Seismic shivers tell of tornado touchdown.

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.

Seismologists weren't alone in seeing these signals. Atmospheric scientists also have reported detecting low-frequency tornado vibrations with avalanche sensors (SN: 9/21/96, p. 186).

Because Doppler radar can't discriminate between a funnel cloud and a tornado, meteorologists must issue tornado warnings for both. This leads to a high number of false alarms, says Tatom. "It's like crying wolf," he says.

The researchers now are using seismic data from their analysis to develop sensors incorporating software that recognizes vibrational patterns specific to tornadoes. The sensors would ignore vibrations caused by a passing truck or train.

Once a tornado touches down, it moves along the ground until it dissipates. Since each sensor could detect a tornado up to 10 miles away, a network of these sensors, spaced 10 miles apart, would enable meteorologists to determine where a tornado is and where it's headed.

The system could be especially useful in the South, where trees and hilly terrain limit the view of volunteers who look for tornadoes after a weather-service warning.

"The early results are promising," says Joseph H. Golden, a research meteorologist with the National Oceanic and Atmospheric Administration in Boulder, Colo. Next, he would like to see a small seismic network undergo field tests.

He may not have to wait long. Tatom and his colleagues plan to install the first network within a year.