The Cascadia Subduction Zone is a signifcant fault running off the coast of the Pacific Northwest, where the Juan de Fuca plate slides under the North American plate. When stress builds up between the plates, an earthquake occurs, which may also be accompanied by one or more tsunamis. This fault represents one of the biggest dangers towards Seattle. The last time it went off was in 1700.
Potential Hazard to Seattle
The Cascadia Subduction Zone is close enough to Seattle to cause serious damage to the city. A Cascadia Subduction Zone earthquake could be as severe as a 9.0 magnitude, resulting in shaking so strong that one would barely be able to stand during the earthquake.
Brick buildings would be likely to collapse, resulting in the potential oblitaration of areas like Pioneer Square. Even with retrofits, brick constructions will still be severely damaged (because bricks move back and forth). Suburban homes with wooden frames might be less damaged if anchored to their foundations.
Seattle also has a lot of area built on fill (which area?). In an earthquake, fill turns to liquid and structures on it can't be supported (how do we know this?) ,so structures collapse as their foundations sink into the ground. Qwest Field, Safeco Field, Harbor Island, the waterfront, and the industrial district are all built on landfill. These areas will be the most damaged. However, Safeco Field was built on pylons which were drilled into bedrock (not sitting on fill) and would be relatively unaffected by an earthquake.
Experts debate what would happen to downtown's skyscrapers. In a normal quake they are some of the safest areas, but a Cascadia quake would be much stronger. Some argue the buildings could not handle so much shaking and would collapse, while others say some should remain standing. Even if the buildings remain standing, the debris from building damage, such as glass or other building materials, could bury the streets along with pedestrians and automobiles.
Other structures at risk include highways and bridges, such as the Alaskan Way Viaduct which is already planned to be replaced due to earthquake risk.
Tsunamis from the fault itself would not pose a risk to Seattle because Puget Sound is protected by the Olympic Peninsula. But landslides in Puget Sound could cause tsunamis that could kill hundreds more and cause more damage. By the time the quake ended, thousands could be dead in Seattle alone. Tens of thousands more could die in other areas. Most likely, the city would never be the same.
The threat is not very well known. So if you are visiting or living in Seattle, be prepared and remember that Seattle is potentially subject to severe earthquakes.