So far, 19 people have sought refuge at a Red Cross shelter that was opened to assist evacuees. That leaves only about 10 to 15 holdouts among the more than 50 residents who live below the 1,758-foot ridge.

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They’re staying put.

That’s the challenge the Yakima Valley Office of Emergency Management is facing while trying to persuade people who live below Rattlesnake Ridge — where a 250-foot-deep crack in the hillside has continued to grow — to evacuate.

Horace Ward, the office’s senior emergency planner, said one of the main reasons some residents are refusing to evacuate is because they say they have no place to go. Ward said he’s hopeful temporary accommodation in hotels and motels will be arranged soon – possibly as early as Thursday night.

“In this state, you cannot force anyone to leave their homes so we are trying to convince them and give them options,” Ward said.

So far, he said, 19 people have sought refuge at a Red Cross shelter that was opened to assist evacuees. That leaves only about 10 to 15 holdouts among the more than 50 residents who live below the 1,758-foot ridge. The residents were advised to leave their homes last month.

Cracks opened up on a ridge above Interstate 82 near Yakima, and has forced some evacuations. (Courtesy Steven Mack)

On Tuesday, scientists from the Pacific Northwest Seismic Laboratory installed three seismometers in the hillside, which might be able to provide at least some warning before a catastrophic slope failure, said Steve Malone, emeritus professor at the University of Washington.

State and county officials are working to address the seemingly inevitable slide of an estimated 4 million cubic yards of soil that threatens the homes at its base as well as Interstate 82 along the Yakima River.

In the 2014 mudslide in Oso, at least 10 million cubic yards of earth cascaded down across the Stillaguamish’s north fork, burying the Steelhead Haven neighborhood, killing 43 people and destroying 36 homes.

Authorities began monitoring the fissure near Yakima — uphill from a rock quarry about 142 miles southeast of Seattle — in October, but they grew more concerned last month when geologists saw increased movement, Ward said.

Officials have said the hillside above the quarry is moving an average of 1.4 feet per week.

On Wednesday, the Washington State Department of Transportation (WSDOT) began setting up a 600-foot barrier made of shipping containers, each filled with 6 tons of concrete. The barrier on Thorp Road, also at the base of the hill and closed, is intended to protect I-82 from rockfall.

WSDOT spokeswoman Summer Derry told the Yakima Herald-Republic that the barrier would not be able to protect the highway from a major landslide, however.

And Ward said that WSDOT had mapped out a “pretty thorough detour route” that will be implemented immediately if I-82 is found to be unsafe.

A geotechnical engineer hired to monitor the crack by the operators of the Anderson Quarry, which is located at one end of the fissure, has said the movement of land has sped up since October. The quarry is owned by LSL Properties and leased to Columbia Asphalt and Gravel, which operates the mine, said Joe Smillie, spokesman for the Washington Department of Natural Resources.

According to Ward, experts predict that the landslide will likely occur sometime between now and the end of February.

“Luckily for us it’s moving slow on a fairly gentle slope and geologists don’t expect it to be a fast-moving event like some of the landslides we’ve seen in Washington,” Ward said.

Laser sensors installed by the geotechnical engineering contractor are able to measure the ground motion, and should detect any acceleration that might presage slope failure, Smillie explained. The sensors transmit data every 30 minutes. “If it speeds up, that would be our cue that something is going to happen,” Smillie said.

The seismometers, which can measure small ground vibrations, provide another potential warning system, Malone said. Currently, the instruments are detecting small pops that probably reflect small slides or rock falls, he said. If the frequency of those vibrations intensifies, that could signal an impending slide.

A major landslide in Greenland last summer was preceded by several hours of increased seismic signals.

“If this Union Gap landslide should let loose catastrophically, we would hope that we might be able to see precursors,” Malone said. “There’s no guarantee.”

Jon Connolly, of the Pacific Northwest Seismic Laboratory, was part of the team that installed the instruments. In places, he said, the crack in the hillside is 30 feet wide. The team also examined the quarry. “The crack goes all the way downhill and comes out on a very steep quarry wall,” he said. “You can see it split out of the wall.”