Science & Technology

Moonquakes Can Last for Hours, Which Doesn't Bode Well for a Lunar Base

Earth isn't the only planet that shakes. Any celestial body can quiver with the right kind of force, including the moon. And because the moon is quite a bit different than our own planet, moonquakes can get really weird — and surprisingly violent.

Buzz Aldrin deploys a seismometer in the Sea of Tranquillity.

Quakes in Four Flavors

Quakes on the moon don't happen quite like they do on Earth. Earthquakes are a result of the movement of tectonic plates: the rocky puzzle pieces that make up our planet's crust. The moon doesn't have tectonic plates, so they're not a result of tectonic activity. Instead, moonquakes happen in one of four ways. Thermal moonquakes result from the strain of dramatic temperature changes over the lunar day (roughly 29 Earth days). Though weak, they're incredibly frequent, happening abruptly two Earth days after sunrise and stopping just as suddenly after sunset.

Deep moonquakes are caused by the tidal pull of the Earth — the same thing that happens in reverse to cause our ocean tides — and happen even more often. Starting in 1972, the Apollo missions used a network of four seismometers to measure moonquakes for about eight years. In that time, they recorded more than 7,000 deep moonquakes. The third type of moonquake occurs as a result of meteoroid impacts.

Representative lunar seismograms from the Apollo 16 station.

Tuning Fork in the Sky

But by far the strongest has to be shallow moonquakes. That same Apollo seismic network saw 28 of these events between 1972 and 1977, and a few registered around 5.5 on the Richter scale — that's strong enough to damage buildings here on Earth. Even worse, they last forever. "Researchers have observed the maximum signal from a shallow moonquake to last up to 10 minutes with a slow tailing off that can continue for hours in total duration," writes lunar researcher Clive Neal.

Why do moonquakes last so much longer than earthquakes? Water. "Water weakens stone, expanding the structure of different minerals," Neal told NASA. "When energy propagates across such a compressible structure, it acts like a foam sponge — it deadens the vibrations." Because the moon is so dry, it vibrates less like a sponge and more like a tuning fork. That means that even if a severe moonquake is only strong enough to do slight damage to any future lunar base — a project some countries are seriously considering — the quake's persistence could be much worse than its magnitude. "Any habitat would have to be built of materials that are somewhat flexible," Neal says. "We'd also need to know the fatigue threshold of building materials." Otherwise, several hours of quaking could leave future moon residents exposed to the moon's virtually nonexistent atmosphere.

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Written by Ashley Hamer May 4, 2017

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