Estimating the inundation limits of small historical tsunamis

Abstract

There has been considerable progress in tsunami research in recent years, yet most work has been focussed on identifying and understanding the evidence of large events. This talk discusses the evidence for small historical tsunamis in Lyttelton Harbour, New Zealand. The study area has been inundated by numerous relatively small historical tsunamis without depositing any notable sedimentary evidence. However, excavations of shallow soil profiles revealed discontinuous layers of small grey mud clasts, most likely transported from the nearby harbour, at various depths across the study area. The origin of these mud clast layers was investigated using a multi-proxy approach comprising sedimentological, geochemical and diatom analyses complemented by radiometric dating and historical data. Subtle variations consistent with inclusions of marine mud, such as a decrease in organic content and magnetic susceptibility and increases in geochemical markers (e.g. potassium, calcium) were found in the sedimentary profile. Variations in diatom assemblages suggesting marine influence were also recorded at similar depths, aligning with layers of mud clasts. Using 137Cs dating and historical data, these deposits were attributed to the 1960 Chile and possibly 1964 Alaska tsunamis. Sedimentary evidence for the 2010 Chile tsunami was not found at the study site, but geochemical analysis of surface samples revealed marked changes in calcium, chlorine, strontium and titanium concentrations, indicative of a change from terrestrial to marine influence. This was used to identify the landward extent of inundation. Ultimately, this study shows that a broad multi-proxy analysis can distinguish even the subtle signatures of an inconspicuous deposit laid down by a small tsunami. Show more