A moonquake is the lunar equivalent of an earthquake, i.e., a quake on the Moon. Moonquakes are much less common and weaker than earthquakes. Information about moonquakes comes from seismometers placed on the Moon by Apollo astronauts from 1969 through 1972. The instruments placed by the Apollo 12, 14, 15 and 16 missions functioned perfectly until they were switched off in 1977.

Classification

• Deep moonquakes (~700 km below the surface, due to tidal stresses from Earth and Sun)
• Meteorite impact vibrations
• Thermal moonquakes (the frigid lunar crust expands when sunlight returns after the two week lunar night)
• Shallow moonquakes (20 or 30 kilometers below the surface, may be caused by large rims on young craters slumping)

The first three kinds of moonquakes mentioned above tend to be mild; however, shallow moonquakes can register up to 5.5 on the Richter scale. Between 1972 and 1977, twenty-eight shallow moonquakes were observed. On Earth, quakes of magnitude 4.5 and above can cause damage to buildings and other rigid structures. Vibration from shallow moonquakes usually continues for more than ten minutes compared to around one or two minutes on Earth.

The moonquakes are much longer than earthquakes because there is no water on the Moon. Water weakens rocks through chemical weathering causing different minerals to expand. This makes the ground act like a sponge and so the vibrations are soon absorbed making the earthquake end sooner. The Moon does not absorb the vibrations as quickly as the Earth as it is more rigid and so keeps vibrating for longer, extending the moonquakes far beyond the biggest earthquakes.

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