Decades ago, seismometers placed on the Moon’s surface during the Apollo missions measured moonquakes reaching upwards of 5.5 on the Richter scale. These tremors last ten minutes or longer under the Moon’s cold, dry surface. Researchers struggled to pinpoint the source of the lunar shaking, but new analysis is shedding light on their origin.
The tremors weren’t the side effects of an asteroid impact. Instead, new research is pointing the finger at fault scarps crisscrossing across the moon’s surface. NASA describes them as sort of “small stair-step shaped cliffs when seen from the lunar surface.” NASA’s Lunar Reconnaissance Orbiter has imaged thousands of fault scarps from orbit such as the one below.
Because there was only a little bit of data to work with, researchers turned to an algorithm to help pinpoint the epicenter of these decades-old moonquakes. Of the 28 quakes registered by the seismometers, eight of them were believed to be centered within 30 kilometers of faults visible in lunar images. And six of those occurred while the Moon was at its farthest point from Earth.
Tidal stresses from Earth’s gravitational pull on the Moon adds a little extra stress to these faults making moonquakes more likely according to the researchers.
What’s the driving force behind what appears to be a still tectonically active Moon? The Moon’s interior continues to cool, and as it does so, shrinks. It’s a slight change (150 feet over the last several hundred million years), but it’s enough for parts of the Moon to push into each other and for one side to thrust upward.
Here’s another image illustrating two sections of the Moon pushing together and causing a fault scarp to form.
“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 Thomas Walters (senior scientist at the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum), lead author on the new study.
What about some other explanation? Meteor impacts? While an impact can cause a moonquake, the seismic signature it produces is different from a tremor coming from within the Moon.
“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 co-author Renee Weber, a planetary seismologist at NASA’s Marshall Space Flight Center.
A still tectonically active Moon would have significant implications for any potential lunar base. Engineers would need to account for not only the strength of the tremors but also how long they last. A 5 on the Richter scale is enough to shift large furniture here on Earth. But they also tend to last just a minute or two. The folks building the lunar base will need to make sure any base could handle the longer shaking.
Thanks to seismometers placed decades ago and images captured by LRO, mission planners have a good idea on where to go next to learn more about the Moon’s potential tectonics.