Pete, thanks for starting this very interesting thread. It has stimulated me to do a bit of research on various earthquake and disaster related items. A couple of points:

Regarding the potential magnitude of strike slip earthquakes, the conventional wisdom has been that they don't get bigger than about M 8. Pete mentioned some recent data suggesting they could be bigger than that. He is probably referring to the 2012 M 8.6 earthquake that struck off the coast of Sumatra. The size of this quake surprised most seismologists. So yes, they can get bigger. However, something in the M 7 range is still probably the most likely case for "the big one" on the Southern San Andreas Fault.

Pete raises the very real issue of supplying potable water to SoCal after a big quake on the San Andreas. As he points out this is not hypothetical nor is it a trivial issue. I found an interesting document from the LA Mayoral Seismic Safety Task Force. The section on water, beginning on page 54 makes interesting reading. The bulk of LA's water comes from 3 aqueducts, all of which cross the San Andreas Fault. The Los Angeles Aqueduct crosses the fault once, in the Elizabeth Tunnel Near Santa Clarita. The other two aqueducts cross it multiple times. All would likely be cut in a major quake, and would take a year or more to repair.

There are some emergency water sources available. The Mayoral report notes "There is an inadequate backup water system available to provide water to Southern California through the one-year aqueduct reconstruction period. Emergency water supplies are stored for use following an earthquake. These supplies are stored at the Stone Canyon, Hollywood, and Encino Reservoirs where large water volumes are stored primarily for emergency purposes. Back-up water sources are also available through connections with other water distribution agencies. Additionally, MWD constructed the large Diamond Valley Reservoir and some local groundwater banks for emergency water storage, some of which can be made available to the City of Los Angeles. Despite these efforts, local water storage is estimated by MWD to last approximately 6 months, and even then only with significant rationing."

Also note that for those area on the east side of the fault, the situation might not be quite so dire. Presumably the portions of the Los Angeles and other aqueducts east of the fault would continue to be operable. Indeed, places like Palmdale might have more water than they know what to do with.

JeffMc was concerned about having to transport water from great distances in a largely desert area. However, if the portion of the aqueducts east of the fault remain intact then it is a somewhat more tractable problem. To be sure there would be great difficulties in distributing the water, especially early on. And it would be a long long time before SoCal was back to normal. However, I think with a large scale mobilization of military and other resources, enough water would reach the people to sustain life.