Actually, it is hard to say what "WILL happen", other than it probably will be different than what any of us imagine. I tend to agree with hikermor that while there may well be some looting and other social unrest in local areas, widespread societal collapse is rather unlikely, in my opinion.

I'm a geoscientist by training, and while my day job doesn't involve earthquakes, I'm very interested in the subject and have read a great deal about both historical earthquakes and basic earthquake science. The thing is that the effects of even a very large quake vary greatly across the region. Besides the size of the quake and distance from the epicenter, much depends on what sort of ground is under you. Areas underlain by bedrock do much better than areas on soft sediments or fill. Likewise, much depends on the quality of construction. Hence the severity of damage, death, and destruction also varies greatly. Likewise, the time it will take local, state, and federal emergency response to reach and help people will also vary over the area.

A few years back, the USGS assembled a large team (geologists, seismologists, engineers, emergency planners, etc) and tried to come up with a realistic scenario for what might happen if a M 7.8 quake started near the Salton Sea and ripped for 200 miles (300 km) to the north along the San Andreas Fault. You might want to take a look at the fact sheet for The ShakeOut Earthquake Scenario — A Story That Southern Californians Are Writing. The full 312 page report with the science behind it is at http://pubs.usgs.gov/of/2008/1150/.