Now, new research has tracked the epicenters of each small moonquake, and found that eight of them could be traced to within 20 miles of so-called fault scarps.
These moonquakes likely happen because the moon is quivering as it shrinks, researchers added.
"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", Thomas Watters, senior scientist in the Center for Earth and Planetary Studies at the Smithsonian's National Air and Space Museum in Washington, said in the press release.
What's more, most of the Moonquakes occurred during times of the month when the tidal stresses between the Moon and Earth were at their greatest, which would make those faults more likely to slip and thus cause a quake.
The scientists discovered that the moon had been shrinking as its interior cooled down and may be getting even smaller as its crust becomes brittle- a bit like what happens to a grape as it dries out to become a raisin. "Just as a grape wrinkles as it shrinks down to a raisin, the moon gets wrinkles as it shrinks".
These faults push one part of the lunar crust up and over the adjoining part, said University of Maryland geologist and study co-author Nicholas Schmerr.More news: Legendary actress and singer Doris Day dead at 97
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As a result, researchers were able to "tentatively attribute" the recorded quakes to the faults.
The moonquakes recorded aren't minor, they are "fairly strong, around five on the Richter scale". "You don't often get to see active tectonics anywhere but Earth, so it's very exciting to think these faults may still be producing moonquakes".
Six out of the eight tectonically active moonquakes occurred when the Moon was at or close to its apogee, the point where it's most distant from Earth and where the diurnal and recession stresses create the most compression near the tidal axis.
US astronauts placed seismometers on the lunar surface during the Apollo 11, 12, 14, 15 and 16 missions, recording 28 shallow quakes up to nearly 5 magnitude, which is moderate strength. These tracks are evidence of a recent quake because they should be erased relatively quickly, in geologic time scales, by the constant rain of micrometeoroid impacts on the Moon. Schmerr designed the algorithm that re-analyzed the Apollo data. The Lunar Reconnaissance Orbiter Camera (LROC) has captured over 3,500 images of the fault scarps on the lunar surface, which show boulders at the bottom of bright patches on the slopes of fault scarps or landslides. The study of Apollo seismic data and analysis of more than 12,000 of the orbiter's photos were published Monday in the journal Nature Geoscience.
"For me, these findings emphasise that we need to go back to the moon". "It is also a testament to how much can be gained by human spaceflight to the surface of other worlds and underlines the unbelievable potential for future missions back to the moon and, hopefully someday, Mars".