Seismic shivers tell of tornado touchdown For years, people unfortunate enough versity in Houghton used physics theo-

to have been standing close to where a ry to estimate how much energy torna- tornado touched down have reported does of various strengths transfer into feeling rumbles beneath them. the earth as vibrations. They found that

Now, researchers say they can use tornadoes could produce ground vi- earthquake-detecting seis- s brations with frequen- mometers to detect and 3 cies between 2 and 269 possibly track all but the :: hertz, the strongest tor- weakest tornadoes. They $ nadoes having the low- report their analysis in the .g est frequencies. January/February SEiSho 2 They compared these LOGICAL &SEARCH LETTERS. f numbers with data col-

Currently, meteorologists Z lected in five southern use so-called Doppler radar 3 US. states between 1971 to detect funnel clouds. .; and 1999 from earth- That system reflects mi- 8 quake seismometers crowaves off particles in that happened to be the sky, but it can’t dis-4 situated near spots tinguish between a funnel 2 where a tornado had cloud and a tornado. Only touched down. The vi- about one in five detect- bration rates predicted ed funnel clouds actually by their model matched touches down to become a those recorded by the bona fide tornado. seismometers, the re-

Seismic readings could searchers found. provide meteorologists with “Seismologists have a ready means for distin- When tornadoes touch been picking up tor- guishing between funnel down, they send vibrations nado signals for years, clouds and tornadoes, says into the ground. but they just didn’t study leader Frank B. Tatom, know what they were,” a mechanical engineer at the company Tatom says. Engineering Analysis in Huntsville, Ala. Seismologists weren’t alone in seeing

Tatom and his coworker Stanley J. these signals. Atmospheric scientists also Vitton of Michigan Technological Uni- have reported detecting low-frequency

Antarctic glacier thins and speeds up One of the largest glaciers in Antarcti-

ca is thinning, according to satellite measurements. The finding spurs con- cerns that changes in the glacier’s ice shelf along the Antarctic coast may in- crease the amount of ice that drains from the interior of the continent and floats out to sea.

Each year, the 200-kilometer-long, 25- km-wide Pine Island Glacier depletes the West Antarctic Ice Sheet of about 69 cu- bic kilometers of ice. Eventually the gla- cier flows into the sea, where it forms a floating ice shelf about 40 km wide, says Andrew Shepherd, a physicist at Univer- sity College London. When he and his colleagues analyzed satellite measure- ments taken between 1992 and 1999, they found that large portions of the Pine Island Glacier thinned during that period. The British scientists report their findings in the Feb. 2 SCIENCE.

At the point called the grounding line, where the glacier reaches the ocean and the ice lifts off the bedrock and floats, the 700-meter-thick ice thinned by about 1.6 m annually during the period studied. As distance inland increased, the glacier showed less thinning.

Much of the Pine Island Glacier rests

on bedrock that lies more than 1 km be- low sea level, says Shepherd. If the gla- cier continues to thin at the current rate,

0 c P ; .- 2 r“ - i 3 3 - - 2 E E

Break-up of this ice sheet along the Antarctic coast may be causing the Pine Island Glacier to become thinner.

large parts of it would be afloat within 600 years-a development that could ac- celerate the flow of ice off the continent.

Because the bedrock under the Pine Is- land Glacier slopes inward toward the center of the continent, the ice stream has to march uphill quite a distance be- fore it again flows downhill at the coast. The resulting resistance to the glacier’s flow will disappear if the ice stream floats free of the bottom, Shepherd notes.

tomado vibrations with avalanche sensors (SN: 9/21/96, p. 186).

Because Doppler radar can’t discrimi- nate between a funnel cloud and a torna- do, 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 seis- mic data from their analysis to develop sensors incorporating software that rec- ognizes 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 mete- orologists to determine where a tornado is and where it’s headed.

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

“The early results are promising,” says Joseph H. Golden, a research me- teorologist 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. -L. Wang

Although the satellite data showed thin- ning ice over a large area, most of the change was confined to the glacier’s fastest-moving portions, which flow about 2.5 km per year. This observation bolsters the team’s contention that changes in ice flow, not melting or decreased precipita- tion, causes the thinning.

Because the rate didn’t change signifi- cantly during the 8 years of observation, the scientists don’t attribute the thinning to a short-term phenomenon such as a glacier surge. Instead, the researchers say any increase in flow could be the re- sponse to the earlier breakup of a more extensive offshore ice shelf that had slowed the glacier’s flow-an idea bol- stered by computer models of glaciers.

If the current ice shelf disappears, the speed of the glacier at the grounding line would increase about 30 percent, says Marjorie Schmeltz of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. She presented the results of her digital simulations at the American Geophysical Union’s annual meeting last December. Similarly, if the current ice shelf had been larger in the past, its shrinkage could have led to the higher flow rates and the thinning that Shep herd and his colleagues have observed, she says. -5 Perkins

70 SCIENCE NEWS, VOL. 159 FEBRUARY 3,2001