MOBILITY SEQUENCE OF THE 2014 SR530 (OSO) LANDSLIDE REVEALED BY STRATIGRAPHIC AND STRUCTURAL MAPPING (Invited Presentation)

The March 22, 2014 landslide near the town of Oso, Washington was remarkable due to its high mobility. Originating on a hillslope only 180 m tall, the landslide mobilized both intact and previously disturbed sequences of glacial stratigraphic units over a distance of 1.4 km, crossing the entire width of the North Fork Stillaguamish River valley and destroying an entire neighborhood in the process. The landslide killed 43 people. The sequence of sliding, including the size, timing, and material characteristics of various geomorphological elements, can greatly influence mobility. To understand the mobility of this landslide, and potentially others like it, we performed detailed stratigraphic and structural mapping of both the source and deposit areas of the landslide.

Our mapping reveals how the major elements of the landslide interacted, with a distal debris flow pushed out in front of a sliding package of previously failed landslide deposits, and with a massive section of downdropped and rotated glacial lacustrine units following just behind. Enhanced mobility of the deposits occurred almost exclusively past (south of) the former position of the North Fork Stillaguamish River, thereby indicating a demarcation point for landslide behavior that is clearly identified by a transition from massive blocks and slices to the discontinuous hummocks of a debris avalanche. Notably, nearly all deposits exhibited indications of extensional mechanics, which infers close timing of the various elements. In addition, large parts of the landslide deposit maintained a high degree of structure, retaining the relative position of originally horizontal-bedded glacial units, even 1 km from the source area. Further, our mapping identified hundreds of liquefaction features (mostly sand boils). The resulting landslide deposits created a structurally complex topography as a consequence of this sliding sequence. However, most of the deposits themselves did not fluidize, but were rafted across the valley on a liquefied base. Overall, understanding the sequencing of the landslide’s elements is imperative to being able to identify causes for the landslide’s high mobility.