In a truly big quake, the water system will not survive. Attended another briefing (a few years later in San Bernardino) where the focus was on a major quake on the southern section of the San Andreas fault.
Bottom line: the Pacific Plate will lift and move North, several feet at a minimum and probably as much as ten feet or more. Every aqueduct, freeway, gas line, railroad, and perhaps the electrical transmission lines will be severed. The LA basin will be cut-off from the east.
Major roads down and no power or water. That’s a pretty poor situation to be in.