No evidence of active faults found in Brightwater seismic investigation

Brightwater neighbors can have added confidence in the treatment plant's design standards after a seismic investigation showed no evidence of active earthquake faults under a critical portion of the site.

In June, King County hired a team of engineering geologists with expertise in surface fault rupture hazards and building code compliance to investigate whether there were active faults under the proposed location of two chemical storage buildings. The three-week investigation involved digging two 200-foot-long trenches ranging in depth from about 8 to 15 feet to examine soil layers.

Though the team found evidence of liquefaction in soils deposited and compacted by heavy glacial ice from at least 16,000 years ago, those deposits would be too dense to liquefy today. The team did not find evidence of features considered to be active faults as defined by the International Building Code (IBC) 2003.

"Even though our geologists did not find evidence of active faults at the site, the reality is that we live in a seismically active region," said Christie True, manager of the Wastewater Treatment Division's Major Capital Improvement Program. "Brightwater will be built to withstand shaking from a very strong earthquake centered anywhere in the Puget Sound Region, including on the treatment plant site itself."

The trenching investigation was conducted as part of a 2005 agreement between King County and Snohomish County to ensure the safety and stability of buildings where treatment process chemicals would be stored

If evidence of active faulting as defined by the IBC were found, King County had committed to redesigning the site and moving the chemical storage buildings. In light of preliminary findings, these design changes will not be necessary. However, King County's engineers and geologists will continue to examine the data, issuing a final report to the public later this summer.

Snohomish County officials visited the trench site last week to observe the data. They are awaiting the final report prepared by the county's engineering geologists.

The county also invited the U.S. Geological Survey, or USGS, to examine the trenches. The USGS discussed a number of possible interpretations for the features found in the glacial soil, though their findings did not reach a particular conclusion.

King County received its binding site plan permit from Snohomish County in May and is now submitting permit applications to begin construction on the treatment plant facilities. Site preparation for Brightwater construction is already under way.

In siting and designing Brightwater, King County has already conducted the most comprehensive seismic investigation for a wastewater treatment plant in the Puget Sound area. The county's analysis greatly exceeds the level of seismic studies for many large projects in the region, including some that are close to or cross the Seattle Fault.

People enjoy clean water and a healthy environment because of King County's wastewater treatment program. The county's Wastewater Treatment Division protects public health and water quality by serving 17 cities, 17 local sewer utilities and more than 1.4 million residents in King, Snohomish and Pierce counties. Formerly called Metro, the regional clean-water agency now operated by King County has been preventing water pollution for more than 40 years.