Apollo-era moonquakes reveal that the moon may be tectonically active

The moon may still be kicking.

Rumbles recorded decades ago by seismometers at Apollo landing sites are probably linked to young faults mapped by NASA’s Lunar Reconnaissance Orbiter, scientists say. Eight of those moonquakes occurred within 30 kilometers of fault scarps, steplike cliffs on the lunar crust that mark places where one side of a fault has thrust up or slipped down. If true, the finding suggests that the moon continues to be tectonically active today, researchers report online May 13 in Nature Geoscience.

Knowing more about that activity, including where the moon’s surface is still on the move, could help scientists identify where — and where not — to land future spacecraft (SN: 11/24/18, p. 14).

Unlike Earth, the moon’s quakes aren’t produced by numerous, large tectonic plates that split apart, collide or slide past one another. Instead, like Mercury and Mars, “the moon is basically a one-plate planet,” says Thomas Watters, a planetary scientist at Smithsonian Institution in Washington, D.C., who led the study.

Still, even one-plate objects can have quakes (SN Online: 4/23/19). As those objects cool over time and the interior contracts, their hard outer shell, or lithosphere, also compresses and cracks. That compression can produce quakes. As the moon’s interior has cooled, its radius is thought to have shrunk by about 100 meters. But whether the moon is still tectonically active today has been a mystery.

In 2010, Watters led a team that examined images from the Lunar Reconnaissance Orbiter, launched in 2009, and identified numerous sinuous cliffs distributed widely across the surface. Called lobate scarps, those features, from tens to a few hundreds of meters high, represent thrust faults, places where the surface is contracting as the moon cools. Ultimately, the team estimated that those scarps were no older than 50 million years.

But that was just the maximum estimation, Watters says. He suspected the faults might be much, much younger.

So the team turned to the thousands of moonquakes detected from 1969 to 1977 by NASA’s Passive Seismic Experiment, consisting of four seismometers installed by astronauts at Apollo landing sites. Most moonquakes were small and originated deep inside the moon. But 28 quakes were larger and shallower, originating within just 200 kilometers of the surface. Even then, some scientists suspected that the moonquakes might be related to ongoing tectonic activity.

“They had the seismic data, but what they didn’t have was potential sources,” Watters says. Now, LRO had provided evidence of abundant faults, “thousands of potential sources.”

But pinpointing the origins of the quakes, and possibly linking them to observed faults, was tricky, because the seismometers were clustered relatively close together at the landing sites. So the team used a mathematical program to better identify the quakes’ epicenters, and then tried to map them to the scarps. Epicenters more than 30 kilometers away from any scarp were considered unrelated.

“We found eight of these within that 30-kilometer, cutoff distance,” Watters says, close matches that suggest that the moon is still actively contracting. “This is data that’s just 40 years old,” Watters says. “If we detected these slip events 40 years ago, then these faults are still active.” That, he says, must also mean that the moon still has a lot of heat in its interior.

Still, the pattern of the faults was puzzling. A global contraction of the moon’s surface should create a random pattern of faults. Instead, the faults had a distinct pattern: In the equatorial and mid-latitude regions, they tended to run north-south. Near the poles, they were oriented east-west.

The only other force big enough and close enough that could act powerfully on the moon is Earth. So the team examined the timing of the moonquakes relative to the moon’s position along its elliptical orbit around Earth. The scientists found, to their surprise, that 18 of the 28 recorded shallow quakes happened when the moon was farthest from Earth, called its apogee.

It’s counterintuitive, but that finding actually supports the idea that Earth is producing additional stress on the moon, Watters says. “Stress is force over a unit area. When the moon is at apogee, the unit area the Earth is acting on is actually greater.” The moon also slows down just a bit as it reaches apogee, giving stresses caused by changes in the pull of Earth’s gravity more time to accumulate, and making quakes more likely.

“I would have been surprised the moon was tectonically active had you asked me 10 years ago,” says Berlin-based planetary geologist Amanda Nahm of the Arctic Planetary Science Institute. “The more we learn about these small bodies, the more we realize that they are so much more interesting and dynamic than previously thought,” says Nahm, who was not involved in the study. “The moon is no longer considered to be ‘dead.’ ”

Mapping out which faults are active could be key to any future plans for a longer-term presence on the moon. “I wouldn’t want to be within 30 kilometers of one of these faults,” Watters says. And the reduced gravity could produce significant shaking from even a weak moonquake. “It’s not going to take a lot of shaking to knock you off your feet.”