May 13 (UPI) -- The moon shrinks? During its more than 4.5 billion years in space, it has slowly shrunk, researchers say.
Over the last few hundred million years, the moon's belt line has slimmed by approximately 150 feet.
According to a new study, the moon's contractions continue today, causing ongoing surface fracturing. These active fractures, or thrust faults, are responsible for moonquakes.
"Some of these quakes can be fairly strong, around five on the Richter scale," Thomas Watters, senior scientist in the Center for Earth and Planetary Studies at the Smithsonian's National Air and Space Museum in Washington, said in a news release.
Using a new algorithm designed to pinpoint the origins of shallow rumblings produced by a sparse fault network, scientists analyzed the data collected by seismometers placed on the lunar surface during the Apollo missions.
The faults created by lunar shrinking take the form of stair-like cliffs. When scientists mapped the origins of the moonquakes recorded by the Apollo seismometers, they found eight of the 28 shallow quakes were within 18 miles of a visible lunar fault.
The new analysis, published this week in the journal Nature Geoscience, showed six of the eight quakes occurred when the moon was at its farthest point from Earth, its apogee, when tidal forces put additional stress on lunar faults.
Simulations determined the chance that coincidence could explain so many shallow quakes occurring near faults during periods of high stress was less than 4 percent.
"We think it's very likely that these eight quakes were produced by faults slipping as stress built up when the lunar crust was compressed by global contraction and tidal forces, indicating that the Apollo seismometers recorded the shrinking moon and the moon is still tectonically active," said Watters.
Authors of the latest study plan to continue analyzing seismic data from the Apollo seismometers, as well as images of the moon's faults, in hopes of uncovering additional evidence of recent moonquakes.
"Establishing a new network of seismometers on the lunar surface should be a priority for human exploration of the Moon, both to learn more about the Moon's interior and to determine how much of a hazard moonquakes present," said Renee Weber, a planetary seismologist at NASA's Marshall Space Flight Center.