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Science | Q & A

Q & A; Noise in Earthquakes


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Q. What causes the noises you hear in an earthquake?

A. The sliding of rocks past one another along a fault causes vibrations. In the rocks themselves, these vibrations are called seismic waves. When they reach the surface of the earth, the waves can be transmitted into the air, where they become sound, explained Dr. Bill Menke, chairman of the department of earth and environmental science at Columbia University. In a similar process, a violin emits a tone as the bow is drawn across the string.

An important part of the process along slipping faults is that the sliding is jerky, as contrasted to, say, the quiet sliding of smooth, lubricated bearings. (A small fraction of an earthquake's sound energy is in the audible range; most is infrasound, too low in pitch for humans to hear.)

As John Armbruster, a seismologist at the Earth Institute at Columbia, noted after a minor earthquake in Manhattan on Jan. 17, 2001, the rock noise can be very loud, and tends to be louder in East Coast quakes than in West Coast quakes. Many people who felt the Manhattan temblor thought that an explosion had occurred, he said, and it produced a sound much more impressive than the vibration that was felt.

In the Eastern United States, he said, the rocks are old and strong and transmit the vibrations from the quake efficiently. The movement is a larger, more sudden jump. Thus, Mr. Armbruster said, the stronger Eastern rock can generate the same amount of energy or sound with a smaller source than a California quake from a larger source. Because the rocks there are younger and not as strong, earthquakes are felt at shorter distances and with less noise. C. CLAIBORNE RAY

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