Geochemical mapping of tsunami inundation : beyond the sand

Tsunami deposits range from mud to boulders, depending upon the material available for transport by these devastating waves. As sandy beaches are most often eroded, many of the reported tsunami deposits are dominated by sand, and are identified in the geological record as an anomalous sand layer within a soil or peat profile, a bit like a layer cake. However, recent posttsunami surveys have shown that the sand layer may extend up to only 60‐70% of the tsunami inundation limit, and is thus not representative of the magnitude of the event. This invariably leads to underestimates of the size of the tsunami and the generating event. As shown by the 2011 Tohoku‐oki tsunami in Japan, this can have devastating outcomes. While the inundation limit can be mapped immediately after the event using the extent of debris, salt‐burned vegetation and water marks, this evidence is often lost over time and is therefore absent in the geological record. Here we report on two studies, where the tsunami inundation limit was mapped using geochemical methods. Six months after the 2010 Maulem Tsunami in Chile, the limit of inundation was identified based on chemical and microfossil data. In Japan, geochemical markers allowed the precise limit of inundation to be identified in the absence of any sedimentological evidence. Follow‐up studies also revealed that the marine geochemical signature was preferentially preserved in the mud component of the tsunami deposit which is found inland beyond the limit of sand deposition. As no marine microfossils were recovered in this area of Japan, geochemical mapping is proving to be the only means by which tsunami inundation maps can be redrawn allowing tsunami mitigation measures to be re‐assessed