Given the geology of the area, Aspect’s Principal Engineering Geologist Dave McCormack summarizes the likely forces behind the slide: “Geologic studies have shown that slides of this nature are fairly common on the flanks of the numerous anticlinal ridges in central Washington. While most are ancient and have not moved during recent history, there are examples, including the Nile Valley landslide of 2009, where old slides have reactivated, or new slides began. These slides occur where basalt flows are interbedded with sedimentary strata. While the basalt strata may be relatively strong, the sedimentary interbeds are often weathered and weak.
When the gravitational driving forces acting on these dipping strata exceed the resisting strength of the weathered sedimentary strata, they begin to slide. There are multiple factors in the delicate balance of gravitational forces versus resisting strength, including the properties of the rock, degree of weathering, groundwater levels, the geometry of the slope, etc.
Triggers for activation of landslides can include increases in groundwater level, strong earthquake shaking, or changes in slope geometry from natural causes like river migration, or human-caused grading. Because of the multiple factors involved, teasing out the exact triggers of a slope failure can be challenging, and the expected type of failure (fast debris runout, slow creeping failure, rockfalls, etc.) difficult to predict.”
Geologic insight will continue to be relied on as the slide keeps moving.