Neotectonic activity in Lake Sils (Engadine, Switzerland) as trigger of tsunamigenic delta collapses?

Tsunamigenic delta collapses in lacustrine environments are still poorly understood phenomena in terms of their recurrence rate, driving mechanism and hazard potential. A partial collapse of the Isola Delta in Lake Sils (Engadine, Switzerland) with an estimated depositional volume of at least 6.5 million m³ is radiocarbon-dated to 548-797 cal CE and may represent a typical tsunamigenic delta collapse in the Alpine environment. Recent studies propose that this basin-wide tsunami with a run-up height of 2–3 m and an inundation distance of 200 m at the lakeshore highlights the importance to better understand these processes and the associated hazards. The collapse was likely triggered by a strong regional earthquake responsible for several simultaneously triggered mass movements in nearby Lake Silvaplana and Lake Como. Increasingly available datasets from Lake Sils (short cores, high-resolution seismic reflection data, numerical tsunami simulations) are now complemented by multibeam swath bathymetry, providing a high-resolution (1 m grid) model of the lake floor that offers new insights into the failed slope masses, and post-failure basin morphology.

Lake Sils is located in the Upper Engadine in southeastern Switzerland at ~1800 m above sea level and has four major sub-basins (Maloja, Central, Sils & Lagrev Basins). A major tectonic element is the Engadine Fault Line (EFL), an oblique sinistral strike-slip fault that runs along the entire Upper Engadine valley. Its influence on the subaqueous morphology of the Maloja Basin in Lake Sils is expressed in the form of several localized troughs and ridges. It is suspected that the fault also cross-cuts the Isola Delta, possibly causing renewed delta failures in case of reactivation. In fact, recent studies have indicated that there is strong evidence for Quaternary left-lateral transcurrent faulting of the EFL, e.g. offsetting a river gully in the Forno Valley close to Lake Sils.

New bathymetric data from Lake Sils and their morphologic interpretations indicate subaquaeous slope failures, the extent of the Isola Delta collapse, and several trough-ridge features within the southwestern Maloja Basin. The latter are possibly indicative of ongoing faulting in the region since such features strongly suggest rhomboidal pull-apart basins within the Maloja Basin along the EFL. In general, such localized troughs within a lacustrine system are expected to level-out over time due to higher sedimentation rates in preferentially deeper regions of the lake. This study thus highlights the use of high-resolution bathymetric data in identifying the combined effects of deep-seated tectonic zones with shallow lake-floor processes, providing new insights into lacustrine hazard studies.