The 2014 Oso landslide in Snohomish County, which killed 43 people and buried an entire community, stunned geologists with its reach, speed and ferocity. That slide struck on a rain-soaked hill composed of glacial sediments that had slid repeatedly in the past.
Rattlesnake Ridge is mostly made up of sturdier basalt, and water doesn't seem to be a factor in the evolving slide. But big landslides can travel fast and far even without water, as the Bingham Mine collapse showed, Moore said.
"We have seen many past cases where large, dry rock slides fail catastrophically," he said.
State officials believe that's unlikely at Rattlesnake Ridge. So far, the slide has been creeping slowly down a slope of about 15 to 20 degrees. It appears to be slipping on a weak layer of sedimentary rock.
The most likely outcome is that the slide will continue to move slowly, with much of the 4 million cubic yards of rock and soil spilling into the quarry pit at the base of the hill, according to Cornforth Consulting, the geotechnical engineering firm hired by quarry operator Columbia Asphalt and Ready-Mix.
Moore describes that as an "optimistic" scenario and says it's important to plan for the possibility of a catastrophic failure.
"If the thing fails all at once, it's well within the range of possibility that it could hit the highway," he said. If the slide slumps in stages, then the potential for damage is likely to be lower.
An independent geologist who raised concerns about the west side of the slope collapsing and spilling across the highway and river said he's less worried now that continued monitoring has shown the 200-foot-deep cracks on the top of the hill are not extending to the north.
"I'm reassured by that, but I still believe a lot of things could happen that we don't expect or haven't predicted," said Bruce Bjornstad.
Washington Department of Natural Resources (DNR) officials say they are planning for a range of scenarios, including a worst-case situation similar to the one Bjornstad outlined. Yakima County and the town of Union Gap issued emergency declarations, and local officials are gearing up to respond if the slide spills into the Yakima River, said Yakima Valley Office of Emergency Management Director Jeff Emmons.