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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!)
Thermal quakes happen in response to the temperature changing from night to day and day to night. And meteroid impacts are pretty much what they sound like—seismic events triggered by the creation of a crater.
The really big lunar quakes, though, are the shallow ones. Scientists still don't know exactly what causes these rarer moonquakes, but seven of the 28 recorded up to 1977 exceeded magnitude 5. A quake of that size on the moon lasts longer than it would on Earth, where water helps tamp down a quake. “Thus, seismic energy is more efficiently propagated through the moon, which is incredibly dry,” Neal wrote. The composition of lunar rocks may also play a role.
Another mystery is why shallow moonquakes contain more energy at higher frequencies than quakes of similar magnitudes on Earth. And the lunar datapoints scientists have, which come from only a handful of sensors over a relatively short period of time, aren’t enough to help explain why. “This is why we need a globally distributed lunar-geophysical network that will last at least a decade,” Neal told me.
Scientists also have their attention on Mars, where they hope to find definitive proof of marsquakes in the coming years. The Mars mission InSight, scheduled to launch in March, will carry a seismometer to the Red Planet, a NASA spokesman told me on Monday.
“We assume that there are quakes on Mars, but none have been measured—as yet!” Neal said. Back on Earth, a rich network of seismometers has been in place for decades. Which means much of what scientists can glean about the seismic activity on other celestial bodies comes from a planet that’s unlike its neighbors in all kinds of ways. “The moon and Mars, being smaller than Earth, have no active plate tectonics,” Neal told me. “So they are ‘one-plate planets.’”
“Therefore, rather than seismicity on Mars, [the] moon and elsewhere telling us about seismicity on Earth, we have used Earth’s seismicity to tell us about other planets,” Neal said. Which is not a perfect analog—but, hey, it’s a start. The unknown, as is its way, far outweighs what little scientists have learned about quakes in space.
“We do not know what causes the largest moonquakes nor where they are precisely located,” Neal said. “We know nothing about Mars, so the InSight mission is critical.”