The preservation potential of onshore tsunami sediments - implications for hazard assessment (Invited)

Over the last decades, numerous post-tsunami surveys documented the characteristics of sedimentary and erosional features caused by onshore tsunami inundation. Even though unique sedimentological characteristics have not been recognized, the available data help to identify ancient tsunami deposits and to extend the tsunami record of a given coastal area back in time. Consequently recurrence intervals of tsunami and related hazards can be better constrained. However, the style of preservation and the extent of post-depositional and diagenetic changes of such deposits have not yet been thoroughly assessed. Here we present results of a survey of Holocene siliciclastic onshore tsunami sediments found in Peru and Chile. We explore how coastal topography, depositional environment, climate, syn- and post-seismic tectonic movements, and the mineralogical composition influence the style and extent of preservation of tsunami deposits. We report that sediments deposited in areas affected by seasonal rainfalls, such as hurricane seasons, wet winter months, or infrequent climate phenomena like El Niño events, will be altered and potentially removed shortly after an event. The rapid burial with post-tsunami sediments helps to protect the event deposits from surface modifications. Tsunami deposits, especially of the last or the greatest wave can possess an increased preservation potential, because they will not be affected by successive waves. This holds especially for sediments deposited on flat coastal plains where the backwash is either weak or channelized. A larger distance to the shoreline places deposits out of reach of normal wave and tide action. Eolian processes can erode sediments quickly, or smoothen and infill scoured surfaces. Coarser deposits may have a higher preservation potential compared to muddy deposits because they are less likely to be reworked by run-off or wind. By contrast, muddy tsunami deposits may escape erosion by wind in some areas because of the cohesion of mud particles and hardening caused by desiccation. In some instances we observed a post-depositional decrease of the thickness of tsunami layers by either surface erosion or compaction. Such changes may cause models inverting tsunami parameters from depositional characteristics to underestimate the magnitude of historical events. Considering these factors, the tsunami record of a coastline is very likely incomplete and the vulnerability of a coastline may consequently be significantly underestimated.