While the Cascadia Subduction Zone should be pretty spectacular when it ruptures, at least a few seismologists don't feel it's the most dangerous earthquake hazard for the city of Seattle. Sure, sure, when it goes, it'll release a massive amount of energy—on par with the Chilean quake. But it's far, far away. Seismic waves move relatively slowly through the crust of the earth. Seattle will probably have a warning before the shaking starts—enough time to get to someplace safe. (Hell, you can have the USGS text or email you when any earthquake is detected nearby.) Provided you have a cache of supplies like food and water (you do have one, right? Right?) it could be survivable. Arguably, the Cascadia Fault is a bigger risk for people in Hawaii and the Pacific rim; this one should be a big Tsunami indeed.
The Seattle Fault—running East to West more or less along I-90 and across Puget Sound—is what will probably doom us. This fault line is quite similar to those responsible for the Northridge earthquake in LA, or the Kobe earthquake in Japan—shallow blind thrust faults with the epicenter of any quake near the surface and right in town. For those of you geographically minded, this fault is more or less underneath Harborview (the region's major trauma center.) No warning, passing right under the primary trauma center and producing shaking most destructive to structures—the Seattle fault is my disaster porn of choice.