Seismic Stations Record Useful Information on Terrorist Bombings, UA Geoscientist Says

Lori Stiles
May 29, 2002

The growing world network of seismic stations is increasingly useful for monitoring more than earthquakes, say university geoscientists who are developing a new specialty called "forensic seismology."

They study seismograms as records of industrial explosions, clandestine nuclear weapons testing and terrorist bombings.

"Forensic seismology has its roots in the verification of small nuclear explosions," says University of Arizona seismologist Terry C. Wallace. "But it clearly is also useful in putting constraints on terrorist bombs."

Wallace will talk about it Tuesday afternoon, May 28, at the American Geophysical Union meeting in Washington, D.C. He also is a panelist at a 9 a.m. news conference, "Geophysics vs. Terrorism." Session panelists also include Gregory van der Vink of the Incorporated Research Institutions for Seismology (IRIS) Consortium, Washington, D.C. and John F. Shroder of the University of Nebraska. The news conference highlights how information collected for scientific purposes can be used to help resolve national security questions.

AGU meeting organizers describe the scientific community as "a largely untapped resource for detecting the signatures of terrorist activity. Researchers operate networks of sensors, and if terrorist activity is detectable, it is quite likely that the evidence will first appear on a data collection system operated for other purposes. Scientists are therefore strongly positioned to serve as the technological equivalent of a neighborhood watch. "

Wallace, his former post-doctoral associate Keith D. Koper, who is now on the faculty at St. Louis University, and van der Vink a few years ago collaborated in research on how seismic records might be used to monitor small or moderate-sized secret nuclear weapons tests. They conclude that the worldwide seismic network has been proved both extensive and sensitive enough to monitor violations of the Comprehensive Nuclear Test Ban Treaty (CTBT).

More recently, Wallace and Koper collaborated with Dirk Hollnack of the University of Nairobi in a seismic analysis of the Aug. 7, 1998, truck-bomb blast at the American Embassy in Nairobi, Kenya. The bomb seriously damaged a dozen buildings, injuring more than 4,000 people, 220 fatally.

The Nairobi attack was recorded by a broadband seismometer operated by the geology department of the University of Nairobi at a site 3 kilometers (less than 2 miles) northwest of the blast. This seismic station was the only station that recorded the embassy attack, but Wallace, Koper and Hollnack used its high-quality data in an analysis that found precisely when the explosion occurred, at 10:39:19.8 local time -- plus or minus two-tenths of a second, and the size of the bomb. The Nairobi bomb was deadly but fairly small, equivalent to 3 metric tonnes of TNT.

"Sometimes what seismology can contribute is the only information we have on the bomb," Wallace said. "That was the case in Nairobi. This was in another country, emergency response was immediate, and by the time the FBI arrived, the crater had been filled in. So it was impossible for the FBI to do its usual forensic analysis.

"In this case, the FBI actually came to us before we had the seismic records. We had to find the records and then do the analysis the FBI needed for their investigation."

"What made it more interesting is that in an effort to fully use seismic recordings from such attacks, we participated in a series of controlled truck bomb explosions conducted at White Sand Missile Range in New Mexico," Wallace said. "The idea was to learn if the type of truck used in the bombing made a difference in the size of the explosion. As a result, we developed a new set of scaling laws that relate seismic and acoustic observations directly to the explosive mass, or bomb 'yield'," he said.

"Now expert seismologists can use these s caling relationships in analyzing any truck bomb explosion and give investigating agents very useful information on the size of bomb. If authorities also know something about the truck, they can then begin to speculate on the type of explosive used, who had access to such explosives, and so on."

The terrorist bombing of the Murrah Federal Building in Oklahoma City on April 19, 1995, was also seismically recorded. The first seismic records looked unusual, and prompted some researchers to conclude two separate blasts – that is, two separate bombs – were involved, Wallace said. But when the Murrah Building later was demolished, the seismic record was identical. leaving experts to conclude there had been a single terrorist bomb. "In some sense this is derivative, but it's really important for providing independent constraints on what happened," Wallace said.

"One of the most remarkable things is – and it's hard for people to visualize – but there's a huge number of seismometers out there, our ears to the ground. They are needed for earthquakes, but anything that makes a thump is going to be recorded," Wallace said.

Geophysicists are increasingly confident when it comes to interpreting curious signals that are unrelated to earthquakes, he added.

"But the most dramatic difference is that we now retrieve most of this data via the Internet. The Internet has provided an incredible pipeline to bring back this information and make it available to anyone. In my lab across the hall we bring in about 550 seismic stations. I look at the record where there's some specific event or region I'm interested in. But I can also get data from places where 1,500 seismic stations report in. "

Wallace has built a search engine that uses keywords like "explosion" to electronically glean news from several newspapers each morning. He then checks the seismic network to see which stations might have recorded events of special interest. In this way he is building a portfolio of seismic signals that can be used to identify mystery events. He'll know if a mystery event is an illicit nuclear weapons test, a coal mine collapse, a pipeline explosion, an airplane crash, some fiery meteor explosion or other occurrence.

On his web site,
Wallace describes what he and his colleagues have learned from seismic analysis about major news events. These include:

  • The accident that sunk the Russian submarine Kursk in the Barents Sea on Aug. 12, 2000
  • The Sept. 11, 2001, World Trade Center terrorist attack
  • An alleged secret 1989 Iraqi nuclear test, reported in the Feb. 25, 2001, Sunday Times (London)