Seattle survives shake-up

August 11, 2010

Scenes from the 1994 Northridge earthquake in California's San Fernando Valley were rekindled as residents of the Seattle area experienced the 6.8-magnitude Nisqually earthquake on Feb. 28.

Although more powerful than Northridge, the quake caused little damage by comparison due to its 36-mile depth. Northridge, which was 11 miles deep, had a magnitude of 6.7 and caused $40 billion in damages. Damages from the Nisqually are estimated at $2 billion.

Damage is mostly nonstructural

By mid-March, 27 buildings in the Seattle area had been red-tagged, or marked as not fit for occupancy. Most damage was nonstructural and limited to older buildings on softer soil, says John Hooper, director of earthquake engineering with Seattle-based Skilling Ward Magnusson Barkshire. The softer the soil, the more amplified the ground motion.

One of the more severely damaged buildings was the six-story Palmer Court Building, an unreinforced masonry structure located in Seattle's historic Pioneer Square district (right). The top portion of its south wall crashed down, crushing several cars. "It came down because it wasn't anchored to the roof," Hooper notes. "Ironically, the structure was scheduled for a seismic retrofit later this year."

Another case of typical damage from the quake occured at the Pacific Medical Center, now headquarters for Amazon.com (see "Seismically serene," page 40). Pieces of the 69-year-old structure's terra-cotta fell onto the roof of an adjoining addition, severing chiller lines. The 63,000-sq.-ft. addition was constructed in 1992 to seismically stabilize the historic building. It passed its first test, since the old structure remained structurally intact.

In Olympia, some 60 miles southwest of Seattle and just 11 miles from the quake's epicenter, the 73-year-old state Capitol building was closed after the temblor knocked 10 columns under the dome out of plumb. Andrew Stepelton, senior property manager with the state's Department of General Administration, says the structure's masonry dome appears to have rocked on top of the columns during the quake, coming to rest 3/4 inch off center while the columns remained crooked.

Other damaged buildings included: Seattle's First Baptist Church, which had two of its four spires shaken loose; the control tower at the Seattle Tacoma International Airport, which had windows blown out; the under-construction Seahawks football stadium, where a vertical form was knocked over; and the King County Courthouse, where windows broke and an interior wall collapsed.

         
 

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