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Did Tiny Quake Cause Mudslide Or Should Officials Have Seen It Coming?

A 1999 report for the Army Corps of Engineers predicted a massive mudslide could hit the area, but county officials said Tuesday they had no advance warning.

A county emergency chief insisted Tuesday that officials could not have anticipated the Washington mudslide that killed at least 16 people — and suggested it might have been triggered by a small earthquake almost two weeks earlier.

"I want to know why this slide went, too," Snohomish County Emergency Management Director John Pennington said at a briefing where he was asked about studies by geomorphologists that highlighted the hillside's instability.

"We're going to get to the bottom of this," Pennington said.

One of the studies was undertaken in 1999 for the Army Corps of Engineers by geomorphologist Daniel Miller, who told NBC News that his warning about the potential for a massive collapse "should have triggered major concerns."

Another study submitted the following year, by engineer and geomorphologist Tracy Drury, said a replay of a 1967 slide that dammed up the river could take a severe toll.

"Based on the available data, and assuming the future resembles the past [the site] poses a significant risk to human lives and private property, since human development of the floodplain in this area has steadily increased since the 1967 event," Drury wrote in the document, which was released by the Army Corp on Tuesday.

Pennington said he had not had a chance to look at the old reports because he was more focused on rescue and recovery.

But on a hunch, his agency did look at recent seismic activity and found that on March 10 there was a 1.1-magnitude earthquake behind the slab of hillside that broke free Saturday and buried a square mile in muck.

It's unknown if the microearthquake, too small to be felt by anyone, played any role in the disaster, which followed a rainy winter.

Pennington said homeowners "were very aware" of the potential for a landslide — there had been at least three smaller collapses at the site since the 1950s — and that the county took steps to reduce the risk.

“My job is advance warning and public information," Pennington told reporters at a morning briefing. "If I had any idea this was going to break on that Saturday afternoon … c'mon guys.”

"This is just one that hit us," he added.

There were 49 homes, cabins or trailers in the path of the mudslide — which tore off a 1,500-foot by 600-foot section of the hillside, spilling earth over the Stillaguamish River and State Route 530.

Miller, who has studied the area extensively, said he never would have built a house there and was surprised it was so developed.

"We did anticipate this event could occur, but we had no idea what the probability was."

He said the 1999 report he prepared was focused on how to protect fish in the river from the sediment that would disrupt their environment in the event of a mudslide.

He said he identified the potential for a collapse that was nearly identical in size to the one that happened in 2006 but also noted that a large block of earth beyond that could pose an even bigger hazard.

"The primary conclusion to be drawn is that mass wasting activity will persist for as long as the river remains at the toe of the landslide," according to his draft analysis, which was first reported by the Seattle Times.

The Army Corps of Engineers said its copy was archived and not immediately available, but that it would have been shared with the county at the time.

Miller said that after the 2006 slide, he went back to the site and later gave a presentation where he again spoke of the potential for a bigger repeat. He could not remember which group he spoke to, but assumed county and tribal representatives were there.

"We did anticipate this event could occur, but we had no idea what the probability was," Miller said.

He said the risk was small enough that he would not have thought twice about driving down Route 530, "but I wouldn't have put a house there."