Abstract: An inertial valve apparatus is disclosed for automatically stopping the flow of liquids or gases in the event of an earthquake or similar vibration, and simultaneously causing the liquid or gas to be vented. The apparatus includes a spring loaded double-seated valve that is held in an unactuated position by a latch. A mass is adjustably suspended, according to the sensitivity to vibrations desired, along a trigger arm connected to the latch, so that movement of the mass in response to vibrations causes the latch to release, permitting the valve to move to an actuated position where the normal flow of liquid or gas is stopped and venting takes place.