Oso landslide hit fast, hard and with no warning

(USA Today) OSO, Wash. -- The massive land slide that killed at least 24 people and destroyed over two dozen homes in Oso, Wash. came without warning, seismic records of the event show.

Seismic record pinpoint that the first slide hit at 10:37 a.m. local time on Saturday.

Local seismic measurement stations clearly show the event happened with no warning and so quickly that victims in its path had little chance to run for safety. Even if it had been slower, there would have been nowhere safe to run—the slide area is almost a mile across.

SEE ALSO: Infant's body found at site of deadly landslide

Oso resident Robin Youngblood was home when the mudslide hit . She heard something strange, looked out her window and saw a mountain of mud barreling down on her house.

"I've never heard anything like it before," Youngblood told KING-TV. "I said, 'What the heck is that?'"

She saw, "a wall, it took me a second to realize it was mud, and it was racing like 150 miles an hours across the far end of the valley," she said.

"I said, 'Oh my God,' and then it hit us," said Youngblood, 63. "I didn't see it hit us, it hit so fast," she said. "The house was moving."

"I remember thinking, 'OK, Creator, if this is it I might as well relax and I just let myself go limp," she said.

Youngblood was the first person rescued after the slide on Saturday. A helicopter plucked her out of the debris.

SEE ALSO: Boy rescued in landslide, search continues for family

The speed Youngblood estimated isn't much of an exaggeration. A landslide at Mount Meager in British Columbia in 2010 reached over 100 miles per hour, said Kate Allstadt, a seismologist at the University of Washington in Seattle.

"Basically half the mountain fell off," she said. "It moved eight miles in about three minutes."

Landslides rarely register as seismic events, despite the amount of earth that moves, because they're more of a slow creep.

The Oso slide was different.

The wall of mud was so big the rumbling sound it created was picked up on 17 different seismometers, some placed as far as 170 miles away, records from the Pacific Northwest Seismic Network show.

There were actually multiple slides over the course of an hour. The first, and biggest, lasted for two and a half minutes, recordings show.

The ground motions recorded show, "the acceleration was very rapid and a large amount of material was involved," said Allstadt.

That first wall of earth, "impacted the neighborhood below at high velocities with no warning," Allstadt said.

There was a brief pause after that first slide, and then another large slide struck at 10:41 a.m.

It was almost as big as the first one and could have been rocks and soil from the newly unstable area above the first slide slumping down into the debris below, she said.

Although the first two major slides did most of the damage, the earth didn't stop moving for several hours.

There were 12 more small landslides recorded on the seismometers over the next hour. Three more came down afterwards. The last one wasn't recorded until 2:10 that afternoon.

Each was likely more soil and rocks breaking off from the scarp, the scar of exposed soil created by the landslide.

By the end, the scene of devastation covered one square mile of land and had rerouted the North fork of the Stillaguamish river.

Despite initial reports, it's unlikely the slide was triggered by an earthquake, experts said.

There was a magnitude 1.1 earthquake in the vicinity of the Oso slide on March 10. The shaking from the quake, which was about 1.2 miles from the slide site, was not strong enough to trigger a landslide, Allstadt said.

"Swarms of small earthquakes like this happen regularly in Washington state and have historically occurred along the Devils Mountain Fault that runs through the valley," she said.

Elizabeth Wiley also reports for KING-TV, Seattle-Tacoma, Wash.


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