I see now that there is land access to Seattle from the southeast corner near the south side of Lake Washington (Renton). But to the south and west there is the Duwamish River. And to the north there is the ship canal, Lake Union, etc. So one way in and out. (And the one major road in and out will be Martin Luther King Boulevard, which may tend to make things interesting with a million-plus people going without the basics of life.)
The tunnel project (Bertha) is what is to replace the Alaska Way Viaduct, and it is way behind schedule. They have modified the bearings and reinforced them, so I hope that does the trick. The bearings that they are replacing failed at the factory! Not sure why they didn’t reinforce them then before they shipped it to Seattle.
A friend of mine was here back in the sixties when Seattle had a big earthquake. He was on a small motorcycle traveling on the Alaskan Way Viaduct and it started to shake! He made it to the other side. It was closed for quite awhile after that one as they did repairs.
I'm not an engineer but it seems to me that the floating bridges across Lake Washington would be more likely to survive a quake than a traditional span bridge. If those bridges survived, Ship Canal Bridge could be by-passed using them and I-405 on the east side of Lake Washington. Very inefficient but possible.
The best way to think of Seattle is not as an island but as two peninsulas whose tips almost touch, only the separated by the ship canal. Access from the west is hindered by the Puget Sound and from the east by Lake Washington.