A 1.1 magnitude earthquake occurred right behind the site of the massive mudslide that decimated the community of Oso in Washington state on Saturday, officials have confirmed.

Snohomish County emergency management director John Pennington said a search of recent records discovered the small earthquake struck on March 10.

It remains unclear what role, if any, the quake played in the slide that struck two weeks later, on March 22.

Previously, officials have blamed groundwater saturation caused by heavy rain over the past month for the slide.

But Pennington said the risk of a slide in the area was well known and ever since a slide hit the area in 2006 officials have been warning people and homeowners were "very aware" of the risk.

"We have pushed message after message that there is high risk of a slide," said Pennington. "There has been warnings and advanced notifications."

Steve Thompson, of the Snohomish public works department, said officials remain "absolutely worried" that there could be more mudflow at the site.

But Thompson said concerns about a flash flood are diminishing as the water caught behind the slip continues to cut a channel through the debris.

The search for survivors was suspended for more than two hours Monday when rescuers were pulled back following concerns the debris field had become unstable.

Thompson said a helicopter went up and a team consisting of three geologists and a hydrologist examined the site and found "some sloughing" off the edge of the slide but nothing to worry about.

Death toll expected to rise

The 2.6-square-kilometre slide destroyed 49 structures when a wall of mud and debris swept across 1.5 kilometres of State Route 530 near the community of Oso, 88 kilometres north of Seattle, Saturday morning.

The death toll is expected to rise today as the search and rescue efforts continue to ramp up.

Officials say geological experts will be monitoring the site from the air as rain today is expected to raise the risk of further slides at the site.

So far 14 people have been reported killed, and 176 people remain on a list of people reported missing to search officials. Several people were rescued on Saturday after the slide struck, but not a single survivor has been found since.

Officials are still trying to confirm the identities and whereabouts of many of those reported missing.

Washington state officials say 150 searchers will be scouring the site on Tuesday, including urban search and rescue experts, dog teams, and 50 members of the National Guard.

They are being supported by a mortuary assistance team, doctors, mobile communications experts, police, fire and emergency medical responders and volunteers.

Oso-landslide-USGS

Photograph from an aerial survey shows the extent of the slide on the hill above a small neighbourhood near Oso. Washington. (King Country Sheriff/USGS)

Snohomish County director of emergency planning John Pennington said organizers were "saturated" with volunteers and made a special plea for people not registered with the search efforts to stay out of the debris field.

"We don't need more volunteers," said Pennington at a briefing on Tuesday morning.

Crews are also expecting to clear snow from the Mountain Loop Highway to provide access to the community of Darrington, which as been cut off by road since the slide left a massive field of debris across Route 530 on Saturday morning.

Animated time lapse images of mudslide site: 1989 to 2014


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Google Map: Oso, Washington mudslide


View Snohomish county landslide near Oso, Wash. (#530slide) in a larger map