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It is thought that "deep" moonquakes are caused by tidal forces exerted on the Moon by the Earth and Sun. Some other sorts of moonquakes are thought to be caused by impacts, or by thermal expansion (when part of the moon rotates into sunlight).

However, there is another type of moonquake that occurs just 20-30 kilometers below the surface. Do we know why these quakes occur?

The article I linked above suggests that the cause is currently unknown, but that article is a few years old and is just a popular news article. As I am not familiar with the literature on this topic, I would like to know if there are any well-substantiated hypotheses about the cause of shallow moonquakes that have been put forth.

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    $\begingroup$ That 20-30 km depth likely has a large uncertainty associate with it. Depths of quakes are much harder to constrain than the epicenter. For one, you need a reasonably accurate velocity model. $\endgroup$ – stali Dec 7 '14 at 20:35
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According to the paper The lunar moho and the internal structure of the Moon: A geophysical perspective (Khan et al. 2013), suggest 2 causes for the less frequent, yet more energetic shallow moonquakes:

  • Similar tectonic movements to intraplate quakes on Earth, also mentioned in the NASA paper Interior of the Moon (Weber, 2013), but this seems unlikely due to the lack of plate tectonics on the Moon, but in an earlier paper, cited by Khan et al. (2013), Shallow moonquakes - How they compare with earthquakes (Nakamura, 1980), suggests that slippage in near surface fracture zones are a possible mechanism.

  • An alternative explanation suggested is the

possibility that shallow moonquakes may instead be triggered by objects emanating from outside the solar system, such as nuggets of strange quark matter.

However, as there have only been 28 recorded since we've been to the moon, it is hard to exactly say.

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