IN IMAGES | & # 39; Moon shrinks and trembles; may be generating moon tremors, "says NASA

The moon shrinks as its interior cools and has lost more than 50 meters in the last few hundred million years, NASA said. Just as a grape contracts and shrinks into grapes, the moon contracts as it contracts. Unlike the supple skin of a grape, the surface crust of the moon is brittle. It breaks as the Moon narrows, forming "thrust faults" that push part of the crust onto a nearby part.

"Our analysis provides the first evidence that these flaws are still active and are likely to produce moonshade today, as the Moon continues to cool and shrink gradually," said Thomas Watters, senior scientist at Center for Earth and Planetary Studies of the Smithsonian's National Air and Space. Museum in Washington. "Some of these earthquakes can be quite strong, about five on the Richter scale."

This is a view of the Taurus-Littrow Valley taken by NASA's Lunar Reconnaissance Orbiter satellite. The valley was explored in 1972 by astronauts Eugene Cernan and Harrison Schmitt of the Apollo 17 mission. They had to zigzag their lunar vehicle up and over the cliff of the Lee-Lincoln fault that runs through this valley. Credits: (NASA / GSFC / Arizona State University)

These fault scarves look like small cliffs in the shape of a stairway seen from the lunar surface, usually several tens of meters high and extending for a few miles (several kilometers). Astronauts Eugene Cernan and Harrison Schmitt had to zigzag their lunar rover on the Lee-Lincoln fault cliff during the Apollo 17 mission that landed in the Taurus-Littrow Valley in 1972.

Watters is the principal author of a study that analyzed data from four seismometers placed on the moon by Apollo astronauts using an algorithm, or mathematical program , developed to locate seismic sites detected by a sparse seismic network. The algorithm gave a better estimate of the location of earthquakes. Seismometers are instruments that measure the quake produced by earthquakes, recording the arrival time and strength of different earthquake waves to obtain an estimate of the position, called the epicenter. The study was published May 13 in Nature Geoscience.

Astronauts placed the instruments on the lunar surface during Apollo missions 11, 12, 14, 15 and 16. The Apollo 11 seismometer only worked for three weeks, but the remaining four recorded 28 shallow earthquakes – expected type of these faults – from 1969 to 1977. The earthquakes ranged from about 2 to about 5 on the Richter scale.

Using the revised location estimates from the new algorithm, the team found that eight of the 28 shallow earthquakes were within 30 kilometers (18.6 miles) of the visible faults in the lunar images. This is close enough to assign tremors to faults, because the modeling done by the team shows that it is the distance over which strong shaking is expected, given the size of these scarves. In addition, the new analysis revealed that six of the eight earthquakes had occurred when the Moon was at its peak or near its peak, the farthest point on Earth's orbit. This is where the extra tidal stress due to the gravity of the Earth causes a peak of total stress, making it more likely slip events along these faults.

"We believe that it is very likely that these eight earthquakes were caused by slippery faults, the stress accumulated when the lunar crust was squeezed by global contraction and tidal forces indicating that Apollo seismometers recorded the contraction of the Moon and that it is still active tectonically, "says Watters. The researchers performed 10,000 simulations to calculate the risk of a coincidence producing the number of earthquakes close to the faults at the time of the strongest stress. They found that it's less than 4 percent. In addition, while other events, such as meteoroid impacts, can produce earthquakes, they produce a seismic signature different from that of earthquakes caused by skid events.

The Taurus-Littrow Valley is the location of the landing site of Apollo 17 (asterisk). The Lee Lincoln Fault is cut off either side of the valley just above the landing site. The movement over the fault was the likely source of many moon tremors that triggered events in the valley. 1) Large landslides on the slopes of the South Massif covered relatively bright rocks and dusts (regoliths) on and above the Lee-Lincoln Escarpment. 2) Rocks have descended the slopes of the north of the Massif, leaving traces or narrow valleys in the regolith on the slopes of the North of the Massif. 3) Landslides on the southeast slopes of the carved hills. Credits: NASA / GSFC / Arizona State University / Smithsonian)