The Two Thousand One Nisqually Earthquake

At ten fifty-four in the morning on February twenty-eighth, two thousand one, the ground beneath southern Puget Sound moved. The shaking lasted nearly a minute. The earthquake measured six point eight on the moment magnitude scale. Its epicenter was northeast of Olympia, below Anderson Island, roughly seventeen kilometers from the city center. The focus was more than fifty kilometers underground, within the subducting Juan de Fuca plate, which makes it an intraslab earthquake, one of the most dangerous types in the Pacific Northwest precisely because the depth tends to spread shaking over a wide area rather than concentrating destruction in a narrow zone.

Olympia took the worst of it. Downtown buildings sustained heavy damage. The Fourth Avenue Bridge was so severely compromised that it was torn down and rebuilt. A buttress under the Washington State Capitol dome splintered. The Legislative Building, the gravity-held masonry dome that opened in nineteen twenty-eight, closed for years for structural repairs.

One person died, of a heart attack during the event. Several hundred were injured across the region. Total property damage was estimated between one billion and four billion dollars.

The earthquake was felt in Oregon, in British Columbia, in eastern Washington, and in Idaho. In Seattle, the Alaskan Way Viaduct sustained damage that ultimately factored into the decision to demolish it and replace it with a tunnel.

The Nisqually Earthquake is the reference event for seismic planning across western Washington. The Juan de Fuca plate is still subducting. Geologists note that a much larger megathrust event, on the Cascadia Subduction Zone, remains a matter of when rather than whether.