Sedimentological signatures of lacustrine tsunamis

Lake tsunamis are a natural hazard with high magnitudes and low recurrence rates. Because of their infrequent occurrence in space and time, little is known about the associated hazard and the risk to the vulnerable coastal areas that are often heavily populated. However, historical reports and recent scientific achievements show that certain Swiss lakes may have been repeatedly affected by tsunamis during the last 15’000 years. This makes Switzerland an ideal case-study area to conduct fundamental research in the field of tsunamis and to gain new knowledge applicable to other lacustrine areas, as well as to the marine environment. Lacustrine tsunamis can be generated by subaqueous and subaerial mass movements, volcanic eruptions, fault displacements within large lakes, and air-pressure disturbances. Mass movements, triggered by strong earthquakes, are considered one of the main causes. However, spontaneous delta collapses and subaerial impact, often related to artificial rock-mining activities, also have induced tsunami events on Swiss lake basins. The geological record of mass-movement deposits in the seismically imaged stratigraphy of deep lake basins provides evidence for the occurrence of prehistoric lake tsunamis. However, because the dimensions (e.g., spatial distribution, volume, etc.) and dynamics (e.g., single-stage or multi-stage failures, initial acceleration, velocity, cohesion etc.) of mass movements strongly influence tsunami generation, which is difficult to estimate, conclusive evidence for prehistoric lake tsunamis is lacking. Therefore, the geological record in the on- and offshore coastal environment may provide further evidence on past lacustrine tsunami events. These sedimentological signatures are examined in this thesis. Recent marine (2018 Sulawesi earthquake and tsunami, Indonesia) and lacustrine (2007 landslide-generated tsunami in Chehalis Lake, Canada) tsunami events indicate that large amounts of sediment are mobilized during tsunami inundation and transported both landward and seaward with backwash currents. To date, a wide variety of sedimentological bed forms and characteristic depositional signatures have been described from various coastal environments. Nevertheless, hardly any tsunami deposits have been described from the on- and near-offshore of lakes, and none were investigated in and around Swiss lakes until today. Yet, historical tsunami hazard descriptions from Swiss lakes provide documentation of inundation distances and run-up, and in specific cases, a limited description of the associated deposits left behind. These descriptions were used to characterize and locate tsunami deposits from lacustrine environments that were compared with descriptions of their marine counterparts. In summary, a combination of geological field- and laboratory analysis, numerical tsunami propagation simulation, and historical documents is used to identify and characterize lacustrine tsunami deposits in several Swiss lakes. At field sites where positive evidence for tsunami deposits was observed, sedimentological characteristics are used to finally validate the robustness of numerical tsunami propagation simulations applied to mass movements observed from bathymetric and seismic reflection data in the lake. Based on numerical tsunami simulation and a suite of sediment cores from the coastal on- and offshore environment of Lake Sils, we were able to reconstruct a prehistoric delta collapse-generated tsunami. An offshore tsunami deposit of the historic 1601 Lake Lucerne event was observed from sediment core transect in a coastal depression in the Lucerne Bay. Another sediment core recovered from the coastal offshore environment contains sedimentary signatures that are likely associated with bottom currents from prehistoric tsunami events at ~2200 and ~5400 Before Present at Lake Lucerne. The observed sedimentological signatures of lake tsunamis were investigated using multi-proxy analysis including whole-core scans (density, magnetic susceptibility, and CT), as well as micro-CT scanning of sediment U-channels, radiocarbon dating, elemental analysis, and grain-size analysis. The identified sedimentological signatures consist of sharp lower and upper sedimentary contacts, successions of single and multiple normal graded sand, massive sand beds, and a characteristic fine-grained top. Based on radiocarbon dating, these signatures can be associated with large mass-movement deposits observed in sediment cores and seismic-reflection data of the deep lake basin.

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