Count on there being no water. Water mains will be broken. The pumps won’t work. The aqueducts that supply water will be ripped apart. Our reservoirs may be breached and the water lost. It will be a long time before they are fixed.
Unless you prepare you will have only the few bottles in your refrigerator, the water in your water heater tank, and the water in your toilet tank. That's it, period.
You should have a good supply, at the minimum enough for at least 2 to 3 weeks. An adult requires one gallon of water a day. Do the math. That’s 21 gallons per person you need to have on hand if the water system is down for 3 weeks.
If you experienced the Northridge and Sylmar quakes you'll remember that within 30 minutes of those earthquakes supermarkets in the San Fernando Valley were stripped of food and water. That's what's going to happen when the big one hits, except it will be all of Southern California, not just the San Fernando Valley. How long would the food you have on hand last?
Lay in a supply. Get bags of dried beans, bags of rice, cans of dried potatoes, and boxes of dried milk. Put them in large plastic containers with airtight lids so insects and rodents can't get to them. Consider investing in freeze-dried backpacking food and military MRE’s [meals ready to eat].
After the quake you'll want to use the food that's in your refrigerator first, then the freezer, because it's going to spoil without electrical power.
As for cooking, the natural gas lines [you do have an earthquake valve on the gas line don't you?] will be down also. You should be OK with a propane BBQ. But store an extra 5 gallon propane bottle or two.