The landslide at Union Gap has been getting attention:
The best coverage is via a series of articles in the Yakima Herald:
The slide has been assessed and monitored by a consultancy out of Portland (they were hired by the rock quarry below the failure once the fractures appeared) as well as Washington State Department of Natural Resources, Washington State Department of Transportation and Yakima County. Pacific Seismic Network has installed seismometers.
The initial imagery and media reports did present a bit of a challenge to discern the direction of slide movement, but there is a general agreement that the slide is moving towards the south, towards the large quarry that has been excavated into the slope to the south. This Picture in the Yakima Herald article provides a good perspective and shows the curvature of the head of the slide.
Based on the reported movement direction, aerial images and geology mapping, the slide may have developed within an old soil horizon between basalt lava flow layers such that it is a dip slope translational failure or the failure could be taking place along a area of fractured bedrock associated with the faulting that has taken place within the ridge. Although basalt is a rather hard rock, basalt lava flows are typically highly jointed and fractured from cooling and/or interaction with water or wet ground. A clip of the local geology map shows the layered basalt units and faulting as well as folding within the Rattlesnake Ridge on the east side of Union Gap.
Clip of the Geologic Map of the east half of the Yakima 1:100,000 Qudrangle (Schuster, 1994)
The map is a bit of a challenge to read. Rattlesnake Ridge is one of several east-west trending ridges in the Yakima Fold Belt. The ridges are generally anticlines, but the folding is tight, and faults are present on these ridges as well.
The layered basalt lava flows with old soil horizons between with subsequent tilting and faulting is a recipe for large scale bedrock landslides. Indeed, the folded and faulted basalt are sites of many very large-scale landslides.
The layering, though faint, can be seen in the image from Steven Mack's video of the slide. The layers of basalt slope towards the quarry that has been excavated into the slope. Hence, the geology as well as the measured slope movement suggests most of the slide will be directed into the quarry.
View of quarry with fractured ground above
Projected movement toward quarry
Homes are located beyond quarry and have been evacuated
An interesting twist to this slide is that the cracked slope extends across the slope break towards the west such that a portion of the failure is located above the very steep slope of the river gap.
In my mind, it is pretty easy to see that as the sliding progresses towards the quarry, the material above the gap will break apart and some material will slide, not toward the quarry but down the steep gap slope. Movement of material down this side slope poses a very distinct threat to the interstate highway at the base of the slope.