Ironically, what helps mitigate the effects that a major earthquake has on a large area, and it's residents and inhabitants, is the very fact that large initial earthquakes strike with little or no warning.
So there is no advance warning on radio and TV (example: Magnitude 7.3 will hit the valley at 9:15 AM Tuesday!) that mobilizes shoppers to go stock up on food and water and generators before the ground shakes, and no 'pre-earthquake' price gouging of generators, plywood, tarps, bottled water, batteries, diapers, tents and gasoline.
Advance notice of huge incoming hurricanes that affect us over in the south and southeast tend to cause a LOT of mass stockpiling of supplies and provisions, traffic gridlock and sometimes millions of motorists on the highways trying to escape natures fury in the last few hours before the major storms make landfall. Not to mention looting.... before, during, and after the event.
And, of course, I was not stating that all of the negative effects (to the supply chain) from an earthquake are permanent...but they will surely be a problem for some people who are directly involved and might have the least available resources. Often, nomadic travelers, van dwellers, and RVers only have a few days of food and water on board...yes, some RVs can carry more than 100 gallons of fresh water...but its more common in a van or small camp trailer to have 20 gallons of potable water or less, and maybe 1 to 2 weeks of groceries. And maybe a couple of 5 gallon gas cans.
Maybe.
Many of these folks (and even the 'preppers') can ride out a short event, but an extended situation with limited or very expensive food, fuel, or water, could be a real problem....if it ever came to that. How many of us carry around 20-50 pounds of dehydrated, shelf-stable food, stocked away for a 'rainy day'? And several water filters/purifiers that we might need to use for water that is from rivers, streams, or lakes? I sure as hell don't.
So, let's hope the Yellowstone Super-Eruption is not gonna happen anytime soon!