Magazine article Science News

Seismic Shivers Tell of Tornado Touchdown

Magazine article Science News

Seismic Shivers Tell of Tornado Touchdown

Article excerpt

For years, people unfortunate enough to have been standing close to where a tornado touched down have reported feeling rumbles beneath them.

Now, researchers say they can use earthquake-detecting seismometers to detect and possibly track all but the weakest tornadoes. They report their analysis in the January/February SEISMOLOGICAL RESEARCH LETTERS.

Currently, meteorologists use so-called Doppler radar to detect funnel clouds. That system reflects microwaves off particles in the sky, but it can't distinguish between a funnel cloud and a tornado. Only about one in five detected funnel clouds actually touches down to become a bona fide tornado.

Seismic readings could provide meteorologists with a ready means for distinguishing between funnel clouds and tornadoes, says study leader Frank B. Tatom, a mechanical engineer at the company Engineering Analysis in Huntsville, Ala.

Tatom and his coworker Stanley J. Vitton of Michigan Technological University in Houghton used physics theory to estimate how much energy tornadoes of various strengths transfer into the earth as vibrations. They found that tornadoes could produce ground vibrations with frequencies between 2 and 269 hertz, the strongest tornadoes having the lowest frequencies.

They compared these numbers with data collected in five southern U.S. states between 1971 and 1999 from earthquake seismometers that happened to be situated near spots where a tornado had touched down. The vibration rates predicted by their model matched those recorded by the seismometers, the researchers found.

"Seismologists have, been picking up tornado signals for years, but they just didn't know what they were," Tatom says. …

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