WHEN the world received the news of the Indian and Pakistani clandestine underground nuclear tests last May, a team of Livermore researchers used the events to validate several seismic methods they have developed over the past decade to monitor the Comprehensive Test Ban Treaty (CTBT). Using data recorded worldwide by a host of seismic monitoring stations, the team successfully differentiated the nuclear blasts from typical regional earthquakes, characterized the yields of the tests, and noted inconsistencies between the announced test yields and the seismic data. In all, the seismic signals from the nuclear tests provided important new data to help calibrate seismic stations in a critically important region of the world.|
The CTBT has been signed by 152 nations, although not by India or Pakistan. The treaty provides for an International Monitoring System (IMS) of automated seismic stations, many of them still to be installed, to record any evidence of clandestine nuclear explosions. These stations transmit data via satellite to the International Data Center in Vienna, Austria, which in turn distributes them to national data centers around the world. Figure 1 shows the location of existing seismic stations in the Southwest Asia area, planned IMS seismic stations, the seismically determined locations of the recent tests by India and Pakistan, and locations of some recent earthquakes in the region.
The U.S. Department of Energy is supporting the U.S. National Data Center (USNDC) at Patrick Air Force Base, Florida, as it prepares to monitor the treaty. As part of DOE's effort, teams at Livermore and Los Alamost have been working to improve ways to seismically characterize clandestine underground nuclear explosions and differentiate them from other sources of seismicity, such as earthquakes and mining explosions. Much of Livermore's work has centered on developing regional discriminants, which are characteristic features of a seismic waveform (for example, the peak amplitude at a particular frequency, within a specific time frame) recorded at distances less than 2,000 kilometers away. These discriminants are used to differentiate between explosions and other types of seismic sources. (See the September 1998 Science & Technology Review, "Forensic Seismology Supports the Comprehensive Test Ban Treaty," pp. 4-11.)
India's nuclear test took place on May 11 and 13, 1998, followed by Pakistan's on May 28 and 30, 1998. None of the planned IMS seismic stations in the region was installed at the time of the tests. Fortunately, stations belonging to IRIS (Incorporated Research Institutions for Seismology), a consortium of U.S. universities, were operating. Two of those stations, called ABKT, in Alibek, Turkmenistan (one of the former Soviet republics), and NIL, in Nilore, Pakistan, were near the sites of two proposed IMS stations GEYT and PRPK. While ABKT data were not available, NIL records of the Indian tests, some 740 kilometers away, were available through the Internet within a few hours, as were data provided by IRIS for other stations throughout the world. The NIL station was turned off during the Pakistan tests, so the data were unavailable.
As part of their calibration work for the USNDC, the Livermore seismologists had already collected and analyzed data recorded by NIL and other seismic stations from more than 200 regional earthquakes between 1995 to 1997 in Iran, Afghanistan, Pakistan, western India, and the surrounding region. Within hours of the announcement of the May 11, 1998, Indian tests, Livermore seismologists were comparing its seismogram with those from nearby earthquakes.
As seen in Figure 2, the seismogram from a representative earthquake clearly differs from that of the May 11 test. Livermore-refined discriminants based on P and S waves were strongly indicative of an explosion, not an earthquake or other seismic source, at all frequencies tested (0.5 to 8 hertz). Livermore seismologist Bill Walter explains that the differences in seismic P- and S-wave energy provide one method of discriminating explosions from earthquakes. Seismic P waves are compressional waves, similar to sound waves in the air. Shear (S) waves are transverse waves, like those that propagate along a rope when one end is shaken. Because underground explosions are spherically symmetric disturbances, they radiate seismic P waves efficiently. In contrast, earthquakes result from sliding or rupture along a buried fault surface and strongly excite the transverse motions of S waves. Thus, we expect that explosions will show strong P waves and weak S waves and that earthquakes will show weak P waves and strong S waves, as seen in Figure 2.|
According to Walter, one way to quantify this difference is by determining the ratio of P-wave to S-wave energy measured from the seismograms. Explosions should have higher P/S ratios than earthquakes, but the frequency at which the best separation occurs varies by region and station. Figure 3 shows the P/S ratio for the May 11 Indian test and for earthquakes shown in Figure 1. The measurements in Figure 3 were made at four different frequencies. The Indian test has a higher P/S ratio than the earthquakes, as expected.
