| Summary: | The Sanford Pasture Landslide Complex (SPLC) is a spectacular gigantic landslide located in south-central Washington 35km northwest of Yakima. The SPLC failed through the southwestern limb of Cleman Mountain, an anticline associated with the Yakima fold and thrust belts in the Grande Ronde basalt flows on the western margin of the Columbia Plateau. The SPLC has a width of 10km, length of 4km, and area of depletion of 32sq-km, and is bordered on the southwest by the Naches River. Surficial geologic mapping indicates the SPLC is dominated by talus with isolated outcrops of basalt and sedimentary interbeds. Based on the locations of interbed outcrops, the vertical displacement of the main failure is approximately 350m. Previous researchers identified the Nile thrust underlying the SPLC and dipping to the northeast. Mapping the landslide revealed an unnamed strike-slip fault on the southeastern boundary of the SPLC. A seismic hazard analysis indicated peak ground accelerations exceeding 0.5g and 1.0g are possible from earthquakes on the unnamed fault and the Nile thrust, respectively. Rock mass strength models were developed for each geologic unit based on laboratory testing, structural data collected in the field, and semi-empirical relationships. By combining the strength models with a geologic cross section developed by previous researchers, limit equilibrium slope stability analyses were performed using the software package RocScience Slide 6.0. By evaluating multiple Nile thrust orientations and peak ground accelerations, the most likely mechanism and conditions of failure were obtained. Landslide initiation occurred near the crest of Cleman Mountain, translating through the upper portion of the Tertiary Tuffaceous Rocks of Wildcat Creek, and daylighting along the Nile thrust under a pseudo-static horizontal acceleration of 0.25g. This mechanism indicates a failed volume of up to 79cu-km. The SPLC's runout suggests the failure was very slow moving and likely coincided with the retreat of the Cordilleran ice sheet. Importantly, the proposed mechanism and acceleration do not induce slip in the presently unfailed portion of Cleman Mountain.
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