During the 1972 Apollo 17 mission to the Moon, astronauts Eugene Cernan and Harrison Schmitt had to maneuver their lunar rover in a zig-zag pattern over the stair-step cliffs of the Taurus-Littrow valley, measuring tens of meters high and extending for several kilometers. Now, experts believe they know what caused such dramatic differences in the lunar landscape: Moonquakes.

According to a new study published in Nature Geoscience, the Moon shrinks as its interior cools, resulting in the loss of around 50 meters (150 feet) over the last several hundred million years. As it shrinks, the brittle lunar surface crust breaks to create “thrust faults” where one section of crust pushes up over another.

“Our analysis gives the first evidence that these faults are still active and likely producing Moonquakes today as the Moon continues to gradually cool and shrink,” said Thomas Watters, senior scientist at the Smithsonian's Center for Earth and Planetary Studies, in a statement. “Some of these quakes can be fairly strong, around five on the Richter scale.”

During several Apollo missions, astronauts placed seismometer instruments on the surface of the Moon that recorded 28 shallow Moonquakes between 1969 and 1977, ranging in magnitude from about 2 to 5 on the Richter scale. Advanced analysis of this data using a mathematical program allowed researchers to pinpoint where and when these quakes occurred.

Eight of the quakes were within 30 kilometers (18.6 miles) of lunar surface fault lines visible from above – a distance researchers say means the quakes can be attributed to the faults. Of those, six earthquakes happened when the Moon was at its furthest point from Earth. Additional stress from Earth’s gravity at this time could have increased the likelihood of these faults giving rise to quakes.

Over the course of 10,000 simulations, researchers determined there is a less than 4 percent chance that the quakes occurring at this fault scarp are purely coincidental. Though meteoroid impacts and other events can cause quakes, their signature is measured differently from quakes that are produced during fault slip events.

“We think it’s very likely that these eight quakes were produced by faults slipping as stress built up when the lunar crust was compressed by global contraction and tidal forces, indicating that the Apollo seismometers recorded the shrinking Moon and the Moon is still tectonically active,” said Watters.

NASA’s Lunar Reconnaissance Orbiter Camera (LROC) has captured over 3,500 images of the fault scarp since 2009, some of which show landslides and boulders over surface that has been recently exposed. Researchers hope to compare pictures of fault regions across different times in order to capture recent Moonquake activity.

“Establishing a new network of seismometers on the lunar surface should be a priority for human exploration of the Moon, both to learn more about the Moon’s interior and to determine how much of a hazard Moonquakes present,” said study co-author Renee Weber.