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

Abstract

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.

Transient Modelling of Permafrost Distribution in Iceland

“…The authors additionally discussed the baseup permafrost thawing as a probable reason for this slide, contributing among other things to lubrication of the base of the colluvium and lowering cohesion. Three-dimensional effects could also play a role, e.g., warming of bedrock at the southern side and propagation of the thermal wave to the northern side, together with warmer water input from the southern side (Saemundsson et al, 2018). The CryoGrid 2 simulations confirm the previous interpretations of the landslide at Móafellshyrna, both the long-term warming of permafrost and the permafrost degradation at the bottom.…”

“…Considering the model uncertainties, the decrease in the depth of permafrost base could be even more severe. Saemundsson et al (2018) argue that the most likely triggering factor of the slide at Móafellshyrna was thawing of the deeper permafrost, implying that the longer-term (decade-scale) ground temperature warming was responsible for this event. The authors additionally discussed the baseup permafrost thawing as a probable reason for this slide, contributing among other things to lubrication of the base of the colluvium and lowering cohesion.…”

“…This has become obvious recently through three events where ice-cemented debris was observed within landslide deposits (Saemundsson et al, 2018; Figure 1). The importance of permafrost thaw for slope processes in Iceland has therefore became a focal point of interest over the past few years, where e.g., Saemundsson et al (2018) urge that the mentioned landslides "have highlighted the need for a more detailed understanding of the distribution and condition of mountain permafrost within perched talus deposits." These previous events occurred in unsettled areas; however, a similar type of landslides may occur in the future in other regions of the country, thus increasing hazard risk for infrastructure and inhabitants.…”

“…Even if the model is not designed to model high topographic heterogeneity, the transient changes in mountain areas can be resolved. In three landslides that occurred at in Iceland, ice-cemented blocks were observed within the deposits (Saemundsson et al, 2014(Saemundsson et al, , 2018. Permafrost was still present at the time of the landslides, since the landslide deposits that were observed contained ice at the time of the slides.…”

“…Permafrost was still present at the time of the landslides, since the landslide deposits that were observed contained ice at the time of the slides. For instance, at Móafellshyrna site, blocks with pore-filling ice of size up to 12 m wide and 10 m high were found (Saemundsson et al, 2018). To test the evolution of the ground thermal regime at the landslide sites we run our model using site-specific parameters (Figure 12).…”