Moonquake Mystery Deepens

From Earth Magazine:

Between 1969 and 1972, five Apollo missions installed seismic stations at their landing sites on the nearside of the moon. Because the moon was thought to be seismically dead, the instruments were left almost as an afterthought to detect meteor strikes. But from the time the stations were switched on until they were decommissioned in 1977, they recorded hundreds of internally generated moonquakes, some as strong as magnitude 5.5 on the Richter scale.

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