Half a century ago, the Apollo astronauts left short-lived seismometerson 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.
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