The physical mechanisms of deep moonquakes and intermediate-depth earthquakes: How similar and how different?

We here review the characteristics of deep moonquakes (DMQ) and conclude that they share many common features with certain categories of intermediate-depth earthquakes, namely, the pressure–temperature conditions where they occur, their occurrence in highly localized nests, their tendency to occur repeatedly at the same or nearly the same location, and their distribution of sizes (b-values). We thus explore various physical mechanisms proposed for earthquakes that might also explain deep moonquakes. These include the possibility that there are partial melts or fluid phases in the lunar mantle that permit DMQ to occur at pressures where ordinary brittle fracture is not possible, the possibility that stress concentration increases tidal stresses at certain locations where moonquakes occur repeatedly, and the possibility that DMQ occur at relatively low stresses because rock strength is lowered by a fatigue process induced by the repetitive application of tidal stresses. The similarities between DMQ and terrestrial earthquakes imply that research focusing on the properties of both phenomena may help us understand the physical origins of both.