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We report the first-ever basin-wide geomorphological characterization of a high-resolution bathymetry map in a steeply incised valley-occupying intra-mountainous lake in the Eastern Alps. The resulting new geomorphological map of Lake Hallstatt is then combined with high-resolution reflection seismic and sedimentary core analyses to document, characterize and date recent (<200 years) subaquatic landslides. The area is located in the UNESCO World Heritage Cultural Landscape of the Hallstatt–Dachstein area, famous for its well-documented medieval and prehistoric human settlement history. This allows us to calibrate observed mass-transport deposits (MTDs) in the high-resolution sedimentary archive (sedimentation rates c. 0.5 cm a−1) of this deep lake dominated by clastic sedimentation.

The hydro-acoustic data document a multitude of different MTDs linked to rock falls, subaqueous slope failures and shore collapses. Sediment cores document laminated background sediments intercalated with distinct event deposits that can be linked to historically documented major flood events and moderately strong earthquakes. Our study suggests that the larger MTDs result from earthquake-triggered subaquatic delta slope instabilities and that the deeper subsurface provides evidence of even larger mass-movement processes, yet to be validated by longer cores. Thus, Lake Hallstatt is a potential natural laboratory for studying causes and consequences of subaquatic landslides in steeply incised intra-mountainous lakes and comparable fjord settings.

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