Abstract

On October 11, 2009, a very large translational landslide, referred to as the Nile Valley Landslide (NVL), destroyed more than 2500 ft (750 m) of State Route 410 near the community of Nile, located about 25 mi northwest of Yakima, Washington. Based on eyewitness accounts, Pacific Northwest Seismic Network records, and an intensive subsurface investigation, we infer two translational failure mechanisms: an initial shallow failure within the unconsolidated surficial deposits, followed by failure within a deep, adversely dipping, claystone interbed between two Grand Ronde Basalt flows. The underlying basalt flow contains a highly pressurized aquifer that significantly reduces the effective stress within the inferred deep failure zone and its stability. Recent inclinometer data showing slope movement within both the shallow and deep failure zones support this interpretation. The NVL is located at the base of Cleman Mountain, an anticlinal ridge that hosts the enormous and currently dormant Sanford Pasture landslide complex (SPLC). Geochemical analyses of basalts encountered in the numerous borings and outcrops within and around the NVL suggest that the basal failure zones of the NVL and SPLC are stratigraphically different, and that the NVL is likely not a reactivation of the SPLC.

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