In the event of a catastrophic earthquake, the region affected would, for all intents and purposes, become an island. If the LA region got hit with the big one, the bridges, both car and railroad, would be damaged or destroyed. That leaves air and sea as the only viable ways of providing resupply in quantities adequate to even begin to approximate that which would be necessary to sustain the huge numbers of folks needing provisioning.
I believe that 30 day supply is probably a minimum number to look at for any area prone to earthquakes. Which pretty much means earth and all the territory encompassed therein.
Even having put the supplies back, we still have to hope, wish, pray and do the best we can that we store them in a location which will not be destroyed when the shaking starts. But it also has to be accessible and not prone to being pillaged by others. In such a situation, as I have said before, it's hard to call someone a bad guy because he is trying to provide for his family.
The Seattle area is another area easily segregated by a good sized earthquake. That's why I am doing my best to get up to that 30 day mark. We don't have a lot of roadways and railways coming in and out of the area. Who knows if the ports would be functioning if such an event occured.