Moonquakes are the lunar analog to terrestrial earthquakes. Though generally not as great in magnitude as terrestrial earthquakes, they could pose difficulty in the construction of long term lunar habitats.
Types of Moonquakes
Seismometers were left by Apollo astronauts at their landing sites between 1969 and 1972. These devices remained functional until their deactivation in 1977. During that time, at least four different types of lunar quakes were observed.
These quakes were found to originate quite deep (around 700 km), and are theorized to be caused by tidal interactions with the earth.
Quakes caused by the impact of lunar meteorites.
Caused by thermal expansion of the crust after the lunar night ends and the surface is warmed by the sun.
These quakes originate 20-30 kilometers below the surface. Their exact cause is unknown, though slumping of young crater rims is suggested as a possible explanation.
Deep Moonquakes, Impact Vibrations, and Thermal Quakes were all found to be quite mild. Shallow Moonquakes on the other hand could be much more intense, as much as 5.5 on the Richter scale. Furthermore, once a shallow moonquake began, it would persist for over ten minutes. This is explained by the lack of chemical weathering on the moon. On earth, chemical weathering has created weaknesses in the surface, causing it to act as as sponge, absorbing vibrations quite effectively. On the moon, the surface behaves more like a solid chuck of rock, and takes significantly longer to absorb the vibrations. Between 1972 and 1977, the Apollo seismometers recorded twenty eight shallow moonquakes.
Effect on Lunar Colonization
A shallow moonquake could spell doom for a poorly designed lunar habitat. Of importance is not only the magnitude of these quakes, but their exceptionally long duration. No earth structure has ever needed to be designed to handle this kind of scenario. Structures would have to be sufficiently strong / flexible to survive this shaking without buckling or leaking atmosphere, and internal furnishings/equipment would need to be secured against the significant motion that would occur from such a long lasting quake. It has been suggested that aluminum would be unsuitable for structural components under these circumstances, as it lacks a well defined fatigue limit, and will fail from even small vibrations given sufficient time. Iron and titanium fare much better in this regard. Buildings composed of lunar brick would need especially careful planning to avoid fractures, and may in fact be impractical in high quake areas. NASA has proposed placing additional seismometers on the lunar surface in order to better map quake occurrence, and perhaps find areas which are more stable.