An Etymological Dictionary of Astronomy and Astrophysics

A → seismic event occurring on the → Moon; the lunar equivalent of an → earthquake.

Moonquakes were first detected by the → seismometers placed on the Moon by Apollo astronauts from 1969 through 1972. The instruments placed by the Apollo 12, 14, 15, and 16 missions were functional until 1977. Unlike earthquakes, moonquakes are not believed to be caused by → tectonic plate movement, but by → tidal forces between Earth and the Moon.

There are at least four different kinds of moonquakes: (1) deep moonquakes about 700 km below the surface.

They occur at monthly intervals at about 100 distinct sites, indicating that these moonquakes are caused by → stresses from changes in lunar tides as the Moon orbits the Earth; (2) vibrations from the impact of → meteorites; (3) thermal quakes caused by the expansion of the frigid crust when first illuminated by the morning sun after two weeks of deep-freeze lunar night; and (4) shallow moonquakes only 20 or 30 km below the surface (science.nasa.gov/science-news).

See also: → moon; → quake.