ProfessionalMackem
Striker
No mystery here mate. The Earth's MEAN density of Earth is indeed circa 5 gm/cm3. That's because of it's relatively large iron/nickel core, surrounded a largely basaltic mantle. The moon possesses a much smaller iron core by comparison, it consists largely of basaltic type material hence the lower MEAN density. The absence of a large iron/nickel core in the moon is due to the impact of Rhea and the proto-Earth where the two cores amalgamated and settled into the large core present in the Earth today. Pretty straight forward really.
As for the Earthquakes, i'd guess it would be something to do with the fact that the p-waves generated by Earthquakes are refracted as they enter a liquid (and the Earth's core is surrounded by a substantial molten outer core.....which gives us our magnetic field effectively) whilst the s-waves cannot pass though a liquid at all. Because the moon has a tiny liquid outer core (if indeed there is anything molten at all) and it is tectonically dead (i.e there is no movement or turnover of mantle) there is no dampening effect that a liquid, molten material provides to a shock force - that is an earthquake.
I'd imagine it would be like hitting a bowling ball and then a water balloon with a hammer. Which one would make your hand vibrate like fuck.
Good post.