As I wrote before, I am more concerned about the short-term psychological reaction of people in LA. And really, the major problem is that few of these people have followed the advisories and put enough water aside to last for a few weeks. That is really the main thing that people need to do to survive. One step!
Agreed, potable water would be a huge issue. Especially from about day 2 until a week or so. Most households can probably find enough fluids to survive for 24 hours or so. After a week or two relief efforts will probably be able to spin up enough to deliver at least survival levels of water. It's the time in between that will be grim. Even in the Pacific NW potable water will be an issue after a Cascadia quake, but at least up there they have numerous natural water sources provided they have means to purify it. In the current drought SoCal is not so fortunate.
Probably the big unknown for LA is the potential of large-scale fires. It was a major problem for San Francisco during the 1906 quake. I think it will also be a major problem - and frightening - for some areas of LA. As residential values have shot up, many developers have built apartment blocks. High-density housing exists in many suburbs. And So Cal is very dry because of the long-term drought. So the fire risk is very real.
I think you nailed it. Fires could well lead to more fatalities than the actual quake itself. I seem to recall that during the flooding from Hurricane Sandy, a whole neighborhood burned to the ground. Not the usual risk one associates with floods, but it happened. SoCal is at orders of magnitude greater risk from fire after a quake.
A major earthquake in a heavily populated area will always be bad. Each region may have special issues which make it worse. In Nepal it was poor building codes, lack of transportation routes and rugged terrain. In SoCal it will likely be lack of water, and fires.
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