Seismic activity on the Moon resulting from internal sources (differential cooling or tidal distortions) or external sources, e.g. meteoritic impact. It is the lunar equivalent of an earthquake. All moonquakes are less than 2 on the Richter magnitude scale. About 3000 per year were recorded by the Apollo seismometers. The lunar seismic signals have very low attenuation and a large degree of wave scattering, consistent with travel through a brecciated crust. Most moonquakes occur at about 1000 km depth and are probably induced by tidal stresses, related to periodic changes in the Earth–Moon distance. No lunar tectonic activity is evident. Rare surface moonquakes are caused by the impacts of meteorites.