As far as damaging quakes go, there have been 25 in Washington and Oregon since 1872 in which about 17 people have been killed.
In the greater Puget Sound region, we've had 12 quakes of magnitude 5.0 or larger since 1900, so that averages out to roughly one every 8 years, but they can come in bunches.
Seattle went 30 years (1965-1995) without a quake over 5.0, but have had four in the eight years since.
For More Information:
UW Seismology Lab -- www.geophys.washington.edu