Courtesy of the U.S. Geological Society with special credit to Art Frankel, Craig Weaver, Brian Sherrod, Lee Liberty and Rick Blakely

This map depicts the three parallel traces of the South Whidbey Island fault as described in the 2008 USGS seismic hazard report.

The U.S. Geological Survey (USGS) released 2008 seismic hazard maps that have implications for Western Washington and the Woodinville area. The new maps represent an update of the 2002 seismic hazard maps and are the basis for seismic design standards in building codes, insurance rate structures, earthquake loss studies, retrofit priorities, and land-use planning.

The multi-year process to develop the maps included work by hundreds of scientists that was rigorously reviewed.

New faults in Pacific Northwest

Two new active faults are included on the maps, one near Port Angeles, one near Bellingham. Each has the potential for a magnitude-6.8 earthquake.

A magnitude-7 (M-7) earthquake is considered a “major” quake capable of widespread heavy damage. Magnitude 8 and above earthquakes are capable of tremendous damage and are considered “great” earthquakes.

Cascadia subduction zone

The Cascadia subduction zone is an offshore fault zone about 700 miles long that runs from mid-Vancouver Island in British Columbia to Cape Mendocino in California. Scientists have given greater weight to the probability of a catastrophic M-9.0 to 9.2 quake along the length of the Cascadia subduction zone compared to a model that allows for a series of more frequent M-8 earthquake ruptures.

Great earthquakes along the Cascadia subduction zone occur on average every 500 years, with the last great rupture occurring in 1700. This event caused a tsunami in Japan.

SWIF

“The South Whidbey Island fault (SWIF) traverses Admiralty Inlet and Whidbey Island before making landfall between the cities of Edmonds and Mukilteo,” according to a 2005 USGS Open File Report.

From there, according to 2004 data, it continues in a northwest to southeast direction crossing through southern Snohomish County and northern King County near Maltby and Woodinville.

The 2008 USGS seismic hazard report shows the SWIF extending into the Snoqualmie Valley. But there is evidence that it extends as far as North Bend.

According to Craig Weaver, Pacific Northwest Coordinator, Earthquake Hazards Program, U.S. Geological Survey, “We have a new hypothesis that the South Whidbey Island Fault continues from the Snoqualmie Valley to, if not well into, the Cascades. We will be doing experiments on this, this summer.”

“The SWIF could extend well over 100 kilometers,” said Dr. Robert Yeats, Senior Consultant and Partner, Earth Consultants International, and former seismic consultant for the City of Woodinville. “It is one of the bigger structures in the area.”

Scientists generally believe that the longer a fault is, the stronger the quake it produces. Given its length, the SWIF has not been investigated in its entirety. It is not known whether it is one continuous fault or whether it stops and starts and is segmented. If it is a continuous fault, it might be assigned a higher estimated magnitude than the one assigned in the 2008 report.

What is known is that the fault does not follow a discrete line.

“My feeling is that the SWIF is a broad zone,” said Yeats.

The 2008 hazard maps changed the single fault location for the South Whidbey Island fault (SWIF) to three faults.

The South Whidbey Island faults in the new 2008 representation are longer than previously designated and are assigned a forecast magnitude of approximately 7.5.

“SWIF has the highest estimated magnitude of any crustal fault in western Washington,” said Yeats.

He believes the newly projected length of the SWIF and its forecast magnitude of 7.5 are “conservative.”

The South Whidbey Island fault is of particular interest because it is long, it crosses populated areas, and it crosses the northern part of the Brightwater sewage plant, which is being built north of Woodinville at the intersection of state routes 9 and 522.

The Little Bear Creek lineament (also referred to as lineament 4) is the active SWIF strand that crosses the Brightwater site beneath the former StockPot soup facility, slated to become the Brightwater Operations Center. In 2004, the USGS confirmed the fault through a trenching exercise mandated by a King County hearing examiner.

Other aeromagnetic expressions of faults have been detected on the sewage plant site but have not been confirmed. All have northwest-trending features, which might mean they are part of the SWIF zone.

For example, there is believed to be another fault on the southern part of the site. King County chose not to investigate the geology of this feature but has assumed it is an active fault and has pushed plant facilities away from it, placing them, essentially, between this assumed fault on the south of the site and the known active fault on the north of the site.

There is aeromagnetic evidence of yet another fault that runs beneath the power plant and the education center on the site. King County did not investigate this potential fault to confirm its existence.

Yeats said, “I (also) identified one (possible fault trace) at Lake Ballinger along the sewage pipeline route based on aeromagnetic (evidence), on disturbance of sediments in boreholes, and on the topography; but King County did not investigate this, stating that they could deal with all faults crossing the pipeline route.”

He said, “Some (of these features) may be faults. Some may not. My view is that Brightwater is a critical facility. We can’t afford for it to go down. If it does, raw sewage will be flowing in Woodinville. We have something in our industry that we call ‘due diligence.’ When we see possible evidence of faulting, it is our responsibility to investigate it.

“The problem is that there is no state law in Washington – as there is in California – that requires them to investigate evidence of surface rupture in the last 11,000 years.

“(The Director of King County’s Wastewater Treatment Division) Christie True has stated that the county will strengthen plant facilities against severe shaking. The hazard we face here is not shaking, but rupture. What are they going to do if the earth pops up six feet or when building foundations move suddenly? King County has downplayed that.

“If I were a consultant, I would be professionally derelict if I advised the county to go forward with this project without further investigation,” said Yeats.

Gunars Sreibers, King County’s project manager for the Brightwater sewage plant, said, “There weren’t any surprises in the new USGS hazards report. We’ve designed the Brightwater facility to withstand potential seismic events. We are very confident that it will withstand a major earthquake.”

According to Emma Dixon, member of the grassroots group Sno-King Environmental Alliance, which has contested the selection of the Route 9 plant site for years, “King?County?cannot say that it has done a thorough site-specific investigation to ensure that there aren’t areas of active faulting directly under the proposed buildings. It has gone to extraordinary lengths to ignore the opinions of the USGS and Dr. Yeats and avoid complete trenching of the site.

“These world experts have identified that the site is literally covered in potential fault traces that should be trenched in accordance with the International Building Code. It is common sense.?

“King?County?cannot ensure that the seismic design is adequate if they have not investigated the geology that the facility is sitting on to know what they need to design for.

“The county might well be able to design to withstand shaking, but the documented behavior of these faults is also folding, warping and surface rupture of the ground,” said Dixon.

For information about limiting damage and loss due to an earthquake, check out “Seven Steps to Earthquake Safety,” which can be accessed at http://www.earthquakecountry.info/roots/seven_steps.html.