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Monitoring the Nuclear Test Ban Treaty


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During the Cold War, the United States and Soviet Union conducted large nuclear weapons tests on average once a week. Although the countries made efforts to keep many of these tests secret, the explosions registered on seismometers around the world. The United States and others established extensive seismic networks to monitor these tests, but no rules were in place to regulate the underground testing of nuclear weapons.

That all changed in the fall of 1996 when the United Nations adopted a new treaty to ban the testing of nuclear weapons worldwide. Although President Clinton was the first head of state to sign the Comprehensive Nuclear Test Ban Treaty (CTBT), the U.S. Senate voted against its ratification in October 1999. One of the key concerns raised by opponents of the treaty was the ability to monitor testing and thus ensure that all countries are playing by the same rules — concerns shared by the current Bush administration. They argued that the United States would be unable to detect clandestine nuclear testing by other countries, thus putting the country at risk.

Since the first nuclear explosion, TRINITY, seismology and weapons testing have gone hand in hand. On July 16, 1945, TRINITY was detonated in central New Mexico with about a 15-kiloton yield. It produced seismic waves that showed up at seismic stations as far away as St. Louis. Jack Abbey/Los Alamos National Laboratory

But a study released by the National Academy of Sciences (NAS) on July 31 says that the detection of clandestine testing is technically manageable. The NAS panel — formed in 2000 at the request of Gen. John Shalikashvili, former chair of the Joint Chiefs of Staff — assessed the effectiveness of the International Monitoring System (IMS) created by CTBT by looking at its ability to detect small, hidden nuclear explosions, in addition to the traditionally larger explosions.
The report says that, using IMS, the United States would be able to detect nuclear tests as low as 1 or 2 kilotons virtually anywhere on or above Earth. By comparison, small nuclear weapons, such as those that devastated Hiroshima and Nagasaki, typically yield 10 to 20 kilotons. Modern strategic thermonuclear weapons have a yield of 100 kilotons or more.

With a nuclear testing ban in place, groups wishing to test weapons in secret have to become more creative — clandestine nuclear testing could happen underground, underwater, in the air and even in space. “There’s really no other treaty like this, that has anything like the level of complexity of monitoring compliance as this treaty has,” says Paul Richards, a seismologist at Columbia University’s Lamont-Doherty Earth Observatory and a member of the NAS panel that compiled the report, Technical Issues Related to the Comprehensive Nuclear Test Ban Treaty.

The report is the first comprehensive assessment of the performance of IMS, a significant part of the CTBT’s verification process. “It appears that IMS is capable of better performance than what had been expected for it in most people’s minds,” Richards says. The panel found that the most effective monitoring of nuclear explosions requires more than just seismology, as reflected in the variety of technologies used by IMS.

IMS, headquartered in Vienna, Austria, consists of 321 stations that include hydroacoustic, infrared, radionuclide and seismic sensors and 16 labs that analyze radionuclide detection signals. Of those stations, 170 are for seismic monitoring. The system is currently three-quarters complete. The International Data Center, also in Vienna, compiles and analyzes the information collected by IMS stations and creates the Reviewed Event Bulletin, a daily list of all seismic signals. So-called verification seismologists must sort through hundreds of seismic signals every day to “verify” natural seismicity vs. nuclear explosions.

Historically, most experience in monitoring nuclear explosions was at distances greater than 2,000 kilometers, using what verification seismologists call teleseismic signals. “For example, when the United States and the Soviet Union used to monitor each other, we didn’t have instruments on the Soviet territory, so it all had to be monitored from a considerable distance. But that’s all changed in the last 10 or 12 years,” Richards explains. Now that IMS has seismometers around the world, verification seismologists can use regional signals — signals from events less than 2,000 kilometers away — to monitor explosions. The ability to monitor signals at these closer distances is quite good, Richards says.

Nuclear weapons experts serving on the NAS panel created and analyzed possible scenarios for clandestine testing. The most difficult situations for monitoring explosions, they found, would come from countries attempting to test weapons in mining regions (where daily blasts occur) and in underground cavities. “It’s well known that such an underground cavity can reduce by a quite large factor the size of the seismic signals,” Richards says. However, the panel concluded that IMS would be able to detect signals in both the mining and cavity scenarios down to low yields.

Ironically, creating small explosions requires good nuclear capability. Therefore the only countries with the ability to conceal small-yield nuclear tests would be highly experienced nuclear-weapon states, such as the United States, Russia, the United Kingdom and France. Yet, for these nuclear-advanced countries, the panel concluded, constrained nuclear testing would provide little value in terms of adding to their nuclear capabilities.

