The May 1998 India and Pakistan Nuclear Tests

Terry C. Wallace
Southern Arizona Seismic Observatory (SASO)
Department of Geosciences
University of Arizona
Tucson, AZ 85721

Introduction

In September 1996 the United Nations voted to adopt a Comprehensive Test Ban Treaty (CTBT), which prohibits any nuclear explosions, whether for weapons or peaceful purposes. The outlawing of nuclear weapons tests significantly limits a country's ability to develop or improve a nuclear weapon, so a CTBT is considered the keystone to stopping nuclear proliferation. The concept of a CTBT started with the Eisenhower administration, so the 1996 vote was truly historic. However, the UN vote to adopt the CTBT was not unanimous: 158 countries voted in favor and 3 opposed (India, Bhutan, and Libya), with 5 abstentions. On May 11, 1998, the one declared nuclear state which was opposed to the CTBT shocked the world by conducting a nuclear test in the Rajasthan desert in northwestern India. Two days later India announced that it had conducted two further tests, and government spokesmen claimed that India needs nuclear weapons to prevent "military adventurism" by neighboring Pakistan.

On May 28, Pakistan's Prime Minister, Nawaz Sharif, announced that his country had conducted five nuclear tests of its own and "settled the score" with India. Although Pakistan had voted in the UN to adopt the CTBT, it had declared it would not sign it as long as India did not. Unlike nuclear tests conducted by the five major nuclear powers, which in the past were shrouded in secrecy, both the Indian and Pakistani governments held several press conferences and announced explosive yields, how many explosions were involved in each test, and even the types of the nuclear weapons. This provided a unique opportunity to compare seismic analysis with governmental pronouncements. This is especially timely, as the U.S. begins debates about ratifying the CTBT. Seismology is the main monitoring technology for buried explosions, and the India and Pakistan tests provide a ground truth test of the as yet incomplete International Monitoring System (IMS), the role of seismic stations which are not part of the IMS, and rapid access through real-time data links.

The India Nuclear Tests

The Indian government held a press conference on May 11, 1998, to announce that they had conducted three nuclear tests at their Pokharan test site in northwestern India . A later press conference on May 17, which included several nuclear scientists, described the tests in detail; the three devices were detonated simultaneously and had yields of 43, 12 and <1.0 kt, respectively. Videotape accompanying the press conference showed ground roll associated with the explosions, and it was apparent that the three devices were exploded in at least two different vertical shafts. The largest explosion (43 kt) was reported to have been a thermonuclear weapon, which purposely had a reduced yield to mitigate damage to nearby towns. Despite these precautions, later Indian press reports stated that more than 40% of the structures in the town of Khetolai suffered some sort of damage. The Indian government announced two further tests on May 13. The announced yields of these tests were subkiloton (0.6 and 0.2 kt), and at least one scientist at the press conference said these events were detonated in "a sand dune."

The May 11 Explosions -- Location

The May 11 tests were detected and located by routine operational algorithms of the Prototype International Data Center (PIDC) and the U.S. Geological Survey. The PIDC posted its preliminary location on a public access seismicity listing (http://www.css.org/products) just over an hour after the detonation. The USGS never did post its location on its public access bulletin (http://geology.usgs.gov/quake.shtml), although it was in the catalog within a few hours after the detonation. Both the PIDC and USGS produced a revised location after collecting more data (Table 1), and these locations along with their 90% confidence ellipses are shown in Figure 2. The locations are in a region of very flat topography and close to a previous "peaceful" nuclear test conducted by India in 1974.
Table 1. Origin Time and Location of Indian Events
  India May 11, 1998 Pakistan May 28, 1998 Pakistan May 30, 1998
  OT Lat Lan OT Lat Lon OT Lat Lon
REB 10:13:44.2 27.072 72.761 10:16:17.0 28.903 64.893 06:54:57.1 28.495 63.781
USGS 10:13:42.0 27.102 71.857 10:16:15.2 28.862 64.818\ 06:54:54.9 28.487 63.787
JHD       10:16:17.8 28.856 64.906\ 06:54:57.1 28.499 63.741
 
