26 July 2018

Detail on the Fagraskógarfjall landslide from the Icelandic Met Office

Posted by Dave Petley

Detail on the Fagraskógarfjall landslide from the Icelandic Met Office

The Icelandic Met Office has published two articles online about the Fagraskógarfjall landslide in Iceland.  These provide a lot more detail about the landslide and its timing, and about possible triggering mechanisms.  The first, published on 10th July, indicates that the slide happened at 05:17 am on 7th July, as measured from seismic data.  I noted at the time that seismic data might provide insight into the slide; I hope that analysis is possible on the dataset.  Interestingly, the report indicates that a smaller landslide may have been noted by a local hunter at about 23:30 the previous evening; this may have been the event that destabilised the main part of the slope.

This report also notes that the debris is up to 20 m thick, and the image below gives a better perspective on spreading mechanism of the slide:-

Fagraskógarfjall landslide

The front of the debris of the Fagraskógarfjall landslide in Iceland. Image by Tómas Jóhannesson via the Icelandic Met Office


The way that the debris has bulldozed the turf is quite interesting, suggesting that the landslide debris may have been sliding rather than flowing, at least in the latter stages of movement.  The article also notes that the run-out angle is 12-13°, which is quite a high level of mobility for a landslide of this volume.  This probably implies quite high velocity.

The second article, published this week, provides detail from a digital elevation model (DEM) of the landslide.  This can be viewed in the following video:-


This gives an initial volume estimate of about 7 million m³ (source volume), and about 10 million m³ for the debris volume (this allows for entrainment of debris along the route and the bulking of the sediment during motion).  Importantly, InSAR analysis of the site by Vincent Drouin at the University of Iceland and the National Land Survey of Iceland suggests that precursory deformation of the landslide mass could be detected from 2015 onwards.  This is not unexpected, but it does provide the potential for detecting these events prior to failure.  This is an exciting area, and one that needs further development.

Finally, the article notes that it is unlikely that this event was associated with permafrost degradation given the elevation of the slope.  As I noted previously, Iceland has had exceptionally wet weather this summer (this is the flip side of the drought in northern Europe).  It is likely that the heavy rainfall accelerated the creep of this large rock mass through its failure point.

Thanks to Harpa Grímsdóttir for highlighting these articles.