A moonquake is a seismic event occurring on the Moon, analogous to an earthquake on our own planet. These tremors reveal that the Moon, long considered geologically inactive, still experiences internal dynamic processes. The existence of these lunar vibrations was first confirmed by the seismometers deployed by astronauts during the Apollo missions, beginning in 1969. Analyzing the data collected from these instruments has provided our knowledge of the Moon’s interior structure.
The Four Primary Sources of Moonquakes
Scientists classify moonquakes into four distinct categories based on their origin. The most frequent type is the deep moonquake, originating hundreds of kilometers below the surface, usually around 700 kilometers deep. These events are primarily caused by the immense tidal forces exerted on the Moon by Earth’s gravity, which continuously tugs and stretches the lunar interior. They tend to occur in predictable clusters, often correlating with the Moon’s closest approach to Earth.
A second type is the shallow moonquake, which originates much closer to the surface, typically between 20 and 220 kilometers deep. These events are far less common than deep quakes, but they are significantly more powerful, sometimes registering up to magnitude 5.5 on the seismic scale. Shallow moonquakes are thought to be caused by the Moon gradually shrinking as its interior cools, a process that creates stress fractures, or thrust faults, in the brittle lunar crust.
Thermal moonquakes represent a third category, driven by the extreme temperature swings between the two-week-long lunar day and night cycles. These quakes are very shallow, occurring near the surface as the crust rapidly expands when exposed to sunlight and then contracts during the frigid night. The constant expansion and contraction of the surface rock creates thermal stress that releases a seismic signal. The fourth source of seismic activity results from meteoroid impacts, where space debris constantly strikes the Moon’s unprotected surface, generating detectable vibrations.
How Moonquakes Differ from Earthquakes
The physical characteristics and behavior of moonquakes are profoundly different from their terrestrial counterparts, largely because of the Moon’s distinct geological structure. Earthquakes are caused by the movement of tectonic plates, but the Moon lacks this global mechanism entirely. The Moon’s interior has cooled much faster than Earth’s, resulting in a thick, dry, and brittle structure.
This structural difference leads to an altered experience of the seismic event. On Earth, water and softer rock materials act to dampen seismic waves, causing an earthquake’s shaking to die out within seconds. In contrast, the Moon’s extremely dry, fractured crust and lack of an atmosphere provide almost no damping effect. As a result, moonquakes cause the Moon to “ring” like a bell, with vibrations continuing for ten minutes or even hours.
While some shallow moonquakes can reach a significant magnitude of 5.5, most lunar seismic events are much weaker than common earthquakes. The propagation of seismic energy is also distinct, with waves scattering differently through the highly fragmented upper layer of the Moon, known as the regolith. This scattering causes the moonquake signals recorded by instruments to lack the clear reflected arrivals that are typical of terrestrial earthquakes, complicating the mapping of the Moon’s internal layers.
Measuring Seismic Activity on the Moon
Our understanding of lunar seismic activity comes from the Apollo Passive Seismic Experiment (PSE), a network of instruments deployed by astronauts. Seismometers were placed on the lunar surface during the Apollo 11, 12, 14, 15, and 16 missions. This network was the first to successfully measure seismic activity on a body other than Earth.
The instruments detected both internally generated moonquakes and external events, such as meteoroid impacts. The seismometers deployed by Apollo 12, 14, 15, and 16 were part of the Apollo Lunar Surface Experiments Package (ALSEP) and were powered by radioisotope thermoelectric generators (RTGs). This power source allowed the four-station network to operate continuously and transmit data back to Earth until the instruments were formally switched off in September 1977.
The Apollo 11 seismometer, which was solar-powered, operated for only three weeks before overheating and ceasing transmission. However, the later, more robust stations provided nearly eight years of continuous data. This long period of data collection provided scientists with thousands of events to analyze, forming the singular database that defined lunar seismology for decades.