You could try asking someone who works at Seattle Community College. There is one deaf teacher who teaches sign language also you can ask the interpreter services office about the deaf events, they might know a thing or two. You can also try HSDC like Missywinks stated. It probably changed since I left Washington state over a year ago. Of course there are many deaf social events. You can also try searching on Facebook to find some chapters that run some events and you can make new friends and pick up some local deaf socials in town.
I know for sure Jillian's run once a month event in downtown Seattle area. There's also the coffee house social at the church near Green Lake and the Northgate Mall social once a month as well. I do not know the dates they run on right now. I just don't want you to end up finding there's no event because of a mistake.