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Seismic data gives insight into Iceland’s largest-ever landslide

The Icelandic Met Office (IMO) has published seismic and monitoring data that provides more details about the failure mechanism of the landslide at Fagraskógarfjall mountain on 7 July, which has been described as Iceland’s largest ever slope failure.

It is estimated that up to 20M.m3 of material has been displaced creating a debris tongue covering an area of 1.5km2 with debris up to 30m thick in places.

Data from IMO shows that there was displacement for some time before the mass failure, which crossed the river Hítará, damming the river and causing a lake to form above the debris tongue.

The failure was picked up by seismographs placing the exact time of failure at 5.17am but local reports suggest there was a smaller failure the previous evening.

Using a digital elevation model of the landslide based on aerial photographs taken from a helicopter, GPS measurements as well as data from terrestrial laser scanner, IMO has created a short video, showing the landslide from various angles.

IMO has said that Sentinel-1 satellite radar images show displacement in the starting area before the landslide occurred according to Insar analysis by Vincent Drouin at the University of Iceland and the National Land Survey of Iceland. The mountainside in the starting area of the landslide is different from the surrounding slopes according to an interferometric analysis of the radar images. The displacement in the last days before the landslide was in the tens of millimetres. The starting area also appears to have been moving since 2015 but at a slower rate.

The IMO said that this detection of movement before the Hítardalur landslide fell indicates that large landslide may be associated with precursors that could be detected by satellite data or other measurements.

Initial findings by IMO suggest that the site may have been a deposit from an historic landslide. According to IMO, rainfall and snowmelt in recent months may have caused unusually high water pressure in cracks and fractures in the area which further weakened the starting area of the landslide.

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