Distinguishing a natural seismic signal from a mine blast or a nuclear explosion is fairly easy if the signal is large enough (Geotimes, August 2000). The signals vary in their frequency content. With smaller signals, frequency analysis is increasingly difficult. Mine blasting can result in seismic events smaller than magnitude 4.0 that superficially appear similar to nuclear explosions. Other, larger signals sometimes also produce initially confusing signals. The NAS report states that IMS can effectively identify trickier signals by looking at a combination of signals, such as an infrasound signal coupled with a seismic one.

For more than 50 years, the former Soviet Union continuously operated the Tyrnyauz Molybdenum Mine in Georgia, shown here. In 1993, an international seismic research team led by Brian Stump brought the first western scientists ever to instrument and document the blasting practices at the Tyrnyauz mine. Such work has helped researchers distinguish mine blasts from nuclear explosions. Image courtesy of Brian Stump, Southern Methodist University


Indeed, seismologist Brian Stump of Southern Methodist University says that using various IMS monitoring techniques together can be extremely useful in distinguishing nuclear explosions from, for example, mine blasts. Stump studies the characteristics of regional seismograms that might be unique to delay-fired mining explosions. “A delay-fired explosion is a series of boreholes — as many as 1,000 —each loaded with explosives and detonated in a timing pattern designed to fracture rock, cast near-surface material, and reduce in-mine ground motions,” Stump explains. His research team identified several criteria unique to mine explosions. Large-scale mining explosions, they found, generate intermediate-period surface waves as well as infrasound signals. “We have just completed an empirical study of infrasound generation by mining explosions. This illustrates how different IMS monitoring technologies may complement one another and improve the overall effectiveness of the monitoring system,” Stump says.

But some explosions simply are not distinguishable from nuclear explosions, Stump adds. On Sept. 22, 1993, the Department of Energy conducted a large, contained, single-fired chemical explosion at the Nevada Test Site, using almost 3 kilotons of commercial blasting agent. This explosion occurred near a previous nuclear detonation. As reported at a 1994 symposium about the Nevada experiment, seismologists could not distinguish between the seismic signals from the single-fired nuclear and chemical explosions. However, it is possible that gases released in such a chemical explosion would leak out, and in a nuclear test, radionuclides might leak out, providing a means to distinguish explosions using IMS. “Fortunately, as the new [NAS] report indicates, such large-scale chemical explosions are quite rare,” Stump says.

Indeed, Richards says, “the search for a technical fix here is not so important as making the point that the 1993 chemical explosion was unique. At the kiloton level, there has not been a commercial/industrial explosion of that type, nor is there any commercial use for such a blast.”

The results of the NAS report come as good news to proponents of the CTBT, who now hope that it will prompt the Bush administration to rethink its opposition to the treaty. While the administration is not pushing for the CTBT, it has not indicated any desire to end the current U.S. moratorium on nuclear testing. “The United States certainly needs to do that work of monitoring for foreign nuclear explosions regardless of whether there’s a treaty or not,” Richards says. “There is still a bipartisan understanding that there’s the need to do this work of monitoring regardless of one’s political opinion about verification capability or opinions on the treaty itself.”

Lisa M. Pinsker

Data Sharing and the CTBT

Every now and then, a seismic signal garners the attention of seismologists worldwide. While outside researchers sometimes contribute their analyses of these "problem events" ad hoc to the IMS, they work with separate stations. The sharing of data between IMS and independent researchers is a hot topic of controversy in the CTBT.

The Vienna-based IDC began as a prototype in 1996, operating near Washington, D.C., for five years {emdash} a test bed to demonstrate that countries could monitor a test-ban treaty, explains Ray Willemann, director of the International Seismological Centre in the United Kingdom. The prototype IDC made the Reviewed Event Bulletin openly available to the research community. Since its move to Vienna, the IDC has given the list to only National Data Centers and select users, says Willemann, who worked at the prototype IDC.

Under the CTBT, participating countries must reach a consensus on all decisions, including the issue of data sharing, a task that has proved particularly difficult. The IMS collects several types of data, including hydroacoustic, infrared and radionuclide, in addition to seismic. Many countries, Willemann explains, have difficulty separating the data when it comes to making decisions about data sharing.

Because CTBT countries are not treating the seismic data separately from the rest of the IMS data, countries that object to making the data openly available are largely concerned about releasing sensitive, largely non-seismic data, such as radionuclide signals, to the public.

Willemann and Richards, like most seismologists, believe that the seismic data should be freely available. Indeed, the NAS report states that a multiplicity of users would greatly enhance the quality of IDC operations. "This is a complicated system, acquiring data of considerable scientific importance. The best way to keep it operating really well is to have users who will occasionally point out if something doesn't appear quite right. There's nothing like having a community of users to keep you awake," Richards says.

Lisa M. Pinsker

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