction of the area of in-undation, however, also requires adjustments for changes of the shoreline over the last few thousand years. In studies of paleotsuna-mi deposits, we need to be aware of the uncertainties inherent in geo-logical 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.