First of all, a 9.1 earthquake would be the largest temblor in the recorded history of the lower 48 United States. Not impossible, but unlikely.
Second not ALL water and food in So Cal comes from somewhere else. I live within walking distance of a fresh water reservoir that is not normally used for drinking but could fill in during an emergency. There are many such man made lakes throughout the area. Depending on the season, runoff could be captured, and there are substantial ground water sources to tap.
As for food, agriculture is one of the most misunderstood facets of life in California. Bakersfield county is the fourth largest food producer in the country. San Diego County ranks number 10. The Chino area still has major dairy farms. Even Orange County still produces over $150 million in food crops each year.
Even if major roads, airports, and rail lines were destroyed, there are major shipping ports from Port Hueneme to San Diego. It's almost unthinkable that any earthquake would destroy them all. With thousands of Marines based at Camp Pendeleton, Barstow, and Twenty Nine Palms, major Naval assets in San Diego, Air Force cargo planes at March Air Force base, and Army personnel at Fort Irwin, there's plenty of military assitance on call.
It will not be pretty when the big one hits, but it won't be the end of the world either.
Kalipornia also has HUGE oil reserves but they won’t allow them to be tapped.