That's debatable. It can mean anything from doomed to in trouble.
IDIOMS FOR TOAST
be toast, Slang. to be doomed, ruined, or in trouble:
>Would you object if he said "utterly destroyed" instead?
Yes I would object.
Does it mean everything west of I5 is going to fall into the ocean? All human structures are going to be levelled? Only structures over 2 stories not made out of wood will be levelled? Or just that everything west of I5 will have more damage than everything east.
And I doubt that EVERYTHING east of I5 is going to share the same fate. The I5 is a human made road, not a geographic feature. It doesn't control earthquake outcomes.
In Oregon, the land to the west of I-5 is clay silt from the Missoula Floods that came down the Columbia Gorge 13,000 - 15,000 years ago. The clay silt is expected to completely liquefy in the event of a major earthquake.
Widespread death and destruction. Seriously, other than the houses on Cooper Mountain, I don't seriously expect anything to still be standing in the Tualatin Basin when the big one hits.
Maybe you think its inappropriately informal, but that is a stylistic concern, not a science concern.