I picked up an earthquake gas shutoff device at Rubenstein Supply Company (96 Woodland Avenue, San Rafael, CA 415-454-1174) - perhaps the best source for plumbing supplies here in southern Marin.

A year or so ago I had picked up a flyer at the County explaining the different gas shutoff devices and indicating that they were mandated in Marin. 

Apparently there are two technologies, one blocks the gas line when the natural resistance of devices is eliminated and the other one is triggered by vibration. 

Rubenstein's had only one device, the Little Firefighter, for $96, and I splurged and bought it. The next time the plumber was out, I had him install it. Mistake! Never install a vibration-triggered shutoff device until your construction is complete - this danged thing has shut off the gas at least 50 times in the past few months...