Massive rock slope failure in Central Switzerland: history, geologic-geomorphological predisposition, types and triggers, and resulting risks

The basis of this article about landslides from massive rock slope failures (MRSFs) in the northern part of the Lake Lucerne area in Central Switzerland is a newly collected inventory of 25 mass-transport deposits (MTDs) within an area of 251 km(2). Clearly dependent on the geologic-geomorphic predisposition, the pertaining processes are to be classified either as rock irregular slides originating from steep rock cliffs or as rock planar slides moving along dip slopes. All the investigated large-scale MTDs with spatial extents from 0.1 up to 14.6 km(2) demonstrate excessive runout distances, especially the deposits associated with rock planar slides. Many of the MRSF are composite, by rock fragmentation into rock avalanches. Some of the mass transport processes involve cascades of events, including debris slides and debris flows. One MRSF was triggered by an earthquake and is thus considered a spontaneous event, whereas another two might be related to a cluster of strong seismic activity. The remaining MTDs cannot be assigned to identifiable triggers, except the Goldau rock slide in the aftermath of a period of heavy rainfall. The MTD inventory of the investigation area is assumed to be incomplete and suggests a low probability of large to very large events; it indicates high hazard ratings, should an event occur. A biasing towards larger and younger events is demonstrated. The M-F plot derived from the inventory, with magnitudes expressed as affected areas, shows two separate populations, distinguished by the occurrence (or absence) of rock avalanches. Using a statistical approach, the hazard associated with MRSF within the study area was rated as high. The societal risks emerging from the examined MRSF would nowadays be classified as unacceptable, especially if the massive spatial impact of rock avalanches is considered.