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Earth is active down to its very rocks. Earthquakes and volcanoes shake our planet, and the continents themselves drift lazily across the surface. Compared to Earth, the Moon is a cold, hard rock. But even lunar rocks are more active than you might think.

Fifty years ago, people walked on the Moon. They weren’t there just for fun – they were there to do science. As well as taking photos and collecting rocks, several Apollo missions placed earthquake detecting seismometers on the Moon.

For almost a decade, these machines sent measurements back to Earth. They detected some small shakes, but at the time, scientists didn’t find anything too exciting in the measurements.

We’ve learned a lot about earthquakes in the fifty years since those missions. We have much better computers and computer programs. So what can we learn re-examining old measurements with new techniques?

Quite a lot, actually! The new analysis, funded by NASA, found much more precise locations for lunar quakes. Eight of those quakes happened very close to visible wrinkles on the Moon’s surface. So those wrinkles aren’t just ancient valleys. They’re active fault lines, able to produce quakes of magnitude 5 or even greater.

The research also uncovered a clue to the origin of these quakes, hidden in the calendar! Six of the eight rift-quakes happened when the Moon was at its apogee, the point in its orbit where it is furthest from Earth. It’s clear that our planet’s gravity is to blame. While the Moon causes high and low tides across our oceans, Earth is causing massive moonquakes in return.

All this seems catastrophic, but it’s not that bad. Earth has more frequent and larger quakes than our Moon. But if humans are headed to the Moon to stay, it’s nice to know where the ground is most stable. After all, it’s a lot easier to fix a crack in your wall on a planet with air!

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