BLOOMINGTON, Ind. -- An Indiana University geophysical experiment
detected unusual seismic signals associated with tornadoes that struck
regions across the Midwest last week -- information that may have value
for meteorologists studying the atmospheric activity that precedes
tornado disasters.
The experiment by IU researchers involves deployment of
more than 100 state-of-the-art digital seismographs in a broad swath of
the U.S. midcontinent. One of the twisters that struck southeastern
Missouri and southern Illinois on Feb. 29 passed through the seismic
detection array.
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| A graphic shows the location of seismographs in eastern Missouri and southern Illinois and the path of the tornado that struck Harrisburg, Ill., on Feb. 29. |
"In examining the seismograms, we recorded
unusual seismic signals on three of our stations in southern Illinois,"
said Michael Hamburger, professor in the department of geological
sciences at IU Bloomington and one of the researchers conducting the
experiment.
"The seismograms show a strong, low-frequency pulse
beginning around 4:45 a.m. on Feb. 29. Our preliminary interpretation,
based on other seismic records of tornadoes, suggests that we were
recording not the tornado itself, but a large atmospheric pressure
transient related to the large thunderstorms that spawned the
tornadoes."
The seismographs that detected the pulse are near
Harrisburg, Ill., a town of 9,000 where a pre-dawn twister caused
extensive damage, killed six people and injured about 100 more.
IU researchers initially feared that some of the
instruments might be damaged by the storm, setting back a National
Science Foundation-funded project that included the investment of
hundreds of thousands of dollars and months of effort. But when
principal investigator Gary Pavlis, an IU professor of geological
sciences, checked the digital recordings of the Illinois stations on
Feb. 29, he found they were still alive and streaming data. As he
checked further, he discovered the strange "tornado seismograms" that
were recorded on seismographs near Harrisburg.
Hamburger said a seismic pressure gradient associated
with the tornado produced a slow, minute tilting of the seismograph that
lasted for several minutes. He said this sort of pressure-related
signal may help scientists better understand atmospheric activity that
takes place right before tornadoes touch down. The IU researchers are
working with colleagues at the University of California San Diego to try
to compare recordings with other tornado-related signals and to dig
deeper into the analysis.
While seismographs have been known to detect seismic
activity related to tornadoes, it is highly unusual to have
state-of-the-art digital instruments recording information in such close
proximity to a tornado, the researchers say.
The IU seismic experiment, dubbed "OIINK" for its
geographic coverage in parts of the Ozarks, Illinois, Indiana and
Kentucky, includes the positioning of 120 seismometers to study
earthquakes and geological structure in a key area of North America.
Installation of the instruments began last summer. They are recording
thousands of earthquakes from the study area and around the world, as
well as nearby mining and quarry explosions.
The $1.3 million, four-year undertaking is part of the
NSF's EarthScope program, which seeks to cover the entire U.S. with a
grid of detection devices for the purpose of better understanding
seismic activity and predicting earthquakes. Researchers liken
EarthScope to "an upside-down telescope" that allows them to look into
the Earth and gain a better understanding of seismic forces.
More details on the experiment are available at newsinfo.iu.edu/news/page/normal/18612.html.
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