"The moon is seismically active," Clive Neal, Associate Professor of Civil Engineering and Geological Sciences at the University of Notre Dame, told a gathering of scientists at NASA's Lunar Exploration Analysis Group (LEAG) meeting in League City, Texas, in 2006.
Between 1969 and 1972, Apollo astronauts placed seismometers at their landing sites around the moon. The Apollo 12, 14, 15, and 16 instruments faithfully radioed data back to Earth until they were switched off in 1977.
What Did These Seismometers Reveal?
Surprising evidence suggests that there are at least four different kinds of moonquakes: (1) deep moonquakes about 700 kilometers below the surface, probably caused by tides; (2) vibrations from the impact of meteorites; (3) thermal quakes caused by the expansion of the frigid crust when first illuminated by the morning sun after two weeks of deep-freeze lunar night; and (4) shallow moonquakes only 20 or 30 kilometers below the surface.
The first three were generally mild and harmless. However, the Apollo seismic network recorded 28 powerful shallow moonquakes between 1972 and 1977-- some of them registering up to 5.5 on the Richter scale. A magnitude 5 quake on Earth is energetic enough to move heavy furniture and crack plaster.
Furthermore, according to Neal, shallow moonquakes tended to last five times longer than on Earth. Recorded shallow moonquakes lasted more than 10 minutes, whereas on Earth, even the biggest, most devastating earthquakes stop shaking after about two minutes.
The reason, according to Neal, has to do with chemical weathering. On Earth, vibrations from quakes usually die away in only half a minute. Water weakens stone, expanding the structure of different minerals. When energy propagates across such a compressible structure, it acts like a foam sponge?it deadens the vibrations.
The moon, however, is dry, cool and mostly rigid, like a chunk of stone or iron. Therefore, moonquakes cause it to vibrate, much like a tuning fork. And with this, even a mild moonquake could cause persistent vibrations that could prove more significant than the moonquake's magnitude.
What Causes Moonquakes?
There has not been significant evidence to pinpoint the exact cause of shallow moonquakes or where they are most likely to occur. Neal explains that because the Apollo seismometers were all in one relatively small area on the front side of the moon, the exact locations of the quakes could not be measured.
Interest in the poorly-understood lunar poles has been growing. This is due to the fact that one prime location for a permanent lunar base is on the rim of Shackleton Crater?a perpetually sunlit region at the moon's south pole. Neal and his colleagues are developing a proposal to deploy a network of 10 to 12 seismometers around the entire moon, to gather data for at least three to five years. This kind of work is necessary, Neal believes, to find the safest spots for permanent lunar bases.
Neal contends that any permanent lunar base would have to be built of materials that are flexible, not unlike skyscrapers in earthquake prone cities on Earth. They would also need to know the fatigue threshold of building materials-- that is, how much repeated bending and shaking they could withstand.
It is just the beginning. The moon may be a technology test bed for the feasibility of creating seismometer networks on other planets such as Mars and beyond-- as they might experience cosmic quakes too.
Sources and Credit: Science@NASA, by Trudy E. Bell
Last Updated: 16 July 2013