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Seismic readings show Washington had 2 landslides

Workers build a temporary road to connect the two sides of a debris field from a deadly mudslide, Thursday, March 27, 2014, in Oso, Wash. The death toll is expected to rise considerably within the next two days as the Snohomish County Medical Examiner’s Office catches up with the recovery effort, Snohomish County District 21 Fire Chief Travis Hots said Thursday. (AP Photo/The Herald, Mark Mulligan, Pool)

Workers build a temporary road to connect the two sides of a debris field from a deadly mudslide, Thursday, March 27, 2014, in Oso, Wash. The death toll is expected to rise considerably within the next two days as the Snohomish County Medical Examiner’s Office catches up with the recovery effort, Snohomish County District 21 Fire Chief Travis Hots said Thursday. (AP Photo/The Herald, Mark Mulligan, Pool)

10ThingsToSee – This aerial photo taken Monday, March 24, 2014 shows the massive mudslide that killed at least eight people Saturday and left dozens missing, near Arlington, Wash. (AP Photo/Ted S. Warren, File)

Searchers examine the area on the western edge of the mudslide where it covers Highway 530 Wednesday morning, March 26, 2014, east of Oso, Wash. The debris field is about a square mile and 30 to 40 feet deep in places, and includes quicksand-like muck, rain-slickened mud and ice. (AP Photo/The Herald, Mark Mulligan )

Rescue workers and work crews toil at the western edge of the mudslide where it covers Highway 530 Wednesday morning, March 26, 2013, east of Oso, Wash. Search crews using dogs, bulldozers and their bare hands kept slogging through the mess of broken wood and mud. (AP Photo/The Herald, Mark Mulligan) Photo taken 20140326

Heavy equipment moves debris on the western edge of the mudslide where it covers Highway 530 Wednesday morning, March 26, 20145, east of Oso, Wash. Washington authorities on Wednesday reduced to 90 the number of people missing from the community wiped out by the mudslide. (AP Photo/The Herald, Mark Mulligan)

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SEATTLE (AP) — Seismic signals showed there were two major slides about four minutes apart during Saturday’s disaster in Washington state, and afterward smaller slides continued for days, University of Washington researchers said.

A report on the landslide readings was sent to the U.S. Geological Survey and posted Wednesday on the Pacific Northwest Seismic Network blog by Kate Allstadt, a university researcher.

The “very rapid and energetic” event was detected on 17 seismic stations ranging from 7 to 170 miles away.

They showed the biggest slide started at 10:37 a.m. and lasted more than two minutes. This is the slide that crossed the North Fork Stillaguamish River and hit the community of Oso, killing at least 25 people

“The big pulse was the main volume of material that broke down from the slope and tumbled down toward that valley,” said Bill Steele, the seismology lab coordinator and spokesman for the seismic network.

“Another big pulse followed that, breaking loose another section of unstable slope,” he said.

The second slide at 10:41 a.m. was material breaking off the steep cliff created by the first slide, Allstadt said Thursday. Smaller slides continued for an hour and occasionally for days later, similar to earthquake aftershocks.

The landslide moved with surprising speed, said Ralph Haugerud, a USGS research geologist at the University of Washington.

“Not very many move this fast,” he said Thursday. Typical landslides in the Nooksack Valley “crept down the hill.”

The vibration of falling can cause a landslide to turn into a debris flow that moves like water.

“My hunch is the slide may have dropped farther than many, and as it did it liquefied,” Haugerud said.

The study of seismic signals showed no earthquake triggered the slide, Allstadt said.

The closest quake was a magnitude 1.1 on March 10. That would have been too weak to lead to a slide, Allstadt said. There have been eight such quakes in the Oso area in the past year.

The tiny tremor was insignificant to the forces of the river and gravity working on the unstable hillside, Steele said. Layers of clay in the hillside collect rainwater, creating a lubricated surface that easily gives way.

“This area is glacial moraine and sandy outwash — very unstable soil being cut by a very active river moving around in the channel,” Steele said.

However, it’s possible the 1.1 tremor was related to some movement in the hillside.

“It could have been an indication of something moving within that slope,” Steele said.

That points to the new area of research called landslide seismology that Allstadt is pursuing.

Landslides are harder to study than earthquakes because the signals are less clear and don’t travel as far, said Allstadt, who used seismology tools to research landslides for her doctorate.

She plans more research that eventually could lead to monitoring that will indicate when slopes are becoming unstable.

Associated Press

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