Browsing by Author "Prasetya, G"

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    Assessing tsunami signatures in the geologic record for long-term risk evaluation, Samoan Islands
    (American Geophysical Union, 2011-12-05) Williams, SP; Goff, JR; Davies, TR; Cheung, KF; Yamazaki, Y; Chagué-Goff, C; Prasetya, G; Wilson, T
    Recent tsunamis worldwide have prompted significant efforts amongst scientific and disaster management authorities to enhance understanding of these processes, and further mitigate their immediate to long-term impacts. The tsunami of 29 September, 2009, which impacted the Samoan Islands, prompted local demand to improve long-term understanding of the risk these processes have on local communities and environment in general. This research aims to address some of this demand through an inter-disciplinary investigation of tsunami (and cyclone) deposits in the Samoan geologic record. The use of tsunami deposit investigations has become a key component in tsunami hazard assessments globally, as they enable long-term understanding of tsunami risk to communities and property, including loss of life. In the Samoan Islands, historical records of tsunamis are meagre and only date back to 1837 AD. This project enables tsunami records to be extended into Samoan pre-history, thereby forming an information basis for long-term risk mitigation in these islands. It also provides an avenue for establishing a suite of multi-proxy criteria for identifying and distinguishing tsunami and cyclone events specific to Samoa. Further, it provides the opportunity for starting to understand likely source and wave characteristics associated with identified tsunamis. In this paper, we provide a discussion on the applications and implications of results yielded thus far in the project to understanding long-term tsunami risk in the Samoan Islands. Current interpretations of empirical stratigraphies, semi-quantitative X-ray fluorescence spectroscopy analysis, geochronology analysis, and preliminary computational modelling of tsunami resonance are discussed for various investigated sites on Savai'i and Upolu (Independent State of Samoa), and Ta'u (Manu'a Group, American Samoa). We show that a long-term geologic record of tsunamis exist on these islands. Further, we discuss the challenges encountered in detailing this record, as well as the challenges that remain in forming definitive interpretations. Ultimately, results from this project will contribute to enhancing our understanding of tsunami processes and their long-term risk in the Samoan Islands, and will contribute to forming a solid foundation for future studies to build on.
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    Characterising diagnostic proxies for identifying palaeotsunamis in a tropical climatic regime, Samoan Islands
    (IEEE, 2011-09-19) Williams, S; Prasetya, G; Chagué-Goff, C; Goff, JR; Cheung, KF; Davis, T; Wilson, T
    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 radiocarbon 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. © Copyright 2020 IEEE