Lacustrine sedimentary evidence of cascading mountain hazards at the inner-Alpine Lake Altaussee (Eastern Alps, Austria) during the Late Holocene

Inner-Alpine or intramountainous lake systems can be formed by glacial, tectonic or landslide processes and typically present complex sediment dynamics that can drastically change over time and can be driven by various natural hazard processes. Their sedimentary archives can therefore provide insights in the past magnitude and frequency of local mountain hazards, which is key for reliable hazard assessment and understanding of mountain landscape evolution. Here we present morphological, seismic-reflection and sedimentary data from Lake Altaussee, a groundwater-fed lake surrounded by steep topography in the Eastern Alps (Austria). The slow organic-rich sedimentation is interrupted by one phase of inflow (by the Augstbach creek), resulting in clastic sediment input between ~1110–1346 cal yr CE. This inflow phase was terminated by an artificial diversion of the creek as documented in historical reports. The combination of large blocks (max. 70 m in diameter), a mass-flow deposit and megaturbidite deposited on deformed basin floor sediments points to multiple terrestrial mass movements with a total volume of ~430,000 m 3 that propagated into the lake and which occurred 755–991 cal yr CE. A 3D hydrodynamic model suggests that the potential mass movement at the northern shore induced an impact wave that inundated the western shoreline with flow depth of up to 9 m and reaching up to ca. 210 m inland. Calculated speed of the backflow indicates that coarse shore sediments can be transported into the lake, which can explain the abundance of gravel/coarse sand at the base of the megaturbidite at the periphery of the basin. Moreover, such impact wave would lead to a standing lake water oscillation (seiching), which is further attested by regular lamination and oscillating grain-size parameters at the base of the main silty unit of the megaturbidite. A persistent change in sediment dynamics around a main subaqueous karst spring pit occurred at 165 cal yr BCE–222 cal yr CE when repeated sediment expulsions from the spring pit started to build up a crater rim. This onset of sediment expulsions is directly overlying a soft-sediment deformation structure (micro-faults). A good age correlation of the two major Altaussee events (i: multiple mass movements, ii: onset of sediment expulsions from spring pit) to large mass-transport deposits in the sedimentary record of the nearby Lake Hallstatt let us hypothesize that two large earthquakes took place in the Late Holocene that led to a multitude of morphological and sedimentary responses in the Upper Traun region. Our findings provide the first indication of an impact wave on a natural lake in the Eastern Alps, highlighting the potential for hazard cascades that remain undocumented in historical records, underscoring the need for reassessing natural hazard risks in alpine lake environments. • Medieval mass movement induced impact wave and lake seiching • 3D hydrodynamic modeling of impact wave dynamics • Repeated sediment expulsion from a major lacustrine karst spring