I have an idea for a novel I want to turn into a reality, and I'm starting some research first to make my book at least plausible in very specific conditions. The main aspect of the story would be focused around the Northwest Coast of the US after an earthquake has completely demolished everything, turning it into a post-apocalyptic wasteland. The main question I'm attempting to answer currently is if this is even possible. I know the Sand Andreas fault line is a major concern presently due to its high capability to release an exorbitant amount of energy in a single quake, but given all the conditions were perfect, is it possible that a single earthquake could cause enough damage to rip apart the entire West Coast? And if not in a single earthquake, can one quake, given it was powerful enough to trigger a series of seismic activity all along the West Coast, cause such a catastrophic, apocalyptic event? Any advice would be greatly appreciated, and thank you all for your time!
Never say never, but it's unlikely.
Every geological fault is not uniform. There are bumps and snags of varying size and strength along its three intentional surfaces. As once fault surface moves over the other some of the snags can break off resulting in localized quakes at the site of the breakages.
Similarly, for very strong, robust snags (protuberances) which ride over the other surface as they suddenly hit a dimple in the other surface or simply just ride along another surface protuberance on the surface of the other side of the fault it slams into the other surface. This causes another form of localized earthquake.
For an earthquake to hit the entire north west region of the US, these type of events would have to occurs along the entire length of the fault simultaneously.
The west coast has had 25 years of preparation to plan for a possible 9.0 above richter scale event. Earthquakes in the industrialized world are not as substantial a deal despite the potential of infrastructure for damage. In the third world where building standards are poor, potential for catastrophic physical damage and deaths in the thousands is more problematic. The most lethal recent quake in US was the 1964 Alaska "Good Friday" Quake which killed 140+ people. Japans 1995 quake killed six thousand! But yes such a quake though possible is statiscally very unlikely. The biggest damage is structures built on land made of silt and sand.
If you need a plausible west coast disaster, consider a fraction km meteor about 1500 km off shore. This would do two things:
A: The Tsunami would hit most of the coast over a span of 4-6 hours.
B: It would rain sea water initially from the splash,killing a lot of vegetation.
C: It would wreck (or at least sharply reduce) crops throughout the northern hemisphere for a year or two. No one would have the resources to help the coastal victims.
Note: You may want to check out the worldbuilding SE for more of this kind of thing.