A Shrinking, Tectonically Active Moon

The Lunar Reconnaissance Orbiter (LRO) began its mission in 2009, almost 40 years after the historic Apollo 11 landing. Now, as we prepare to celebrate the 50th Anniversary of Apollo 11 this summer, a new link between the science of Apollo and LRO is changing our understanding of the Moon.  

One of the most unexpected discoveries of the LRO mission has been the detection of a vast network of small fault scarps, cliff-like landforms, virtually everywhere on the Moon. These fault scarps, formed as the lunar crust contracts and is pushed together, breaks and is thrust upward, are evidence of a shrinking Moon. Prior to LRO, only a relatively few of these small fault scarps were found in our highest resolution photographs of the lunar surface, mostly taken by the Apollo Panoramic Cameras. The most noteworthy of these is the Lee-Lincoln fault scarp in the Taurus-Littrow valley, very near the Apollo 17 landing site.

In images returned by the Lunar Reconnaissance Orbiter Camera (LROC) with its two Narrow Angle Cameras that can resolve objects on the surface as small a few meters, thousands of small fault scarps have now been mapped. As extraordinary as the sheer number of small fault scarps is their apparent age – they are young, formed very recently in the Moon’s geologic past. But just how recent, is recent? This is a difficult question to answer. Our ability to precisely date surfaces and landforms on the Moon and the other solid bodies in our solar system is limited, particularly if they are very young. So, how can data from the Apollo missions help?

The Apollo astronauts placed seismometers on the Moon that recorded thousands of moonquakes between 1969 and 1977. Most of the moonquakes recorded by Apollo seismometers were from sources deep in the Moon’s interior, but a small number of quakes were from sources at shallow depths. Faults on Earth are known to be currently active because they are the source of earthquakes. Could some of the shallow moonquakes recorded by the Apollo seismometers be from activity on the young thrust faults discovered by LRO? Because of the limitations of the Apollo seismometers, the locations or epicenters were not known with a high degree of accuracy making a comparison with mapped fault scarps problematic.

Using a new algorithm developed for sparse seismic networks, the location and depth of the seismic sources of shallow moonquakes can be more accurately determined. We found that eight of the relocated shallow moonquakes are within 30 km of a mapped young fault scarp. This is the distance over which our modeling indicates that shallow moonquakes are predicted to produce strong ground shaking. If we are correct, some of the shallow moonquakes were caused by active lunar faults.

Active faulting is likely driven primarily by ongoing cooling of the Moon’s still-hot interior, but the broad pattern of the young fault scarps suggests tidal forces exerted by Earth also contribute. Tidal forces reach their maximum when the Moon’s orbit takes it furthest from Earth. Quakes are expected as forces build on a fault’s surface until it is released by slip along the fault. Our modeling indicates that six of the eight shallow moonquakes with epicenters nearest to a fault scarp occurred when tidal forces were at or near their peak.

Connecting shallow moonquakes to young fault scarps strongly suggests current lunar tectonic activity and that the Apollo seismometers recorded temblors of the shrinking Moon! The significance of this finding goes beyond the realization that a relatively small rocky body has remained, like Earth, tectonically active over billions of years. Just as on Earth, it is important to know the locations of active faults on the Moon because shallow moonquakes can result in moderate to strong seismic shaking - something to keep in mind when selecting sites for long-term lunar outposts.