Study links 'moonquakes' with lunar shrinking

You’ve heard of earthquakes, but you probably haven’t heard of moonquakes. A new study involving researchers from UBC, NASA and the Smithsonian has found that shallow quakes on the moon can be linked to young faults — cracks in the moon’s crust — on the lunar surface and the moon’s slow but gradual shrinking.

“This was actually a study using a really neat combination of old[er] data and new[er] data,” said Dr. Catherine Johnson, an author of the study and professor in UBC’s department of earth, ocean and atmospheric sciences.

The researchers analyzed existing data collected from five of NASA’s Apollo missions to the moon, during which astronauts installed seismometers — instruments that measure ground motion — on its surface. Four of these seismometers operating from 1969 to 1977 recorded 28 shallow moonquakes. Although the Apollo data dates back 40 or 50 years, Johnson explained that in the geological history of the moon, it is still considered very recent.

Other researchers had looked at the Apollo data before and had roughly estimated where these quakes took place, but whether they occurred due to faults in the moon’s surface was unclear. Newer, higher resolution lunar images from NASA’s Lunar Reconnaissance Orbiter satellite show what appear to be young faults on the moon’s surface, but researchers did not know whether these faults were active and were capable of producing shallow moonquakes.

To find answers, Johnson and her colleagues put these two datasets together. “It was a really nice study that says that basically you can associate the locations of these quakes with mapped faults [from satellite images],” she explained.

Using modern analysis techniques on the Apollo data, the researchers tried to get better location estimates of where the quakes occurred. They then compared these new locations to the locations of faults from the recent satellite images. Lastly, the researchers compared the amount of observed shaking from seismometer data to the amount of shaking that would be expected if a quake occurred at a fault.

They found that eight of the twenty-eight shallow quakes were within thirty kilometres of faults visible in lunar images, which is close enough that they could attribute the shallow moonquakes to the activity of the faults.

“It was wonderful for me personally to see this come to some fruition,” said Johnson, who had previously worked on the same research question with Jean-Francois Blanchette-Guertin, a former student, using the old location estimates and old mapped faults. Using the newer location estimates and satellite images in the current study helped her find answers that she and Blanchette-Guertin had not been able to get.

['auto'] Courtesy Charles Kosman

“It was a circle in terms of ideas that you have that you can’t really make progress on, and a few years later they come back around again … it just needed a new dataset and a new technique!”

According to Johnson and the study, these quakes seem to occur because the moon is shrinking. Due to the moon’s cooling interior, the moon contracts very slowly, having shrunk less than 0.06 per cent in size over the course of 4 billion years. This shrinking — along with the earth’s gravitational forces on the moon — builds up stress and creates what are called “young thrust faults” on the moon’s surface. When enough stress builds, the faults slip past each other and moonquakes are produced.

“We know that this is true, for example, on Mercury … Mercury has these enormous faults — we don’t know whether they’re active today, of course — but it has enormous faults on its surface that are the result of this shrinking and cooling,” Johnson added.

Although the characteristics of shallow moonquakes are not fully known, Johnson said that they are distinct from moonquakes that occur very deep in the lunar interior, and are likely similar to the kind of quakes that occur in the middle of Earth’s continents. She hopes that the study’s findings can help inform future exploration of the moon, especially for astronaut safety.

“Oh, it’s fun, it’s always fun to discover things,” Johnson said, when reflecting on her part in the study’s discovery.

“I would say [the results were] not unexpected to me because I’d already looked at this a long time ago with another student, and so I wasn’t surprised. But it’s nice to be able to say something more definitive … To actually be able to put some numbers on it and, you know, quantify how certain you are.”