Seismic monitoring stations detected the event, which appeared to be in the vicinity of Novaya Zemlya, an island that is the site of a Russian nuclear weapons testing facility. Newspaper reports quoted unnamed sources describing the event as having "explosive characteristics" and a Clinton Administration spokesman maintained that technical data "lend themselves to alternative explanations."
Had the event indeed been a nuclear explosion, it would mean the Russian government had violated the terms of the Comprehensive Test Ban Treaty (CTBT), an agreement laboriously negotiated over the last two decades by the United States, Russia and other nations that bans such testing.
Analysis by the two Columbia researchers, Paul Richards and Won-Young Kim, seismologists who specialize in nuclear test verification, shows the seismic event was a small earthquake. The research, published in the Oct. 23 issue of Nature, demonstrates that scientists have "excellent" capability to monitor such events in the vicinity of Novaya Zemlya, they said.
Other scientific views at odds with the U.S. government's interpretation of the Aug. 16 event have been reported in the Washington Post and the New York Times. But the Nature study is one of the first independent tests of a new International Monitoring System that is being developed to verify the CTBT. The system, created by signatories to the treaty, is a global arrangement of sensitive instruments designed to detect the various physical and chemical fingerprints of nuclear weapons tests.
Data from these instruments are to be analyzed by the nations participating in the treaty in order to determine compliance with the test ban. Since nuclear explosions can produce signals that may look like an earthquake, "we need to be able to differentiate the two," Dr. Richards said.
Dr. Richards saw the Aug. 16 event as an opportunity to test the capabilities of seismological data to monitor nuclear tests under the test ban treaty. In the Nature article, he and Kim state: "The ease of location and identification of the 16 August event, with a magnitude of about 3.5, demonstrates that the CTBT can be monitored near the Russian test site down to magnitude 3, and maybe even lower. . . . So the 16 August event indicates excellent capability to monitor the CTBT." Seismologists use the earthquake Richter scale to measure the size of both explosions and earthquakes; a magnitude of 3.5 corresponds to an explosive yield of about one-twentieth of a kiloton.
The Columbia seismologists drew on several lines of evidence to verify that the event was not an explosion. First, the scientists compared seismic recordings of the questioned event with the recordings of a known nuclear explosion set off by the former Soviet Union in 1990. Drs. Richards and Kim noted differences in the delay between the first arriving seismic shocks (P-waves) and ones arriving later (S-waves). The recordings, from a Finnish monitoring station west of Novaya Zemlya, showed that S-waves from the 1997 event took 8 seconds longer to reach the station than those from the 1990 nuclear test, indicating that the more recent event was in fact nearly 100 kilometers, or about 62 miles, east of the test site.
Additional work by other seismologists located the epicenter some 50 kilometers to the southeast of Novaya Zemlya, in a part of the Kara Sea nearly 400 meters deep. An explosion of this size located under water would be associated with a large acoustic signature, as sound waves produced by the blast were picked up by a network of sensitive seafloor microphones maintained by the U.S. government. No such acoustic signature was observed.
Neither the large underwater drilling apparatus that would have been necessary for such a test, nor any radioactivity, was observed. Drs. Richards and Kim further noted the presence of several aftershocks in the region, which would not be expected if the event had been a nuclear test.
"Aftershocks are common for earthquakes, but would not be detectable at IMS stations after small nuclear explosions at Novaya Zemlya," Dr. Richards said.
Drs. Richards and Kim also analyzed the seismic waves produced by the event and recorded on seismological monitoring stations scattered throughout Eurasia. These data are collected by various U.S. government and international programs and are openly available to the scientific research community. Using a method of analysis that carefully measures the size of shock waves at high frequencies, the seismologists found that the ratios of P to S waves were consistent with ratios for known earthquakes with epicenters near Novaya Zemlya. This technique, which has been carefully refined through years of research at Lamont-Doherty and elsewhere, provides the most definitive evidence that the Aug. 16 event does not have "explosive characteristics," the scientists said.
To Drs. Richards and Kim, the data are clear: the event was an earthquake. However, they had the luxury of time to track down available data and use the most recent methods of data analysis. U.S. government officials, they said, must work within the framework of treaty monitoring.
"It may be difficult for these officials to accept that data from non-IMS stations and data analysis by independent groups of seismologists have had such prominence," Drs. Richards and Kim write in Nature. "When the research community is able to demonstrate a good new method of discrimination, or the need for good communication to non-IMS stations with openly available data, the development must be assessed and operational procedures perhaps revised."
The U.S. government needs to provide mechanisms that allow officials to draw on new research in their decision-making, the scientists said. "But there are more than ten thousand seismographic stations deployed around the world for the general purpose of research into earthquake hazard and the structure of the Earth's interior, so that supplementary data on problem events in the CTBT context will often be available," they wrote.
Despite the U.S. government's refusal to acknowledge a mistake, both that government and the IMS have the technology to discern earthquakes from nuclear explosions, the scientists said. "Our capability to monitor the CTBT is magnificent. We have all these discriminants upon which knowledgeable decisions can be based," Dr. Richards said.
Lamont-Doherty Earth Observatory is a part of the Columbia Earth Institute, founded this year to promote wise stewardship of the Earth's resources.