The first thing to know about moonquakes is this: They last forever. While most earthquakes are over in under a minute, moonquakes can last for an afternoon. In the 1970s, at least one 5.5-magnitude moonquake shook the lunar surface at full force for more than 10 minutes straight, then tapered off gradually over the course of several hours.
“The moon was ringing like a bell,” Clive Neal, a geological sciences professor at Notre Dame, told NASA about the Apollo-era lunar seismic data he and his colleagues examined. A strong moonquake would be enough to devastate a hypothetical human settlement—breaching a moon base’s seal and causing a catastrophic loss of oxygen—which is part of why scientists became interested in studying the phenomenon in the first place.
Beginning in 1972, astronauts left seismic sensors on the moon, where they gathered data for about five years until the network was shutdown amid budgetary concerns in 1977. Still, the sensors transmitted evidence of more than 12,000 moonquakes in the time they were running.
Scientists have identified four classifications of moonquakes: deep moonquakes, thermal moonquakes, meteroid impacts, and shallow moonquakes.
Deep moonquakes are the most commonly occurring—scientists counted about 7,000 of them in under a decade, according to an article Neal wrote for Geotimes. They’re just seismic blips, usually measuring 2 or smaller in magnitude. And they happen with such regularity, about every 27 days, that scientists believe they're caused by Earth’s tidal pulls. (Next time you have the opportunity to gaze at the ocean, just imagine the quakes rippling across the moon!)