I'm considering a move/vacation to Seattle, and I'm concerned it may be dangerous due to the risk of an earthquake?
Yes. Earthquakes are Dangerous
No. Because big earthquakes happen so infrequently (~300 years in Seattle's case) that one shouldn't really worry about it. Engineers design buildings to withstand large earthquakes. Of course there is the possibility of failure, but generally the US is very good about their infrastructure. Seattle is not like Oklahoma, where they are completely unprepared for earthquakes. I lived in Los Angeles in 18 years, and while my time there was during the Northridge 1994 earthquake, most of the damage was to the buildings (which were insured) and not to the people. Still there is some risk, but I think the risk of driving in Seattle is probably riskier than living in Seattle for 50 years of earthquakes.
Earthquakes happen everywhere, even in places where some people think earthquakes are "not supposed to happen".
Tectonic boundaries, where crustal plates meet, such as the west coast of North America (San Andreas fault), the Himalayas, New Zealand and Japan are more prone to earthquakes than places in the middle of continental plates, such as Australia.
But places far away from plate boundaries also experience earthquakes, sometimes large quakes of magnitudes 6 or 7, but such quakes in those sorts of locations are infrequent.
Ground stresses permeate the entire crust and there are faults of varying lengths and activities everywhere. The ground stresses can cause faults, whether major or minor, to move, which causes earthquakes - even far away from major faults like the San Andreas.
So to your question, will Seattle experience an earthquake? Yes. When? That's unknown, because the timing, strength and exact location of earthquakes cannot be predicted.
The best thing you can do, wherever you decide to live, is to avoid living in a dwelling that is built on soil that could liquify should an earthquake eventuate.