THE MOON is shrinking as its interior cools, becoming about 150 feet thinner over the last several hundred million years. Just as a grape wrinkles as it shrinks down to a raisin, the moon gets wrinkles as it shrinks. However, unlike the flexible skin on a grape, the lunar surface crust is brittle, so it breaks as the moon shrinks, forming "thrust faults" where one section of crust is pushed up over a neighboring part
"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," says Thomas Watters, senior scientist in the Center for Earth and Planetary Studies at the Smithsonian's National Air and Space Museum. "Some of these quakes can be fairly strong--around five on the Richter scale."
These fault scarps resemble small stair-step shaped cliffs when seen from the lunar surface, typically tens of yards high and extending for a few miles. Astronauts Eugene Ceman and Harrison Schmitt had to zig-zag their lunar rover up and over the cliff face of the Lee-Lincoln fault scarp during the Apollo 17 mission that landed in the Taurus-Littrow valley in 1972.
Wafters is lead author of a study published in Nature Geoscience that analyzed data from four seismometers placed on the moon by the Apollo astronauts using an algorithm developed to pinpoint quake locations detected by a sparse seismic network. The algorithm gave a better estimate of moonquake locations. Seismometers are instruments that measure the shaking produced by quakes, recording the arrival time and strength of various quake waves to get a location estimate, called an epicenter.
Astronauts placed the instruments on the lunar surface during the Apollo 11, 12, 14, 15, and 16 missions. The Apollo 11 seismometer operated for only three weeks, but the four remaining recorded 28 shallow moonquakes --the type expected to be produced by these faults--from 1969-77. The quakes ranged from about two to around five on the Richter scale.
Using the revised location estimates from the new algorithm, the team found that eight of the 28 shallow quakes were within 18.6 miles of faults visible in lunar images. This is close enough to tentatively attribute the quakes to the faults, since modeling by the team shows that this is the distance over which strong shaking is expected to occur, given the size of these fault scarps.
Moreover, the new analysis found that six of the eight quakes happened when the...