Coastal Lake Hamana is located near the convergent tectonic boundary of the Nankai-Suruga Trough, along which the Philippine Sea slab subducts underneath the Eurasian Plate, giving rise to repeated tsunamigenic megathrust earthquakes (Mw≥8). A good understanding of the earthquake- and tsunami-triggering mechanisms in terms of rupture mode and recurrence pattern in time and space, is crucial in order to better estimate the complexity of seismic risks for the densely populated Enshu-nada coast. Based on existing historical data of paleoseismicity (last ~1300 years), the easternmost segment (Tōkai segment) of the Nankai-Suruga Trough appears to exhibit a seismic gap and is expected to rupture in the near future, causing the next “Tōkai earthquake”. Studying the sedimentary infill of Lake Hamana may help to fine-tune hazard assessment in the area of interest. Thanks to its extensive accommodation space, the Hamana lake basin is considered to be a good archive of past “big wave” events. Fieldwork (Oct.-Nov. 2014) comprised a reflection-seismic survey for imaging the lake’s stratigraphic features, based on which favourable locations for gravity coring were selected. A systematic sampling of bottom sediments from different sites enables us to evaluate vertical as well as lateral changes in depositional environment, including event deposits generated by tsunamis and tropical storms (i.e. typhoons). An important part of the study is dedicated to qualitatively distinguish sedimentary facies of storm deposits from the ones generated by tsunamis, since this is an essential step in correctly assessing future hazards. For identification of marine tsunami incursions, a set of sedimentological, geophysical, geochemical and micropaleontological analyses are applied on the core sediments in a multi-proxy approach. Radionuclide dating provides the necessary timeframe and information on prevailing sedimentation rates. Sites bearing the potential of recording complete and long event histories will be sampled with long cores.
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The east coast of Japan is prone to tsunamigenic megathrust earthquakes, as tragically demonstrated in 2011 by the Tōhoku earthquake (Mw 9.0) and tsunami. The Nankai Trough subduction zone, to the southwest of the area affected by the Tōhoku disaster and facing the densely populated and heavily industrialized southern coastline of central and west Japan, is expected to generate another megathrust earthquake and tsunami in the near future. This subduction zone is, however, segmented and appears to be characterized by a variable rupture mode, involving single- as well as multi-segment ruptures, which has immediate implications for their tsunamigenic potential, and also renders the collection of sufficiently long time records of past earthquakes and tsunami in this region fundamental for an adequate hazard and risk assessment. Over the past three decades, Japanese researchers have acquired a large amount of geological evidence of past earthquakes and tsunami, in many cases extending back in time for several thousands of years. This evidence includes uplifted marine terraces, turbidites, liquefaction features, subsided marshes and tsunami deposits in coastal lakes and lowlands. Despite these efforts, current understanding of the behaviour of the subduction zone still remains limited, due to site-specific evidence creation and preservation thresholds and issues over alternative hypotheses for proposed palaeoseismic evidence and insufficiently precise chronological control. Within the QuakeRecNankai project we are generating a long and coherent time series of megathrust earthquake and tsunami recurrences along the Nankai Trough subduction zone by integrating all existing evidence with new geological records of paleo-tsunami in the Lake Hamana region and of paleo-earthquakes from selected lakes in the Mount Fuji area. We combine extensive fieldwork in coastal plain areas and lakes, with advanced sedimentological and geochemical analyses and innovative dating techniques.
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