The Moon is actively shrinking and experiencing its own quakes
The Moon is not a dead, static rock. It experiences moonquakes that can last for up to ten minutes—far longer than typical earthquakes on our damp planet. Some of these tremors are caused by tidal forces from Earth's gravity, while others occur as the Moon's interior cools down, causing its crust to contract and wrinkle like a drying grape.
Listening to the Lunar Interior
For centuries, the Moon was widely perceived as a geologically inert world—a cold, rigid sphere whose internal fires had completely burned out billions of years ago. That view shifted dramatically during the Apollo program. Between 1969 and 1972, astronauts deployed passive seismometer packages across several landing sites, establishing the first extraterrestrial seismic network. These instruments transmitted continuous data back to Earth until they were officially turned off in late 1977, revealing that the Moon possesses a complex, active seismic life of its own.
The seismic records collected over those eight years demonstrated that the lunar interior is continuously generating tremors. Far from being an unchanging expanse of dust and ancient craters, the Moon vibrates with thousands of distinct seismic events each year. These tremors, termed moonquakes, originate from a variety of distinct physical processes occurring across different depths and environments, demonstrating that internal dynamic forces remain at work beneath the lunar surface.
Four Varieties of Tremors
Planetary scientists categorised the signals recorded by the Apollo seismometers into four distinct classes of moonquakes. The first and most frequent are deep moonquakes, which originate roughly 700 kilometres beneath the lunar surface. These deep tremors occur with remarkable regularity and correlate closely with the Moon's monthly orbit around Earth, indicating that they are driven by tidal gravitational stresses that repeatedly stretch and compress the lunar mantle.
The second class consists of shallow moonquakes, which occur at depths of only tens of kilometres below the surface. Though far less frequent than deep events, shallow moonquakes are the most powerful seismic phenomena on the Moon, capable of reaching estimated magnitudes above 5.5 on the Richter scale. Their exact origins are associated with structural stresses within the crust, including localized faulting and tectonic adjustments.
The remaining two classes are driven by external and surface-level phenomena. Thermal moonquakes are generated when the intensely cold lunar crust is suddenly heated by the rising sun after the two-week lunar night, causing the surface rocks and soil to expand and crack. Finally, meteoroid impacts produce distinct seismic signatures when extraterrestrial debris strikes the surface at high velocities, sending shockwaves through the surrounding regolith.
Contraction and the Wrinkling Crust
Among the most significant discoveries regarding shallow moonquakes is their link to the Moon's ongoing thermal evolution. Although the lunar core and mantle have cooled considerably since their formation, the interior continues to lose residual heat into space. As this heat escapes, the Moon's interior slowly cools and contracts. Because the brittle outer crust cannot simply shrink smoothly to accommodate the decreasing volume beneath it, it fractures and breaks.
This global contraction forces sections of the crust to push against one another, forming thrust faults known as lobate scarps. These scarps appear as stepped cliffs and winding ridges extending across the lunar landscape, often rising tens of metres high and stretching for kilometres. The formation and ongoing movement along these fault lines generate shallow moonquakes, confirming that the Moon is actively shrinking over geological timescales and that its tectonic landscape remains dynamic.
The Ringing Bell Phenomenon
While earthquakes on Earth typically release their energy in a matter of tens of seconds to a couple of minutes, moonquakes behave in a completely different manner. Once a moonquake begins, the shaking can persist for ten minutes or even longer, with vibrations gradually decaying over extended periods. This prolonged duration caused early mission scientists to famously describe the Moon as ringing like a bell when struck.
This dramatic contrast in seismic behaviour stems directly from differences in moisture and internal structure. Earth's crust is permeated with water and hydrated minerals, which rapidly absorb and dissipate seismic energy through viscous damping. In contrast, the Moon is exceptionally dry and heavily fractured by billions of years of meteorite bombardment. Without water to attenuate the shockwaves, seismic energy bounces and scatters endlessly through the dry, fractured rock layer, sustaining the shaking for extraordinary lengths of time.
Implications for Future Exploration
The reality of an active, vibrating Moon presents critical considerations for long-term human exploration and base construction. Because shallow moonquakes can produce strong, prolonged surface accelerations, permanent structures erected on the lunar surface cannot simply be engineered for static vacuum conditions. Habitats, power systems, and scientific facilities must be capable of enduring sustained shaking that far exceeds the duration of terrestrial earthquakes.
Furthermore, mapping the distribution of lobate scarps and identifying active fault zones has become essential for mission planning. Siting lunar outposts too close to young, geologically active fault lines could expose installations to recurrent shallow tremors and potential surface displacement. As future missions return to the Moon, deploying more advanced and widespread seismometer arrays will be crucial for understanding the complete structure of the lunar interior and mitigating seismic risks.
Key takeaways
•The Moon experiences four distinct types of moonquakes: deep tidal quakes, powerful shallow tectonic quakes, diurnal thermal quakes, and meteoroid impacts.
•Shallow moonquakes are driven in part by global thermal contraction, as the cooling lunar interior causes the brittle crust to buckle into thrust faults called lobate scarps.
•Because the Moon is completely dry and heavily fractured, seismic vibrations are not quickly absorbed, causing moonquakes to ring out for ten minutes or more.