IDENTIFYING KEY FACTORS OF TRANSLATIONAL LANDSLIDES IN THE YAKIMA FOLD BELT

The Rattlesnake Hills translational landslide in Yakima County, Washington made national news in 2017 when it was first noted as steadily sliding toward several homes and an interstate highway. This study catalogues and systematically assesses characteristics of similar translational landslides in the Yakima area. We identified a set of factors that are most commonly associated with translational landslides in the folded Columbia River basalt lava flows (CRBs) and interbedded sediments of this region. These folded units belong to the northwest-southeast trending anticlines of the Yakima Fold belt. An improved understanding of where and why these events occur will provide a basis to identify locations susceptible to landslides in this broad geological province of central Washington.

We developed a database of 36 translational landslides west of the Yakima River Canyon, and used LiDAR and satellite imagery to map and measure physical characteristics of each landslide, such as slope angle, aspect and prior slope conditions below the failure sites. The geologic units associated with the landslides were identified in the field and from existing geologic maps. Petrographic thin sections of sandstone and volcanic ash interbeds associated with failure surfaces were evaluated for induration and grain size homogeneity. Shear strength was measured in the laboratory for the two most common failure-plain interbeds.

Each slide involves basalt lava rock sliding over loose sediment. Translational slides are more commonly associated with contacts at the Roza basalt/ Squaw Creek interbed and the Pomona basalt/ Selah interbed than any of the other six basalt/interbed couplets in the study area. An airfall tuff from southwest Idaho known as the Cougar Point Tuff Member VII caps the Selah member sediments and appears to be among the more likely sediments to induce a failure surface. The Rattlesnake Hills landslide is an example of a translational slide active at the interface between the Pomona basalt and the underlying Selah sediments. The landslides of this study show a tendency to develop along the generally shallow dipping south flanks of anticlines at angles as low as 8 degrees.