Trudy E. Bell

1260 Andrews Ave.

Lakewood, OH 44107

216-221-5008 (fax 5088)

t.e.bell@ieee.org

\[Word count: 626; target: 500\]

**Moonquake!**

By Trudy E. Bell

NASA astronauts are going back to the moon and when they get there they
may need quake-proof housing.

That's the surprising conclusion of Clive R. Neal, associate professor
of civil engineering and geological sciences at the University of Notre
Dame after he and a team of 15 other geologists and planetary scientists
reexamined Apollo data from the 1970s. "The moon is seismically active,"
he told a gathering of scientists at NASA's Lunar Exploration Analysis
Group (LEAG) meeting in League City, Texas, last October.

Between 1969 and 1972, five of the six Apollo missions each placed a
seismometer on the moon. Although the one from the Apollo 11 failed
after a few months, the ones from Apollo 12, 14, 15, and 16 faithfully
radioed data back to Earth until they were switched off in 1977.

And what did they reveal?

There are at least four different kinds of moonquakes: (1) deep
moonquakes from about 700 km below the surface; (2) vibrations from the
impact of meteorites; (3) thermal quakes from 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. The shallow
moonquakes, however, were doozies. A few "registered up to 5.5 on the
Richter scale," Neal said. A magnitude 5 earthquake on Earth is
energetic enough to move heavy furniture and crack plaster. Apollo
seismometers recorded shallow moonquakes about five times a year.

The worst moonquakes (magnitude 5) recorded so far aren't nearly as
powerful as the worst quakes we can experience on Earth (magnitude 9).
But moonquakes *are* worse in one way: they last a long time.

Shallow moonquakes all continued longer than 10 minutes, because "the
Moon rings like a bell," Neal said.

On Earth, vibrations from quakes usually die away in half a minute
because of chemical weathering, Neal explained. "Water weakens stone,
expanding the structure of different minerals, and forming layers of
clays. When energy propagates across such a compressible structure, the
soil is like a foam sponge---it deadens the vibrations." Even the
biggest earthquakes stop shaking in less than 2 minutes.

The Moon, however, is dry, cool, and rigid, like a chunk of stone or
iron. So moonquakes set it vibrating like a tuning fork. Even if a
moonquake isn't intense, "it just keeps going and going," Neal said. And
for a lunar habitat, that persistence could be more significant than a
moonquake's magnitude.

"Any habitat would have to be built of materials that are somewhat
flexible," Neal said, so no air-leaking cracks would develop. "We'd also
need to know the fatigue threshold of building materials," that is, how
much repeated bending and shaking they could withstand.

What causes the shallow moonquakes? And where do they occur?

"We're blind here," Neal said. "We don't have any knowledge. The Apollo
seismometers were all in one relatively small region on the front side
of the Moon, so we can't pinpoint where or why." He and his colleagues
do have good guesses, among them being the rims of large and relatively
young craters that may occasionally slump.

"We especially can't tell about the lunar poles," Neal continued. That's
important, because one leading candidate location for a permanent lunar
habitat is on a permanently sunlit region on the rim of Shackleton
Crater at the Moon's south pole.

Neal and his colleagues are now preparing a proposal for deploying a
network of 10 to 12 seismometers around the entire Moon, to gather data
for at least three to five years to help select the safest location for
the first lunar habitat.

\--Trudy E. Bell

More about Clive R. Neal appears at <http://www.nd.edu/~cneal/> .

Abstracts from Neal's presentations include "The Importance of
Establishing a Global Lunar Seismic Network," available at
<http://www.lpi.usra.edu/meetings/leag2005/pdf/2065.pdf> and "The Lunar
Seismic Network: Mission Update" at
<http://www.lpi.usra.edu/meetings/lpsc2004/pdf/2093.pdf> .

One of the scientists working with Neal is Yosio Nakamura; a report on
his work appears at <http://www.jsg.utexas.edu/news/feats/nakamura.html>
.