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/.

Ah, yes- nature of the ground and quality of construction. Much of my community is built on marine sediments, susceptible to liquefaction (buildings may sink into the ground when shaken). The local hospital is building an extensive addition and a major part of the project has been digging a mammoth hole and preparing the extensive subfoundation - as I understand it, a mammoth concrete plug on which the foundation will be placed - all to counter the effects of potential liquefaction.

AKSAR - I'm hoping that the Shakeout is the "most probable" outcome ... and not something worse. But if you look at the ground accelerations predicted for the Shakeout - they are still very high across large parts of the LA Basin. I think I saw comments recently that hundreds, and maybe thousands, of buildings will be badly damaged (or come down). We have still not brought all properties up to code, and some are also historic buildings. My main concern in terms of disasters is probably the fire potential - since water mains will be broken. we have many neighborhoods in LA now which have been built up to very high population densities ... almost no room between adjacent houses and apartments.

Ultimately, it is the human reaction to the earthquake which I think is our biggest problem. there are very few fresh water sources in L.A., and it is impossible for the Gov't to get supplies to millions of people. so if the freeways are down, and people run out of water & food, that is the real problem (along with a climate of fear created by the earthquake and the subsequent fires).

Pete

It will suck to be in LA and other points covered in red when San Andreas cuts loose. Fortunately I live in an area covered by blue-green on that last graphic and should have no structural damage in that scenario. There are lots of faults throughout SOCAL though; San Andreas could trigger other faults which may put more red on that map.

Yes, Southern California does have its faults, and new ones are discovered all the time when they cause an EQ. Basically, I think we are all pretty much at risk to about the same degree.

The "same degree", I wouldn't take it that far. Just saying that not being in a Red zone on the graphic doesn't make you safe, just less threatened; a 5.8 EQ is not a minor shake, but it's way better than a 7.8 EQ. In general the entire SOCAL region will be in bad shape, but I have confidence this humble dwelling will mostly remain standing.

If I had my choice, I would rather live out the aftermath of an earthquake. Which although has the potential to destroy a lot of property and infrastructure in the actual earthquake and the immediate in terms of fire etc, it sure beats living in the aftermath of a flood where water can damage everything beyond any semblance of repair.

As for the question of water. Yes it may be hard to come by in the immediate quake center but there is enough US logistical support such as the military that has the ability to not only clean and sanitize water but also desalinate salt water if required. Also I am sure other countries such as Canada would deploy their DART team which can provide up to 50,000 liters of clean water per day.

Interesting related note to this. My wife who went over with a NGO to the Philippines immediately after Typhoon Haiyan, relayed to me that once aid from countries from around the world ramped up, they were almost overwhelmed in terms of logistics and supplies pouring in, even though that country does not even have close to the same infrastructure as California or any other major US regions.

In the event of a real major earthquake, I would suspect the same logistics and support would flood into the California area and there would be more then a couple of aircraft carriers and other navy ships anchored off the coast along with 1000's of hands of help coming onshore in a very short time. Not to mention the vast US civilian and military air support that would be put to use.

Teslinhiker - I do agree that the arrival of major naval ships is likely to help Los Angeles a lot. I'm not sure, though, that the "swing into action" phase can reach people in the first 10 days. so there is a time window there of about a week where ugliness could prevail.

I just read a nice article today by a lady who is a geologist from Oregon. she quoted another eminent seismologist (geophysicist) as saying something like this: "For those cities in the world where the population is high and earthquake risks are real - it would be appropriate to spend as much on 'earthquake defense' as they do on military defense". that's not an exact quote, but that was the meaning. I thought it was a very good comment, and it is applicable to Los Angeles and San Francisco right now. but no-one is doing it.

Pete

Pete, I think you greatly overestimate the time it would take the US military to respond to a major quake in Socal, and greatly underestimate the level of support they could provide.

You seem to be forgetting that there is a very large military footprint already in California, which could swing into action quite rapidly. And, it should be pointed out that the military bases in CA are dispersed over a wide area. Even assuming one or more bases were themselves hard hit by the quake, the others could still respond very quickly.

Just a short and by no means complete list:

Camp Pendleton - the 1st Marine Division is practically next door to LA.
Naval Base San Diego - Just a short sail down the coast, San Diego is home port to more than 50 navy ships, numerous other commands, and thousands of sailors. The first ships could arrive in LA in a couple of days.
Marine Corps Air Station Miramar - Also just down the coast near San Diego, home of the 3rd Marine Aircraft Wing.
Fort Irwin - out in the desert, home of the Blackhorse Regiment.
Marine Corps Air Ground Combat Center Twentynine Palms - Also out in the desert, lots Marines out there too.
Marine Corps Logistics Base Barstow - More Marines, and lots of stuff.
Point Magu - big navy base with lots of sailors and planes.
Port Hueneme - lots of Seabees and their construction equipment.
Travis AFB - a bit further away, but how long does it take to fly one of their C-5s or C-17s to LA? (Not long!)

The point being that Socal has a great deal of military support very close at hand, which could be quickly mobilized in the event of a major disaster. The rest of the country should be so lucky!

By the way, did you mention civil unrest? Just how long do you think that would last when the Marines arrive? Not very damn long, I will wager.

None of this is meant to imply that a M 7 or 8 earthquake in Socal would not be a terrible thing. It would be extremely bad. The USGS scenario I linked upthread talks about thousands killed, many thousands more injured, major fires burning out of control, services severely disrupted for many months, and years for the economy to begin to recover. However, it is well to keep things in perspective. In many ways Socal is much better prepared than many other regions of the country (such as Oregon, for example).

I'm less confident. the reason why - is because we have millions of people in the disaster zone. Literally millions. It's simply IMPOSSIBLE for relief agencies to pass out water and food to millions of people over the space of a few days. it cant be done ... its a logistics impossibility. No doubt there will be some quick-action responders. They might set up some distribution points - but those places will get flooded with people. and many more people will never have access. The whole system would work fine if everyone in L.A. stored 2-3 weeks worth of food and water in their houses. Then it should be possible to get by. But as I said, we're lucky if people have 2-3 days worth of stuff. I do not think it will be possible for the military to enforce martial law on Los Angeles during the 1'st week after the Big Quake. More likely, they will be told to guard specific keypoints that need protection. But the city will be very messed up, it's a very big area, and there are far too many confused, frightened and rioting people. I still think "anything is possible" in that first 7-10 day period.

AND just to add something ... food for thought. The Marines at Camp Pendleton - yes they're a "force for good". But the problem for them is that the "escape route" for L.A. that goes south might be intact ... meaning that thousands and thousands of desperate people from L.A. will head towards San Diego. The Pendleton Marines will have their hands completely full - just dealing with those refugees, getting them supplies, and keeping law & order. And the Marines at Twenty-Nine Palms have a totally different headache. First, they are right beside the San Andreas fault - so they will take damage themselves. And then next ... their immediate neighbors are Palm Springs and Palm Desert. Those two populated areas are sitting on TOP of the San Andreas - they will be rubble. So the Twenty-Nine Palms Marines probably cant get any further than Palm Springs - so much help will be needed right there. And if they do go deeper into L.A. - they will just encounter huge crowds of refugees who are trying to escape by going east. It's a mess. No matter what the Marines commanders decide - its Mission Impossible.

Just my $0.02.
And I would certainly like to be wrong about this.

Pete