Moonquakes fall broadly into three categories. Deep moonquakes occur hundreds of miles beneath the surface of the moon, triggered by the tidal pull of Earth’s gravity tugging and stretching the Moon’s internal structures. Shallow moonquakes originate near the surface, depths of 20-30 km and can be more intense. These are caused by the Moon shrinking as it cools, a process which has continued since the Moon first formed nearly 4.5 billion years ago. Finally, impacts from rocks on or near the surface, caused due to thermal expansion or contraction, produce surface quakes. The shallow quakes detected by Apollo instruments have been recorded at strengths up to around 5.5 on the Richter scale, and some shake the lunar ground for over ten minutes, far longer than typical terrestrial tremors.
