OSL Dating and GPR Mapping of Palaeotsunami Inundation: A 4000-Year History of Indian Ocean Tsunamis as recorded in Sri Lanka

To evaluate and mitigate tsunami hazard, as long as possible records of inundations and dates of past events are needed. Coastal sediments deposited by tsunamis (tsunamites) can potentially provide this information. However, of the three key elements needed for reconstruction of palaeotsunamis (identification of sediments, dating and finding the inundation distance) the latter remains the most difficult. The existing methods for estimating the extent of a palaeotsunami inundation rely on extensive excavation, which is not always possible. Here, by analysing tsunamites from Sri Lanka identified using sedimentological and paleontological characteristics, we show that their internal dielectric properties differ significantly from surrounding sediments. The significant difference in the value of dielectric constant of the otherwise almost indistinguishable sediments is due to higher water content of tsunamites. The contrasts were found to be sharp and not to erode over thousands of years; they cause sizeable electromagnetic wave reflections from tsunamite sediments, which permit the use of ground-penetrating radar (GPR) to trace their extent and morphology. In this study of the 2004 Boxing Day Indian Ocean tsunami, we use GPR in two locations in Sri Lanka to trace four identified major palaeotsunami deposits for at least 400 m inland (investigation inland was constrained by inaccessible security zones). The subsurface extent of tsunamites (not available without extensive excavation) provides a good proxy for inundation. The deposits were dated using the established method of optically stimulated luminescence (OSL). This dating, partly corroborated by available historical records and independent studies, contributes to the global picture of tsunami hazard in the Indian Ocean. The proposed method of combined GPR/OSL-based reconstruction of palaeotsunami deposits enables estimates of inundation, recurrence and, therefore, tsunami hazard for any sandy coast with identifiable tsunamite deposits. The method could be also used for anchoring and synchronizing chronologies of ancient civilisations adjacent to the ocean shores.