Earthquake

Key Points

Earthquakes are the most serious hazard facing Seattle. Unlike other potentially catastrophic hazards, Seattle has had and will experience powerful earthquakes.

The Seattle area experiences three earthquake types with three very different consequences.

Crustal or Shallow Quakes  occur in the North American plate at 0-30 km near the crust's surface along faults. Intense shaking occurs near the epicenter but usually diminishes quickly with distance relative to the other earthquake types. Shallow quakes are the type expected on the Seattle Fault zone, which is the primary but not only source for shallow quakes in Seattle.

Intraplate or Deep Quakes occur at depths of 30-70 km in oceanic crust as it dives under lighter continental crust. Because of the depth, even buildings located right above them are far enough away that ground motions are attentuated. The 2001 Nisqually Earthquake was a deep quake.

Subduction Zone or Megathrust Quakes occur on the interface between the North American plate and the San Juan de Fuca plate, a small plate extending from northern California to British Columbia. These are the largest type of earthquakes in the world.

  • An earthquake on the Seattle Fault poses the greatest risk to Seattle. Deep quakes are the most common large earthquakes that occur in the Puget Sound region. Quakes larger than 6.0Mm occurred in 1909, 1939, 1946, 1949, 1965 and 2001.

    • The Seattle Fault zone extends east-west through the middle of the city

    • A Seattle Fault quake could be as large as magnitude 7.5[i], but less than 7.0 mm is more probable.

    • The most recent Seattle Fault earthquake was about 1,100 years ago;

    • The Seattle Fault has been active about three or four times in the past 3,000 years.

  • Megathrust earthquakes are the greatest risk to the region as a whole. A megathrust earthquake could reach M 9.0+ and affect an area from Canada to northern California. Shaking in Seattle would be violent and prolonged, but not as intense as a Seattle Fault quake. This area has a megathrust earthquake about every 500 years.

  • About 15% of Seattle's total area is soil that is prone to ground failure in earthquakes. The Duwamish Valley, Interbay and Rainier Valley are vulnerable to ground failure and shaking because of the liquefiable soils in these areas.

  • Seattle has an estimated 819 Seattle unreinforced masonry buildings that perform poorly in earthquakes. These older brick buildings tend to be concentrated in areas expected to experience the strongest ground motion during earthquakes. Other vulnerable building types exist, too.

  • Seattle is heavily dependent on its bridges. Damage to them would impair emergency services and the economy. The city has launched a multi-year effort to retrofit them to a life-safety standard so they won't collapse. Despite the retrofits, many will not be usable after a strong earthquake. Most of the critical bridges were retrofitted by 2009.

  • Combined property damage for quakes in 1949 and 1965 in the region amounted to roughly $400 million (2010 dollars). The 2001 Nisqually Earthquake resulted in damage to City of Seattle buildings, infrastructure and response costs that exceeded $20 million. Adding in the costs of repairing arterial road structures, the figure topped $36 million.

  • Secondary impacts such as landslides, tsunami, fires, and hazardous materials releases could become disasters themselves. In many earthquakes more people die from fire than building collapse.

    • 2013 research finds that Seattle is at risk of thousands of landslides following a strong (magnitude 7) Seattle Fault earthquake.  Estimates range from 5,000 if soils are dry to 30,000 if soils at saturated.

    • A large Seattle Fault earthquake could trigger a 16 foot tsunami that would strike the Seattle shoreline within seconds of the earthquake and flood it within 5 minutes. Although megathrust and deep earthquakes will not directly cause tsunamis in Seattle these sources could initiate landslides that result in local tsunamis.

    • A magnitude 7 Seattle Fault earthquake could cause dozens of fires. Suppressing the fires would be more difficult because damage to the water system would reduce water pressure in many parts of the city.

    • Structural failure and fires would probably causes multiple hazardous materials releases. They could range from minor spills to major incidents.

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                [i] Vidale, 2013