Thanks to the seismometers placed on the lunar surface during the Apollo program, we know that several thousand moonquakes occur each year. They fall into two categories. Most are deep events from 400 - 600 miles ( 600 - 900 kilometers) at about 50 identified centers. These reoccur in time with the monthly Earth tidal cycle. In addition, there are rare near-surface events caused by crustal stresses, rock slides, meteor impacts and such within the outer 100 kilometers.