A gas shut-off valve assembly designed to automatically close gas flow in a gas conduit in response to a seismic vibration comprises a door which may be parallel to a port in the conduit and a weight above the door. In some embodiments, the door may be mostly out of the conduit passageway and out of the gas flow allowing for improved gas flow efficiency in the normally open state. Upon sensing a seismic vibration, the weight falls from a platform onto the door causing the door to move into the conduit passageway sealing the passageway. In some embodiments, a port in the passageway is perpendicular to the door's default position in the open state. When the door closes the passageway, the door rotates ninety degrees about an axis into the second closed position to become parallel with the port.