India conducted its first nuclear test on May 18, 1974. The Bhabha Atomic Research Center (BARC) released a report on the explosion which stated: (1) The device was exploded underground, at a depth of 107 m, in a vertical shaft. (2) The device was detonated at approximately 23:45 GMT. (3) The yield of the explosion was approximately 12 kt (later revised to 8 kt). Although the exact location of the explosion was not revealed, the Indian government did release a photograph of a subsidence crater formed shortly after the explosion. Gupta and Pabian (1996) located the subsidence crater on commercial satellite photographs (27.095 0.001N, 71.752 0.001E), providing ground truth for seismic locations  shows a picture release by the Indian government of the subsidence crater above the 1974 test). The locations are approximately 25 km from the town of Pokharan, and the area was therefore referred to as the Pokharan test site. In late 1995,  The New York Times  ran a story which stated that there was activity at the Pokharan test site consistent with possible preparations for another nuclear weapons test. Gupta and Pabian (1996) collected and analyzed the commercial satellite imagery to investigate these allegations, and they came to the conclusion that they could identify two vertical shafts, separated by about 1 km, located approximately 3 km southwest of the 1974 crater (plain star, Figure 2). The location of the midpoint between these shafts is plotted on Figure 2. It now appears that these shafts were used in the May 11 tests, providing a calibration for the seismic locations. The REB and USGS locations are within 3 and 12 km of the star in figure 2, which is presumed ground truth. Since the seismic locations are tied with ground truth the bias in travel time residuals can be calculated and any further Indian tests will be located with relatively small uncertainty.

The May 11 Explosions -- Seismic Yield

The seismic waves that are generated by nuclear explosions have been the subject of extensive research for nearly 40 years. One of the most remarkable facts derived from this research is that the high-frequency seismic radiation from well-tamped nuclear events is nearly independent of source rock type. This is true for granite, water saturated tuff, shale, and limestone. There are some exceptions, including weak clays and some very porous materials, but this generation allows for the calculation of explosive yield from a high frequency measurement of seismic size like mb. Mathematically this is expressed as a proportionality between mb and explosive yield: mb scales as log Y. If yield is expressed in kilotons, the constant of proportionality is approximately 0.8. Using this fact, it is possible to write a general relationship between yield and mb:
mb = C + 0.8 log Y
(1)

The constant C in (1) is very dependent on the seismic attenuation of the source region, in particular the attenuation associated with the upper mantle beneath the explosion source. It is possible to determine C by measuring the seismic spectra at teleseismic distances. In general, C is smaller for tectonically active regions, like the Basin and Range Province, as compared to stable tectonic environments like the Indian Shield. The value of C varies from about 3.9 to 4.5. If mb is well determined (averaged over a large number of stations), it is possible to get a robust estimate of explosive yield.

The Indian test site lies within a very stable tectonic province, the Indian Shield. Examination of the spectra from the May 11 explosion indicates that attenuation in the test region is very similar to that seen beneath the former Soviet Union test sites in Kazakhstan. The standard relationship used for stable environments is (Ringdal et al., 1992):

mb = 4.45 + 0.75 log Y
(2)

This relationship was developed from mb calculated with the traditional definition, which requires a period of approximately 1 Hz. Great care must be taken in applying this formula, especially since many of the mb measurements that are now reported are not made at 1 Hz.

The USGS reported a mb = 5.2 for the May 11 explosions (preliminary magnitudes were slightly higher). This implies a yield of approximately 10 kt. Considering uncertainty in the yield relation and magnitude variability, we determine that the May 11 event had a yield of 10-15 kt. This value is much smaller than that reported by the Indian government; if all three reported tests were detonated simultaneously, the yield would be the sum of the individual explosions as announced by the Indians, or approximately 55 kt (55 kt corresponds to a mb = 5.76). On the other hand, if the explosions were delayed by several tenths of a second, then the mb would only represent the largest of the explosions. However, the reported yield of the thermonuclear device was 43 kt, which is still much larger than expected from the observations.

Although there is uncertainty in the absolute determination of the seismic yield, the relative yield between two tests can be calculated with a much higher precision. If Equation (2) is used to calculate the yield from two explosions at the same test site, then the differences in magnitude can be expressed as

mb = 0.8 log (Y1 / Y2)
(3)

where Y1 and Y2 are the yields of the first and second events respectively. The mb reported for the 1974 Indian test was 4.9; applying Equation (2) gives a ratio of yields of 2.4. If the yield determination of May 11 is correct, then the yield of the 1974 explosion would be 4 to 6 kt. The Indian government announced the yield for 1974 as 8 to 12 kt; however, there have been reports that the actual yield of the 1974 test was really less than 5 kt, and possibly as small as 2 kt.

