The Moon looks unchanging from Earth, but its interior is slowly cooling and contracting. As the Moon shrinks, pressure builds in its brittle crust. When sections of that crust suddenly slip along a fault, the ground shakes: a moonquake. Scientists estimate that the Moon’s diameter has decreased by about 150 feet (50 meters) over the past several hundred million years.
Wrinkles in a rocky world
Imagine a grape drying into a raisin. The Moon also develops wrinkles as it contracts, but its crust cannot bend like grape skin. Instead, it breaks. One piece of rock can be pushed up over another, creating a fault and a step-like cliff called a scarp. NASA’s Lunar Reconnaissance Orbiter has photographed thousands of these features across the lunar surface.
There is evidence that some of these faults may still move. Apollo astronauts left seismometers on the Moon, and four of them recorded 28 shallow moonquakes between 1969 and 1977. A later analysis found that eight occurred near faults visible in lunar images—a strong clue linking the shaking to the Moon’s contraction, though Earth’s gravitational pull also adds stress.
Why moonquakes matter
Not every moonquake comes from shrinking. Earth’s gravity can trigger deeper quakes, while meteorite impacts and rapid temperature changes can also shake the surface. The contraction-related quakes are especially interesting because they show that the Moon is not entirely geologically quiet.
They matter for future visitors, too. NASA’s study of the lunar south pole found that shaking could destabilize loose soil on some slopes. Understanding where faults lie can help planners choose safer places for equipment and long-term habitats. The next time the Moon appears perfectly still overhead, remember: beneath that familiar face, an ancient world is cooling, cracking and occasionally trembling.
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