Oso, Washington, landslide of March 22, 2014: Dynamic analysis

This paper describes and explains the spectacular mobility of the 2014 Oso landslide, which was the cause of its fatal consequences. A geomorphic interpretation of the site conditions is used to reconstruct the landslide failure mechanism. Two numerical models are used to conduct an inverse runout analysis. The models implement a newly defined rheology appropriate for liquefied soils. It is shown that this landslide occurred in two phases, characterized by different material strengths. Although the temporal sequencing of the two phases remains somewhat ambiguous, it is clear that the distal phase underwent significant undrained strength loss (liquefaction) and travelled more than 1.4 km over a nearly horizontal surface. The proximal phase underwent brittle failure, with much less strength loss than the first phase. The parent material forming the slide mass was composed of insensitive, overconsolidated glaciolacustrine silt and clay, material not traditionally recognized as liquefiable. It is hypothesized that a substantial volume of liquefaction-prone soil was formed by colluvial softening of the parent material during the process of slope development prior to 2014.