California residents are used to the duck and cover rhetoric and are probably conditioned on what to do during an earthquake. For the rest of us, ahem, New Madrid fault line, who are not accustomed to quakes. . .if you're not around tall buildings, it's best to get outside of your domicile if you can do so safely and get away from buildings into the open. If you're stuck inside, head for a door frame to stand under, or find a heavy duty piece of furniture like a sturdy dining table to get under. Stay away from windows, glass, bookshelves, or anything else that might shatter and/or fall on you.
Once the shaking has stopped, it's a good idea to smell for leaking gas and have a utility shut-off wrench on hand so that you can shut off gas flow to your own home if you're smelling a leak.
Most of the time, thankfully, earthquakes cause little or no damage. If one were to damage your home severly, however, where would you go? Spend hundreds or thousands on a hotel for weeks or longer? If a friendly relative or even neighbor doesn't mind putting you up, that's great, but that situation can get very tense for everyone very quickly. If you have the means, consider a camping trailer or even RV as a back up "home" for emergencies. If you don't have the means for a camper, a simple tent and camping supplies can keep you more comfortable immediately after losing your home -- at least until you figure something out.
Food / Water.
There's the trend. It always comes back to food and water.
It's great to have bottled water on hand. It's great for short term planning, as well as for morale boosting and stress reliever in a long term scenario. However, bottles of water run out quickly. We need a way to procure water anywhere, anytime. A simple LifeStraw or kitchen-use Berkey water filter allow you to have water no matter what. You can stand in line for hours to have a few bottles of water, or you can store some ahead of time and get a water filter to produce hundreds of gallons yourself.
Again, most quakes are not doomsday events. Locals are used to the drill and usually can step up. Supplies are usually distributed effectively on a local basis and this is usually a localized disaster. Food supply will likely not be interrupted, nor will water.
So with earthquakes you usually will want supplies for immediate relief. Think about storing your quake kit (with extra clothes) somewhere out of your area (2-3 hours), preferably at the relative or friend's house where you plan to go if you're homeless temporarily.
On that note, is your gas tank full or do you run it on fumes regularly? Have an emergency supply of 20 gallons which should be enough to get you to your backup location and supplies.
Get your doom fix here. Worldwide quakes would overwhelm services. Most hospitals are running at or close to full capacity on a daily basis under normal everyday conditions. Medical attention would be nearly impossible to get during a nationwide or world wide emergency. You're on your own.
In the megaquake scenario food supplies WILL be interrupted. Water supply WILL be interrupted. Anything that is frequently delivered will not be delivered. That means gas. Everyone will be confused and dazed.
During hurricane Katrina, looting ensued, many police and EMS decided to stay with their families. When local services failed to provide direction, help, and food and water were not handed out, people took it upon themselves to survive by any means necessary. If it looks like services are overwhelmed, you've now got personal security issues to worry about. Assess the situation. Does it look like a temporary condition, or is this not going to be cleaned up any time soon?
Decide whether to stay in your area/home and try to disappear, or whether to get out of town.
The universe is capable of slamming Earth back into the stone ages at ANY time. It's hard, even for me, to imagine world wide quakes that would decimate all population centers. . .but anything is possible.
Quakes, for me, are a quick temporary issue similar to power outages. If you are worried about the "big one" though, massive decimation would certainly lead (eventually) to a loss of emergency services, social unrest, and interruption of supplies. The good news is that food could still be mass produced most likely, although transporting it anywhere might be impossible. Infrastructure such as roads and bridges may be destroyed, making the disaster felt far away from the epicenter(s) due to supply corridors being closed.