While other seismic disturbances measured by the Apollo-era seismometers could be attributed to Earth's own gravity making itself felt through space - in the same way that the moon's gravitational pull causes the oceans to rise and fall, so the Earth's pull actually deforms the moon's surface, stretching it into a slightly oblong shape before releasing it back into a sphere - asteroid impacts, or temperature shifts, these "shallow" moonquakes had previously frustrated all attempts to study them.
According to scientists, the moon has shrunk about 50 metres over the last several hundred million years. The fault scarp or cliff is like a stair-step in the lunar landscape, formed when the near-surface crust is pushed together, breaks, and is thrust upward along a fault as the Moon contracts.
"Our analysis gives the first evidence that these faults are still active and likely producing moonquakes today as the Moon continues to gradually cool and shrink", Thomas Watters, a senior scientist from the Center for Earth and Planetary Studies said. A team of researchers including Nicholas Schmerr, an assistant professor of geology at the University of Maryland, designed a new algorithm to re-analyze seismic data from instruments placed by NASA'sApollo missions in the 1960s and '70s.
Watters said some of the quakes "can be fairly strong" - as high as a 5 on the Richter scale.
Data from the seismometers placed on the moon during the Apollo 11, 12, 14, 15 and 16 missions revealed 28 moonquakes recorded between 1969 and 1977.
Using the revised location estimates from the new algorithm, the team found that eight of the 28 shallow quakes were within 30 kilometers (18.6 miles) of faults visible in lunar images. This was close enough for the team to conclude that the faults likely caused the quakes.
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"This is exciting as it wasn't clear if the moon had already gone through this period billions of years ago and was tectonically dead, or if it was still active in the present", Schmerr said.
To check their findings, researchers ran 10,000 simulations to see if this amount of quakes near faults can be coincidental, however, they found the chances are less than 4 per cent.
By combining information gathered by LRO and data from the various Apollo missions, researchers were able to "advance our understanding of the Moon while suggesting where future missions intent on studying the Moon's interior processes should go", according to LRO project scientist John Keller.
The findings suggested that these "cliffs", or thrust faults, had formed as a result of tectonic activity related to the moon contracting in size as it cooled. Some of these images show landslides or boulders at the bottom of relatively bright patches on the slopes of fault scarps or nearby terrain. These American astronauts will take a human landing system from the Gateway in lunar orbit, and land on the lunar South Pole.
The Apollo missions also detected about one moonquake per day resulting from space rocks hitting the lunar surface.