If SeaTac and McChord are unusable, help is about a month away by sea and much depends on a usable port.
Trucks could get there sooner but most of the lead trucks would have to carry heavy equipment to clear the roads. And would take at least 1 if not two weeks.
I practice medicine in north Idaho. This past summer my facility participated in the “Cascadia Rising” exercise. This simulated a 9.something quake and tsunami and was a really big deal in the Pacific NW. The storyline on most of the “victims” that were sent to us was that they “drove” from Seattle. Not bloody likely!
I would imagine that they will be. Along with Boeing Field, Everett, etc.
Before they knew about the magnitude of this quake the emergency response was coordinated between Portland, Seattle and Vancouver, B.C. If one city got hit, the others would help.
But now that they now about this one the aid will come from east of the Cascade mountains. BUT - the various bridges west of the mountains will be blocked, so they will have to chopper stuff in. For Seattle the plan is to use the large runways at Moses Lake (3 hour drive from Seattle) to land stuff.
Then it was, IIRC, truck things the two hours west to the east side of Snoqualmie Pass to a staging area, then chopper things from there into Seattle. As you said, I'm sure they will have over-water routes set up - but from where? Probably California.
Seattle itself will be very difficult to get any vehicles in at first as it is pretty much going over large bodies of water and the bridges will be knocked out. Although they did just complete the new 520 floating bridge - I'll have to look if they designed it for Cascadia Fault - I'm guessing that they would have. Of course that bridge goes across Lake Washington, and the Seattle fault could setup a large tsunamis in the lake (20 to 30 feet iirc?), which I'm guessing would take out the bridge. I think the Seattle Fault, because it is shallower and near the city would take out more bridges than the Cascadia one would.
Anyway - it will be a huge catastrophe. And it WILL take time for logistics to get things (and medical help) to people. What we need is another one like the Nisqually E.Q. to wake everybody up again and get folks prepared. (But not do too much damage. Me to to a certain degree as well, I know I'm not as well-stocked as I used to be after the last one.