Lunar Seismic Actvity

Marvin Ostrega

Seismic activity on the Moon is very low, basically insignificant. Due to the lack of plate tectonics, Lunar seismic activity is about a 100 millionth of Earth's, or ~2 x 10¹⁰ J/yr (excluding impacts). In eight years of monitoring, a large-but-rare moonquake has not been recorded, but it is possible to have up to 1 x 10¹⁴ J/yr if such events were recorded. The largest recorded seismic activities are approximately equivalent to a 4 on the richter scale, with 1-2 being typical.

The lunar seismic activity is usually caused from tidal forces and secondary effects from impacts. Secondary effects includes fresh crater ejecta cracking due to thermal stresses and disruption of slopes with high angles of repose. Other, non-seismic activity includes astronaut activity and impacts (both meteorite and artificial).

The Moon has very low elastic wave propagation losses, and seismic activity is thus clearly registered over long distances. This low attenuation also results in a long half-lives of seismic energy, in the tens of minutes, and the phrase "rang like a bell" after an Saturn upper stage lunar impact. Incidently, this long seismic signature suggests itself as a possible communication device, albeit with bandwidth issues.

There are secondary effects from lunar seismic activity that can create hazards to a presence on the Moon. Because of the excellent wave propagation, seismic activity can create widespread secondary activity, such as crater wall slumping and landslides. Apollo 17 astronauts visited a landslide which probably resulted from the impact that created the Tycho crater, 100 million years ago, and 2000 kilometers away.

Although the effect of lunar seismic activity is widespread, a lunar astronaut would hardly notice a moonquake happening, and precautions for moonquakes are probably unnecessatry. There is a small probability of a major event, large ones almost certainly being impact-related, but even then precautions, if used, will possibly be unnecessary. A site selection criteria may be to areas of potential secondary activity, such as those near high angles of repose (although this may be a given).