When we picture a landslide, we often think of a torrent of rocks, dirt, and vegetation as part of a hillside gives way. In fact, not all landslides are sudden or fast-moving. Also known as earth flows, slow-motion landslides are actually more common than fast-moving landslides.
As the name suggests, slow-motion landslides are landslides that are slow moving. Some of these types of landslides move as slowly as one meter per year. These landslides can start and stop over time and can endure for hundreds of years. One such slow-motion landslide is the Slumgullion landslide in southwestern Colorado. Slumgullion measures about 6.8-kilometer-long and is roughly 522 hectares (1,291 acres) in area. The landslide gets its name from the Middle English word slum meaning “slime” and the English word gullion meaning “cesspool”. This massive landslide first occurred seven hundred years. For the last three hundred years, a 3 .9-kilometer-long portion of the landslide is still moving slowly. The rate at which the land is sliding various from barely perceptible to a rate of 2 centimeters a day.
Mapping Slow-Motion Landslides
Scientists used to pound stakes into slow moving landslides and then come back months or years later to measure how far the stake had moved. With geospatial technologies available today, scientists can use satellite data to remotely measure ground movement of these landslides. To measure the movement of slumgullion, scientists collected data over multiple flights using Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR). UAVSAR data collects highly detailed ground movement data down to the centimeter. By mapping a region multiple times over a period of time, researchers can analysis the rate and location of ground movement with precise detail. Between 2011 and 2018, scientists were able to create a highly detailed map of Slumgullion showing the different rates of ground movement of the slow-motion landslide.