If gas leakage is detected, shut off the gas at the valve which is on the pipe that comes up out of the ground and goes into your gas meter. This valve can be turned with a crescent wrench so the flat part of the valve is perpendicular to the direction of the pipe (this is the closed position). Better yet, get and keep a simple gas shut-off wrench tied to the gas meter with twine. (Relight any pilots after turning back on) Run a moderate quantity of cold water into your bathtub and other containers in case service will be disrupted. Shut off the incoming (cold) valve to your water heater in case you need the water in it. If your water is contaminated, or it sputters or stops, shut off the water service line valve where it comes into your home. You should know where all these valves are and how to operate them before an earthquake.