NASA released a statement this week saying the moon is shrinking which may be causing "moonquakes." According to this statement from NASA, the moon has gotten more than 150 feet (50 meters) skinnier 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. Unlike the flexible skin on a grape, the Moon’s 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," explain Bill Steigerwald and Nancy Jones, authors of this release.
Those thrust faults are triggering quakes like faults do on Earth. Thomas Watters, senior scientist in the Center for Earth and Planetary Studies at the Smithsonian’s National Air and Space Museum in Washington, said some of the stronger quakes are around 5 on the Richter scale.
During the Apollo 11, 12, 14, 15, and 16 missions, astronauts placed seismometers on the Moon's surface. The measurements from those instruments have been used in the past and are being used currently to study location and intensity of moonquakes. Studying data from the seismometers and images from NASA's Lunar Reconnaissance Orbiter Camera prove the moon is still tectonically active. These sensors can also tell the difference between an impact from space debris (external) and internal shaking from a quake.
For future study NASA would like to compare those LROC images from different times to see how the landscape has changed and to create a better network of seismometers on the moon during future exploration. According to this release, which you can read here, "NASA will send the first woman, and next man, to the Moon by 2024...The agency will establish sustainable missions by 2028, then we’ll take what we learn on the Moon, and go to Mars."
NASA adds Mercury also experiences quakes and has similar, though much larger, thrust faults to the moon. The fact they are so much larger indicates Mercury has shrunk significantly more than the moon has. Scientists speculate this suggests Mercury was molten rock after it formed and has since cooled since rock expands when heated and contracts when cooled.