What primary mechanism is thought to cause the shallow moonquakes detected by Apollo seismometers?
Answer
Tidal flexing from Earth's gravity combined with crustal contraction
The seismic data returned from the seismometers left on the Moon by the Apollo missions revealed continuous, albeit minor, internal vibrations known as shallow moonquakes. These tremors are not indicative of large-scale volcanism or major quakes expected from a very hot interior. Instead, the prevailing theory suggests they are generated by two primary ongoing stresses: the constant flexing of the Moon caused by the gravitational influence of the Earth (tidal flexing), and the ongoing slow compression of the solid shell as the interior continues to contract due to cooling. The existence of these tremors implies the Moon remains physically dynamic.
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