Abstract: A seismic activated gas valve includes a valve body defining a vertically oriented interior chamber having a input port formed in the upper portion thereof and an output port formed in the lower portion thereof. A magnetical ball retainer is supported in the uppermost portion of the valve body and supports a steel closure ball by magnetic attraction under normal conditions. An annular steel washer and a spaced apart magnetic washer captivate a sealing O-ring to form a valve seat between the input port and output ports of the valve body. Under conditions of substantial seismic activity, the steel closure ball is separated from the magnetic ball retainer and falls downwardly through the vertical chamber and comes to rest against the valve seat in a sealing attachment. The magnetic washer beneath the sealing O-ring attracts the steel ball and resists subsequent reopening of the valve due to vibrations imparted to the closure ball.