Fallout From a Nuclear Nonevent

On Aug. 16, a seismic event occurred in the vicinity of the Russian nuclear test site in Novaya Zemlya, a large island in the Arctic Ocean. According to the Washington Times newspaper, an initial identification of “explosive characteristics” was made, raising the possibility of a clandestine nuclear test by the Russians. On Sept. 13, the Associated Press reported that a “secret” follow-up Air Force study had located the event well offshore, which points to a small earthquake, not an explosion.

What has gone unreported is that information to assess the character of this event is in the public domain. When the prospect of a clandestine explosion became known in late August, a number of university seismologists obtained data from nearby “open” seismic stations in Norway, Finland and in Russia itself--data available to anyone with a modem or an Internet connection. The publicly available evidence argues strongly that the Aug. 16 event was an earthquake of roughly magnitude 3.3, not a clandestine nuclear blast.

All this suggests that existing seismic verification technology is equal to the task of effectively monitoring the Comprehensive Test Ban Treaty, which President Clinton sent to the Senate this week, urging ratification. The president, in a U.N. speech on Monday, also called on other nations to go forward with ratification--146 have signed the treaty, but it has been ratified by only four.

The controversy over the Aug. 16 seismic event indicates that we need to review carefully the U.S. procedures for detecting, identifying and publicizing suspected treaty violations. It also demonstrates the importance of maintaining a global network of open seismic observatories so that future suspect seismic events can be characterized as rapidly as this one has been, without revealing classified data sources.

There are two quick methods for assessing whether a seismic event is a possible clandestine nuclear test: the location of the event and the relative amplitudes of seismic P (compressional) and S (shear) waves. Location is determined by triangulating the arrival times of seismic waves at seismic observatories. Regarding the Aug. 16 event, if data from only one or two stations are used, the “uncertainty ellipse” can straddle much of Novaya Zemlya and the surrounding ocean, possibly suggesting a location at the Russian nuclear test site. As data from more stations are included, however, the location is put off the east coast of Novaya Zemlya, with a tight uncertainty ellipse.

Explosions and earthquakes stress the surrounding rock differently, and so excite P and S waves differently. P waves are like sound waves, involving the compression of rock, and are generated by explosions, in either rock or water, more strongly than S waves. S waves involve shearing, where rock moves sideways as the wave passes through. Earthquakes involve sliding along a fault surface and can generate large S waves in relation to P waves.

Novaya Zemlya has a long record of both small earthquakes and Russian nuclear tests. Examples of seismograms from the Aug. 16 event and previous Novaya Zemlya earthquakes and explosions were faxed among several university seismologists so that we could judge for ourselves whether the event “looks like an explosion.” It does not. The S wave is large, similar to previous earthquakes in the area.

Press reports that the seismic signals “look like an explosion” are suspect. My colleagues have attempted to identify the source of this opinion, but no U.S. government seismologist would make such a claim publicly or privately. In fact, sources within the government confirm observations made by academic seismologists from public data. The evidence is strong that the Novaya Zemlya event was an earthquake beneath the sea floor, far from known nuclear test sites.

The controversy should not cloud the Senate’s debate over the test ban treaty. The Aug. 16 event stems from an incorrect preliminary assessment of the event’s character, not a genuine disagreement among experts. If a healthy network of open seismic stations is maintained, the vast majority of suspect seismic events will be examined and disposed of before security alerts develop into diplomatic imbroglios. Freely accessible information will greatly aid the monitoring of the Comprehensive Test Ban Treaty for all signatory nations.