The entire pacific rim is a pretty seismically active area, nicknamed the 'ring of fire'.

Seattle and Portland are relatively close to the Western US's subduction zone (one plate sliding under another), though not nearly so as Southern California, which actually straddles the plate junction.
Source(s):
http://www.ess.washington.edu/SEIS/PNSN/INFO_GENERAL/faq.html

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I heard on the History, Science or Discovery channel that there was an earthquake so huge in exactly 1700 that it produced a bad tsunami in Japan that was recorded. It moved the land up or down 3 feet or something like that somewhere in Oregon or Washington area. The white man was not really living there except maybe as trappers so it was not part of any "recorded history."  However Indian dances record the event.

I think the scientists looked through the mud and saw that these events occur every 300-600 years.
Source(s):
http://en.wikipedia.org/wiki/1700_Cascadia_earthquake
http://bulletin.geoscienceworld.org/cgi/content/abstract/115/10/1289 - apparently they occur ever 570-590 years on average over the last 6,700 years. It turns out they have dropped the land. not raised it.
Discovery, Science or History Channel.

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