Part of being prepared is being educated. It's a good idea to know how to turn off your gas in case of a gas-leak, which can happen at any time, but may be more likely in an earthquake. You should store a wrench next to the gas meter or in an easily accessible location so that you can turn the gas off if needed.
"Check for gas leaks. If you smell gas or hear blowing or hissing noise, open a window and quickly leave the building. Turn off the gas at the outside main valve if you can and call the gas company from a neighbor's home. If you turn off the gas for any reason, it must be turned back on by a professional."1
From The San Fransisco Chronicle:
"It's estimated that 90 percent of the damage caused by the 1906 San Francisco earthquake was due to fires caused by ruptured gas lines - over 30 fires leveled 490 city blocks. In light of this, it would seem prudent to shut off the gas main in the event of a major earthquake, right? Not so fast says Pacific Gas and Electric Co. spokesman David Eisenhauer. "If you do not smell or hear gas do not shut off," says Eisenhauer. "If you shut it off and there's no damage it could take days to turn it back on." Once shut off, PG&E also does not recommend residents turn it back on themselves. This is because either someone from PG&E or another trained professional should inspect the house first for any damage to gas lines."2
If you determine that you need to shut off your gas, simply turn the valve one quarter-turn with a wrench (as shown in the diagram above). You can significantly reduce the likelihood of a gas leak in an earthquake by securing your water heater. Water heaters often rock loose during and earthquake and sever the gas connection.
2- How To Prepare For An Earthquake (Paul Kilduff, special to The San Franscisco Chronicle)
Diagram Source: http://www.doh.wa.gov/phepr/handbook/utility.htm