Using “boots-on-the-ground” geologic mapping techniques, combined with high-resolution orthoimagery and airborne LiDAR data, they reconstructed the likely sequence of events that led to the landslide’s large mobility. Some of the data they collected were highly ephemeral, being obscured by erosion and vegetation within one year of the landslide and highlighting the need to record many observations within a few months of the disaster. Some landslides stop moving close to where they began, and others are very mobile and can travel long distances, affecting not only what is located at the base of the slope, but also farther away.Ĭollins and Reid mapped the geology and structure of the Oso landslide deposit by making multiple visits to the site over the course of three years. How far a landslide moves from the site where it began can, of course, vastly amplify the consequences of slope failure. Geological Survey conducted detailed post-event field investigations and material testing of soils involved in the failure. To resolve causes for the landslide’s behavior and mobility, Brian Collins and Mark Reid of the U.S. Credit: Stephen Slaughter (Washington Geological Survey, Washington Department of Natural Resources)Īs a compelling example of a large-mobility landslide, the 22 March 2014 landslide near Oso, Washington, USA, was particularly devastating, traveling across a 1-km-plus-wide river valley, killing 43 people, destroying dozens of homes, and temporarily closing a well-traveled highway. Photo by Stephen Slaughter (Washington Geological Survey, Washington Department of Natural Resources). Collins and Reid attribute extensional hummock formation to widespread basal liquefaction of underlying alluvial sediments in the river valley. Nearly the entire landslide deposit exhibits indications of extension. Here, several geomorphological components of the landslide are visible, with a hummock field in the foreground transitioning upslope to larger slices of deposit separated by multiple scarps, which then transition to a fallen-tree covered, back-rotated block downdropped from the headscarp in the far field. This photo was taken the day after the catastrophic slide, before the river cut through the landslide deposit. The MaSR530 landslide near Oso, Washington, caused 43 fatalities, destroyed a neighborhood, blocked a state highway, and temporarily dammed the North Fork Stillaguamish River.
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