On Monday 11 October 2015 Colin Stark and Göran Ekström of the Lamont-Doherty Earth Observatory and Department of Earth and Environmental Sciences at Columbia University detected a landslide of about 45 000 tons in the region of Mount Steele in the southwestern Yukon Territory, Canada, using a network of seismometers belonging to the Global Seismographic Network. However this only enabled them to pinpoint the event to within an area of about 500 km², and due to the remoteness of the location they were highly unlikely to find any witnesses to the event. They therefore turned to satellite data to find the location of the event, and were able to locate it in an image of the area taken by the Operational Land Imager on Landsat 8 on 13 October 2015, in an image showing extensive darker material on the flank of the mountain and on the surface of the nearby Steele Glacier.
Landslide debris on the surface of Mount Steele, southwestern Yukon Territory. NASA/Earth Observatory.
Without access to the site determining the cause of the event is not likely to be possible, and the cause of very large landslides and avalanches is often very difficult to determine even with close inspection. Such events are generally caused by the sudden release of a source of tectonic stress that has built up over a long period of time, with the actual trigger often being a very small event, the traces of which are typically wiped out by a large landslide.
Image of the same location taken on 6 October 2015, demonstrating that the surface debris is a new feature. NASA/Earth Observatory.
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