The landslide is fairly easy to see on Figure 1. The first signs of problems came in the weeks leading up to the collapse, when the coal tunnels started to show signs of movement. The final collapse occurred at 4:10 am when a block of about 650 m in height, 900 m in width and with a maximum thickness of about 150 m thick broke off the hillside and thundered into the valley below. The failure, which had an estimated volume of 30 million cubic metres, apparently took about 100 seconds, killing 76 people. Most of the victims were killed in their beds and their bodies could not be recovered as they were buried to a depth of about 30 m. Figure 2, from Natural Resources Canada, captures the scale of the event quite well. The landslide deposit extended over about 3 sq km, blocking the river and flooding about 2 km of the railway line. Seventeen coal miners were buried in the coal mine by the landslide but were able to dig themselves out over a 14 hour period.
The Frank landslide remains of great interest for a number of reasons:
- There has been considerable controversy over the cause of the landslide. Initially the blame was pinned squarely on the mining activities, but more recently it has been increasingly accepted that in fact the geological structure, which was unfavourable for stability, was probably the primary cause. The mining probably did not help, and other factors such as rainfall may have also had a role.
- There has also been a great deal of speculation over the the rate at which the landslide debris spread itself over such a large area. This very rapid runout behaviour is a somewhat enigmatic process in many large rockslides. Theories have abounded over the role of air entrainment, reduced basal friction, etc. The jury is still out - this remains one of the great questions in landslide science.
- Parts of the mountain are probably still potentially unstable. In 1911 part of the town was relocated over fears about the stability of the slope. Today, real time monitoring is undertaken by the Alberta Geological Survey using a wide range of sensors. The aim is not to prevent a failure event but to provide a warning that it might occur. This is fine for a progressive failure or for one triggered by rainfall, but it offers little if there should be a large earthquake. Nonetheless, this is important work that is undertaken to a world leading standard. The work is described exceptionally well here