The discovery of an active earthquake fault line under the Brightwater sewage treatment plant site hasn’t slowed King County, but it has piqued the interest of seismologists.
King County announced Tuesday that discovery of the fault has forced it to do a second environmental review of the 114-acre site at Highway 9 and Highway 522. But it said the review won’t consider whether the county should look for another site.
Brightwater opponents say discovery of the fault – found in the fall after seismologists dug a trench on the Brightwater site – should be enough to force King County to pick another site for its plant.
“We’re not happy that we live on top of these faults,” but this is proof that “maybe they need to go back to one of the other sites,” said Corinne Hensley, a spokeswoman for the Sno-King Environmental Alliance, which has sued King County in an effort to force it to build Brightwater somewhere else.
Instead, King County plans to construct the proposed sewage plant so it can withstand a magnitude 7.5 earthquake, the largest earthquake U.S. Geological Survey scientists are now saying is likely on the fault.
Lost in the shuffle is the discovery of a system of active faults that puts most of south Snohomish County on shaky ground.
“I think people are (now) realizing that this is earthquake country,” said Craig Weaver, a local coordinator for the USGS National Earthquake Program.
The fault found under the Brightwater site has experienced two and perhaps three major earthquakes in the last 10,000 years, and a nearby fault found at Cottage Lake has experienced at least one major quake during that span.
“We know we’re going to have another one of those babies,” Weaver said. “It could happen right now.”
If one occurs, the earthquake could easily be more powerful than the magnitude 6.8 Nisqually earthquake of 2001, which caused up to $4 billion damage in the Puget Sound region.
The Nisqually quake wasn’t nearly as damaging as it could have been because it was centered 30 miles below the surface. That distance lessened the impact considerably by the time the shaking worked its way to the surface.
That would not be the case with the surface-level fault under the Brightwater site that is part of the South Whidbey Island Fault Zone.
“What we’re looking at here is a Kobe-like situation, or a Northridge,” Weaver said of recent earthquakes in Japan and California that were particularly destructive because of how close to they occurred to the earth’s surface.
Seismologists had long thought that the Whidbey fault system extended southeast under south Snohomish County, but that was only recently confirmed with the discovery of active faults at the Brightwater and Cottage Lake sites, Weaver said.
Weaver said the USGS does not take a position on whether King County should build at the Highway 9 site. He did say the building standards that King County is using to build the plant are designed to account for up to a 7.5 earthquake, the maximum expected at the Brightwater site.
King County planned to build to the highest earthquake standard even before the fault was found, said Christie True, Brightwater project director.
“That means our structures will be beefed up” with extra concrete and steel reinforcement, she said.
The supplemental environmental impact statement that King County agreed to do will simply spell out how the county will make the plant earthquake safe.
King County resisted the added review for fear that it would slow down the project, but decided to do it now so it doesn’t affect its planned start of construction in the summer. The King County hearing examiner also had ordered the county do the review if an active fault was found under the site.
The draft of the supplemental review should be released to the public at the end of February or in early March, True said. It will cost abut $500,000.
Construction is on schedule to start in the summer on the $1.48 billion project. The plant is scheduled to open in 2010.
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