Half a century ago, the Apollo astronauts left short-lived seismometers on the lunar surface. They found that the moon was alive and kicking. Some tremors deep below the surface likely were caused by Earth’s gravitational pull. Others were vibrations from meteorite impacts. Still others resulted from expansion of the moon’s chilly surface every two weeks when the sun rose.

There were also shallow moonquakes, just a few miles beneath the surface. Unlike the other categories of quake, these convulsions couldn’t be satisfactorily explained. But a study published Monday in Nature Geoscience suggests that they were triggered across myriad young faults by a combination of escaping internal heat and Earth’s gravitational pull.

The discovery suggests that the moon is still tectonically active, and raises the possibility that future lunar bases may be vulnerable to shallow moonquakes. It also prompts questions about the moon’s evolution.

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The moon, likely born from a violent impact four billion years ago, is just over one-quarter the radius of Earth. Its diminutive size led to the belief that any internal heat should have escaped into space long ago. As a result, the driving engine of most major geological activity should have shut down. But this new evidence suggests it hasn’t given up the ghost just yet.