Assessing natural hazards with lake sediments: Reconstructing histories of floods, earthquakes and tsunamis

Natural hazard assessments rely on high-quality time series of catastrophic events in order to constrain their recurrence rates and intensities. In this context, lake sediments provide continuously recording, datable and sensitive geologic archives that are capable of extending instrumental and historic time series towards prehistoric epochs. Such time series allow thus detailed and unprecedented analysis of past hazard activities and their temporal variations, eventually allowing projections into the future. Hazardous events to be recorded in lake sediments are for instance i) floods, producing prominent clastic layers (Glur et al, 2013), or ii) earthquakes, inducing subaquatic mass movements, related turbidites, or in-situ sediment deformations (Strasser et al., 2013). The sensitivity and response of each lacustrine system to record such natural hazards, however, are different and lake-specific, implying that i) each record needs to be calibrated with instrumental/historic data, and that ii) lakes should be weighed by their sensitivity before compared in regional multiple lake studies. While recurrence intervals can be determined by identifying and dating event layers, reconstructing hazard intensity is more complex. Flood intensity, for instance, is rather reflected in the maximal grain size of a flood layer and not by the layer thickness. The earthquake's epicentral location, magnitude and epicentral intensity are even more challenging and rely on complex analyses within multiple records and on empiric attenuation equations of seismic shaking that are furthermore masked by site effects. Projections of future hazards need to include human-induced changes, such as catchment-wide land-use or changing climate forcings. Forward numeric models that consider the external forcings can successfully simulate the hazard in a probabilistic manner, as will be shown with a tsunami hazard analysis, that quantifies the impact of these seismically-triggered lacustrine events on the lake shore communities through simulations, providing a timely approach to use lake sediments in a quantitative fashion (Hilbe and Anselmetti, 2015). References: Glur, L., Wirth, S.B., Büntgen, U., Gilli, A., Haug, G.H., Schär, C., Beer, J., and Anselmetti, F.S., 2013, Frequent floods in the European Alps coincide with cooler periods of the past 2500 years: Nature Scientific Reports, 3, 2770, 1-5. Hilbe, M. and Anselmetti, F.S., 2015, Mass movement-induced tsunami hazard on perialpine Lake Lucerne (Switzerland): Scenarios and numerical experiments: Pure and Applied Geophysics 172, 545-568.