Abstract: A gas shut-off valve is provided for closing off a natural gas pipeline in the event of an earthquake or other seismic disturbance. The valve has an inlet and an outlet aperture communicating with a central chamber, and a valve seat provided between the inlet and the outlet apertures. An annular channel is located above the chamber, and at least one ball rolls along the channel. A guide ramp is provided which allows a ball to leave the channel and fall onto the valve seat during the earthquake, blocking gas flow between the inlet and the outlet apertures. The guide ramp has a concave surface which provides a continuous path for the ball from the channel. The ball has a center of gravity substantially displaced from an initial position over the channel to a second position directly over the valve seat when it has rolled to a crest of the guide ramp under influence of the earthquake.