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I was under the impression that the bigger danger was the expected eventual earthquake of the Cascadia Subduction Zone, potentially leading to tens of thousands killed or injured and millions more displaced.[1][2]

[1] http://www.newyorker.com/magazine/2015/07/20/the-really-big-...

[2] https://en.wikipedia.org/wiki/Cascadia_subduction_zone#Earth...

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The damage from a Cascadia rupture is going to be more widespread but the ground shaking in the metro areas may be less than for the San Andreas because the San Andreas hits the surface quite close to populated areas (LA also has more people that WA and OR combined). The intensity of ground shaking decreases with distance due to geometric spreading and energy absorption by the intervening rock mass. Cascading ruptures are also every 500 years (or so) vs every 100 years, so the hazard per year is less. But yes, both are major concerns.

Also, speaking of other faults, WA also has the smaller, but much more shallow, Seattle fault. Because it's so much closer to the surface, and right under the city, it can potentially do more localized damage than Cascadia.

(There are other dangerous faults like that in Puget Sound region, but Seattle fault gets special attention because of how many densely populated Seattle metro areas sit literally right on top of it.)

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