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I am guessing that Seattle, which can only be accessed by bridges, will be an island unto itself for quite awhile with probably all of the bridges destroyed if it is a 9.0.

Seattle can only be conveniently accessed from the east by bridges due to Lake Washington. Access from the north or south doesn't require crossing any large body of water. When the I-90 floating bridge sunk in the early 1990s, it made for inconvenience but did not paralyze the city.

It is hard to tell what the total damage a 9.0 earthquake would be but it would be massive. I'm sure that both floating bridges are gone in such an earthquake. Is the Alaska Way Viaduct since in use? I wouldn't want to be on it or under it if such an earthquake hit. I'm sure it collapses. It is possible or likely that I-5 and other major roads would be damaged. Transportation around the Puget Sound would be certainly disrupted for weeks or months.

Here is a video put out by WSDOT of what might happen in the event of an earthquake to the Alaskan Way Viaduct and the Seattle waterfront. It was done a few years ago. In it, the earthquake was supposed to be around the same magnitude as the 2001 Nisqually Quake, but closer to Seattle. If the Cascadia lets go with the full-margin rupture (also called the 'Full 9 Rip'), the results would likely be much, much worse.

Computer Simulation of Seattle Waterfront in an Earthquake

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.