Characterising diagnostic proxies for identifying palaeotsunamis in a tropical climatic regime, Samoan Islands

The September 2009 South Pacific Tsunami (2009 SPT) in the Samoa Islands resulted in local public and national calls to improve understanding of the medium- to long-term risks of tsunamis in these islands in order to further mitigate their impacts. This research addresses some of these calls through an interdisciplinary palaeotsunami investigation. Historical data beginning in 1837 indicate that the Samoan Islands have been impacted by tsunamis from all the major tsunamigenic zones within the Pacific Rim of Fire, making it an ideal location for starting to understand tsunami frequency and distribution within this region. Furthermore, the region has an historical record of extreme tropical cyclones. The overarching concept of this study is that tsunamis, like cyclones, leave a distinct geological deposit within coastal landscapes they impact. The origin of a high-energy geological deposit, be it storm or tsunami, can be determined by using a suite of diagnostic criteria. However, the origin of a deposit can still be ambiguous, because some of the diagnostic criteria (e.g. grain size, microfossil assemblages and characteristics) can be extremely similar for both processes. Moreover, local factors can also influence the characteristics of deposits. This project aims to elucidate this enigma by establishing a suite of diagnostic criteria (e.g., stratigraphy, lithology, macro- and micro-palaeontology, geomorphology, grain size characteristics, geochemistry, anthropology, archaeology, numerical modelling) to distinguish between tsunamis and cyclones in this tropical climatic regime. Preliminary studies show that a geological record of historical /palaeotsunamis and storms/palaeostorms is preserved on the south and south east coast of Upolu, west and northeast coast of Savai´i (Independent State of Samoa), and northwest coast of Ta´u in the Manu´a Group (American Samoa). We present preliminary X-ray fluorescence spectroscopy (XRF) and geochronological results (C-14 rad- ocarbon dates) conducted on samples (sands and paleosols) collected from various sites on Upolu, Savaii, and Ta´u islands. These serve as a starting point for developing a suite of diagnostic proxies for identifying and distinguishing tsunami from storm deposits in the Samoan Islands, and establishing the geochronology of identified events. Numerical modelling of wave resonance around these islands, as well as identified palaotsunamis will form an additional proxy for interpreting the palaeotsunami data. Further, it forms a basis for starting to understand the likely sources of these events, forming a basis for refining the frequency and (likely) magnitude distributions associated with these events. Planned Pb-210, Cs-137 and C-14 dating will enable a detailed interpretation of the chronology of specific events identified in the geologic record. Furthermore, they will enable a correlation of deposits with known historical events, providing a control on distinguishing recent tsunami from storm deposits (subsequent to 1837 AD), and enabling palaeo-events to be identified. This will form a basis for identifying similar events within the geologic record in similar environmental regimes. Ultimately, this work will significantly improve understanding of the nature and risks of coastal hazards in Samoa, thereby improving local capability to mitigate their medium to long-term impacts. It will also contribute to tsunami hazard mitigation efforts within the broader SW Pacific through a strengthened tsunami database in the region.