Show Date: May 31, 2006 
Pam Eastlick for THE DEEP on line

A WHOLE LOT OF SHAKING

Welcome to The Deep science and technology column where we cover topics from the deep sea to deep space and beyond. Join us each week on Newstalk K57 on Wednesday night from 7 to 8 p.m. for exciting live science expeditions or listen live on our web site www.thedeepradioshow.com

Earthquakes have definitely been in the news. The massive one in Indonesia has killed at least 3,000 people and we even had a nice little shaker here on Guam last week.Earthquakes are caused by lava movement inside the Earth and the Indonesian earthquake and our earthquakes here have the same cause. The gigantic slab that underlies the Pacific Ocean is being overridden on all its sides by the floating masses of the continents and as the Pacific plate is driven downward, the movement causes earthquakes.

But Earthquakes aren’t the only quakes in the solar system. There are also Marsquakes and interestingly enough, Moonquakes. Our moon is also subject to those nasty shaking events. (And you can’t call them ‘Earth’ quakes on the moon!)

We know there are quakes on the moon because the Apollo astronauts placed seismometers (the instruments that measure quakes), at their landing sites on the moon. The Apollo 12, 14, 15, and 16 instruments radioed data back to Earth until they were switched off in 1977. What did they find?

They discovered there are at least four different kinds of moonquakes: (1) deep moonquakes about 400 miles below the surface, probably caused by the tidal attraction raised by the Earth and the Sun; (2) vibrations from meteor impacts; (3) thermal quakes caused by the expansion of the frigid crust when it is first lit by the morning sun after two weeks of deep-freeze lunar night (-250 degrees!); and (4) shallow moonquakes only 15 miles or so below the surface. The first three were generally mild and harmless. Shallow moonquakes on the other hand were doozies. Between 1972 and 1977, the Apollo seismic network saw twenty-eight of them; a few of them registered up to 5.5 on the Richter scale. The earthquake we had on Guam last week was 4.8 on the Richter scale. Furthermore, shallow moonquakes lasted a remarkably long time. Once they got going, all continued more than 10 minutes and made the moon ring like a bell.

Buzz Aldrin deploys a seismometer in the Sea of Tranquility.

On Earth, vibrations from quakes usually die away in only half a minute. The reason has to do with chemical weathering. Water weakens stone, and expands its structure. When energy propagates across water-weakened stone, it acts like a foam sponge and deadens the vibrations. Even the biggest earthquakes stop shaking in less than 2 minutes.
The moon, however, is dry, cold and mostly rigid, almost like a single chunk of stone or iron; and moonquakes make it vibrate like a tuning fork. Even if a moonquake isn't intense, it shakes for a very long time. For people colonizing the moon, that long-term shaking could be more significant than a moonquake's magnitude. Join us this week on The Deep when we talk about quaking and shaking both here and on our closest neighbor. Don’t miss it!

The Deep is broadcast on Newstalk K57 every Wednesday night at 7:00 p.m. You can also listen live from our web site www.thedeepradioshow.com. Join Jim Sullivan, Pam Eastlick, and Peter Melyan on the deepest radio show on Earth.