The triggering factors of the Móafellshyrna debris slide in northern Iceland: Intense precipitation, earthquake activity and thawing of mountain permafrost.

On the 20th September 2012, a large debris slide occurred in the Móafellshyrna Mountain in the Tröllaskagi peninsula, central north Iceland. Our work describes and discusses the relative importance of the three factors that may have contributed to the failure of the slope: intense precipitation, earthquake activity and thawing of ground ice. We use data from weather stations, seismometers, witness reports and field observations to examine these factors. The slide initiated after an unusually warm and dry summer followed by a month of heavy precipitation. Furthermore, the slide occurred after three seismic episodes, whose epicentres were located ~60km NNE of Móafellshyrna Mountain. The main source of material for the slide was ice-rich colluvium perched on a topographic bench. Blocks of ice-cemented colluvium slid and then broke off the frontal part of the talus slope, and the landslide also involved a component of debris slide, which mobilized around 312,000-480,000m (as estimated from field data and aerial images of erosional morphologies). From our analysis we infer that intense precipitation and seismic activity prior to the slide are the main preparatory factors for the slide. The presence of ice-cemented blocks in the slide's deposits leads us to infer that deep thawing of ground ice was likely the final triggering factor. Ice-cemented blocks of debris have been observed in the deposits of two other recent landslides in northern Iceland, in the Torfufell Mountain and the Árnesfjall Mountain. This suggests that discontinuous mountain permafrost is degrading in Iceland, consistent with the decadal trend of increasing atmospheric temperature in Iceland. This study highlights a newly identified hazard in Iceland: landslides as a result of ground ice thaw. Knowledge of the detailed distribution of mountain permafrost in colluvium on the island is poorly constrained and should be a priority for future research in order to identify zones at risk from this hazard.