Sedimentological post-tsunami survey of the deposits formed by the September 16, 2015 Illapel tsunami in central Chile

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On September 16, 2015 a Mw 8.3 earthquake off the coast of Illapel, central Chile, triggered a tsunami reaching wave heights exceeding 4.5 m at the town of Coquimbo. Eye witness reports indicate that the tsunami deposited sediments at several coastal sites including the coast near the village of Tongoy south of Coquimbo. Post-tsunami surveys conducted in the wake of a larger tsunami depositing sediment onshore are an indispensable tool in the study of tsunami deposits. They are instrumental not only in encompassing the large and still not completely explored variety of sediments deposited by tsunamis but also for gauging the difference between modern deposits and those contained in the paleotsunami record. Recent studies indicate that sediment thicknesses and extent, sedimentary structures, grain-size distributions and petrographical composition maybe significantly altered already in the first few years after deposition. These differences are largely due to post-depositional modification including early diagenesis. Presently there are only two studies exploring these changes by following the deposits fate in the years following a tsunami event. In the proposed study we will map the spatial distribution of the deposits laid down by the September 16, 2015, Illapel tsunami. Distance from the coast to the line of maximum inflow as well as the relationship between this distance and the landward end of sediment deposition are of particular interest. If present we will record markers of the inundation height. We will record the sedimentological features of the deposits by digging pits and trenches. We will sample the deposits in grids covering the lateral extent of the sediments to map the spatial arrangement of grain-size distributions. If more than one tsunami layer is present we will sample each layer, or, if possible, each lamina. We will also sample for later OSL studies to explore the reasons for inhomogenous bleaching of datable grains in tsunami deposits observed in several studies. Where possible we will interview eye witnesses to obtain information on inundation heights and other observations.