Seismic gas valves shut off the natural gas flow to a building during a significant earthquake or seismic event. These valves most commonly use either an excessive gas flow sensor or a motion sensing ball mechanism to detect dangerous conditions.
A seismic gas valve, using an excessive gas flow sensor, closes when a gas pressure surge or leak occurs in a pipe or appliance beyond the point where the valve is installed. A seismic valve that detects motion usually consists of a metal ball on a ledge inside a cage. When the mechanism is shaken, the ball falls from the ledge, blocking the intake and preventing natural gas from flowing into the home.