Is it dangerous to live in Seattle due to the earthquakes?
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.