India reported that its nuclear testing on May 11, 1998, was composed of three almost simultaneous explosions with yields of 45, 15, and 0.2 kilotons and that the two larger tests were separated by about a kilometer. According to Walter, the team's examination of regional data recorded at NIL and at teleseismic stations thousands of kilometers away did not reveal obvious signs of multiple shots. The U.S. Geological Survey reported a teleseismic magnitude of mb 5.2 (mb is the bodywave magnitude and is roughly related to the Richter scale). Assuming simultaneous detonation of the three tests and using published magnitude-yield formulas for a stable region, the announced total yield of 55 to 60 kilotons appears to be at least three times larger than the yield indicated by the seismic data.|
Livermore researchers then compared the seismogram from the May 11, 1998, tests with India's May 18, 1974, single test (its only previous nuclear test) using data from stations in Canada and Scotland that recorded both events. The 1974 test generated a clearly detected teleseismic signal with an mb of 4.9. Because India declared the 1974 explosion a "peaceful nuclear explosion," some information about it was reported, such as the fact that it was a single explosion at a depth of 107 meters. However, Indian scientists and officials stated a large range in the yield estimate-4 to 12 kilotons.
Figure 4 shows the seismograms from the 1974 and 1998 tests using data from the Canadian station (for ease of comparison, the 1974 test's amplitude is doubled to match that of the 1998 test.) The two seismic waveforms show remarkable similarity.
Several interpretations of the seismic observations are possible. According to Livermore seismologist Arthur Rodgers, if the three 1998 shots were indeed detonated nearly simultaneously and separated by less than a few kilometers, "We would probably see just one large shot in the seismic waves." Rodgers also says that the second and third shots could have been so small compared to the first that they were overwhelmed in the seismogram. Also, a cavity or substantial amount of porous material near the explosive site could, if present, have reduced the coupling of energy into seismic waves, thereby significantly reducing the seismic magnitude of all three tests. Finally, it is possible that the yield announced by the Indian scientists was simply three to six times too large.|
On May 13, India announced two additional low-yield tests totaling 800 tons. The Livermore team examined data provided by the NIL station, but none showed any obvious seismic signal. Using the largest amplitude of the background earth noise around the time of the test as an upper bound for the signals from the event, the Livermore researchers determined that the event must have produced an mb of less than 2.8. The two tests were said to be conducted in a sand dune, which might poorly couple the explosive energy into seismic waves and thus reduce the strength of any recorded seismic signal. Adjusting for this geologic condition, says Walter, a signal should have been observable at NIL if the yield was 100 tons or more.
Walter says that the nuclear tests in India provided valuable data in a region with only a single previous nuclear test. This data will be used to help calibrate the CTBT monitoring network.
The data from the Indian tests will also improve scientists' understanding of the physical basis of the regional discriminants developed at Livermore. As a result of the tests, the discriminants may be applied with greater confidence to much lower yield explosions than the Indian tests and in South Asia and other regions where no nuclear test data are available to calibrate nearby monitoring stations.
The Livermore team plans to conduct more research to further characterize the May events as additional seismic data and information on emplacement conditions become available from Indian and Pakistani officials and scientists. In the meantime, researchers are hopeful that their detailed analysis of the nuclear tests, done without the forthcoming IMS stations, shows that the planned international network will indeed be effective in detecting and identifying clandestine nuclear tests.
Key Words: Comprehensive Test Ban Treaty (CTBT), discriminants, U. S. National Data Center (USNDC), nuclear test.
For further reading:
W. R. Walter, A. J. Rodgers, K. Mayeda, S. Myers, M. Pasyanos, and M. Denny, Preliminary Regional Seismic Analysis of Nuclear Explosions and Earthquakes in Southwest Asia, Lawrence Livermore National Laboratory, Livermore, CA, UCRL-JC-130745, July 1998.
For further information contact Bill Walter (925) 423-8777 (email@example.com) or Arthur Rodgers (925) 423-5018 (firstname.lastname@example.org). Information on DOE's overall CTBT program may be found at www.ctbt.rnd.doe.gov.