What is the age of the buildings (I'm guessing old from your comments, but not sure), are they seismic retrofitted, and what part of town are they in? There is a big range of peak ground motions possible in the Seattle area depending on underlying geologic materials (till vs fill or other soft deposits), location, and which "big one" EQ you are talking about (Cascadia vs Seattle fault). Single story wood frame structures tend to do well in earthquakes as long as they don't have fundamental structural issues like no foundation, poor attachment to foundation, unreinforced chimneys, 3rd floor water heaters that are not strapped in, etc. For the likely transaction costs of selling a building in Seattle, you could do a decent seismic retrofit. Also, it is worth asking questions about if there is an EQ and it makes a tree fall on your house, would THAT be covered even if you can't get EQ insurance (ie, what about secondary effects such as trees and fire).
There are hazards everywhere ranging from tornadoes to floods to landslides to hurricanes to.... you get the picture. Plenty of houses will fall down the next time the New Madrid fault goes.
If you are deeply concerned about owning a duplex in this area, I'd be happy to take it off your hands. :)