The May 11 Explosions -- Crater Morphology

It is possible to calibrate the seismic yields with other features caused by the explosion, in particular the shape, size, and character of the crater formed above the detonation. Several investigators (Houser, 1970; Weaver et al., 1990) have looked at the nature of craters as a function of the depth of burial for explosions at the Nevada Test Site and several of the former Soviet Union test sites. For explosions which are buried at shallow depths, material is ejected after detonation and a "throw-out" crater is formed (the evolution of craters as a function of depth of burial can be viewed by following the link to . Throw-out craters are easily identified on satellite photographs from an ejecta blanket, not unlike the ejecta associated with a meteorite impact. As the depth of burial increases, the aspect ratio (crater depth versus crater diameter) changes until some critical depth at which instead of material being ejected, it is pushed up into a dome-like structure, referred to as a retarc (this word comes from "crater" spelled backward, from P. Richards, personal communication). Increasing the depth of burial results in a contained event in which a cavity is formed around the detonation point. This cavity is highly pressurized immediately after detonation; as time passes the pressure in the cavity decreases, and eventually the cavity collapses and material from above the cavity falls into the cavity. The region above the cavity which fails and falls into the cavity is called the "chimney." For a certain range of burial depths the chimney eventually reaches the surface, producing a subsidence crater. Subsidence craters can be differentiated from throw-out craters by the lack of ejecta and the presence of circular tension fractures. Continuing to increase the depth of burial inhibits the development of the chimney. As depth increases, the chimney does not reach the surface or develop a crater; eventually a depth is reached at which no chimney is formed because the cavity stresses are sufficient to totally support the overburden.

The goal of the U.S. nuclear test program was to design the collapse chimney to assure no release of radioactive by-products of the explosion into the atmosphere. The optimal design is for the chimney to reach the surface and form a subsidence crater. Experience from the Nevada Test Site (NTS) led to the development of a scaling relation between depth of burial and optimal containment of radioactive gases:

d = 122 Y1/3
(3)

where d is the depth of burial in meters and Y is the yield of the explosion in kilotons. In other words, at the NTS a 1 kt explosion would be buried 122 m, and it would be expected that a clear subsidence crater would be developed. The relationship in Equation (3) does depend on rock type. However, rock type, and consequent rock strength, influence the depth-of-burial relation, although only by a relatively small factor. When a test is designed, the device is buried at the maximum credible yield, which is larger than the expected yield.

The Indian government released a photograph of the crater formed by the 1974 explosion. In that photo it was possible to identify tension cracks, and therefore the crater was classified as a subsidence crater. BARC stated that the depth of burial for the 1974 event was 107 m. Using the U.S. depth-of-burial formula, the 1974 test implies a subkiloton size. Even allowing a factor of two adjustment for the strength of materials at the Pokharan test site, the announced depth of burial and subsequent crater indicate a yield of < 5 kt. This is very consistent with the seismic estimates of the yield using the teleseismic mb determination of 4.9.

The Indian government released several photographs of the test areas for the May 11 explosions. At least one of the tests was accompanied by a subsidence crater; it is likely that this crater is associated with the largest of the tests on May 11. Assuming that the Indian scientists used the same depth-of-burial formula for 1998 as they did in 1974, one can infer that the seismic and crater estimates of yield for 1974 can be used to calibrate the 1998 explosion/yield relationship. Thus, the magnitude difference formulation (Equation 2) implies a total yield for May 11 of <12 kt. In summary, the seismic yield equation, character of subsidence crater, and comparison with the 1974 explosion all give a consistent value of yield of 10 to 15 kt.

The May 11 Explosions -- Multiple Tests?

The announcement that the May 11 test actually involved 3 different explosions was a source of great interest in the press. However, detonating multiple explosions simultaneously was a fairly standard practice for both the US and the former Soviet Union. In fact, the Threshold Test Ban Treaty (TTBT), a bilateral agreement between the US and the Soviet Union, defined a nuclear test as (1) a single underground explosion, or (2) two or more explosions detonated within 2 km of each other and separated in time by no more that 0.1 seconds. There are several reasons for conducting simultaneous explosions, including economy and the effects of radiation on weapons or other structures.

 

We conducted two simple tests to look for multiple explosions within the waveforms of the May 11 event. First, we examined the teleseismic determination of mb as a function of azimuth. We took all values of mb reported in the USGS EDR (Earthquake Data Report) and looked for trends; if two explosions were separated by a kilometer the relative arrival times of the P waves would shift as function of azimuth. There should be two azimuthal ranges for which the interference is optimal and the magnitude is enhanced. We did not observe any coherent trend. This does not preclude separated explosions, but does limit the tempo-spatial distribution of sources to be a fraction of the period of the mb measurement (1 second). The second procedure we tried was a comparison of the May 11 waveforms with the 1974 waveforms at stations which recorded both. An examination of the EDR suggests that this comparison should be possible at approximately 10 stations. However, we could only make the comparison at one station, Yellowknife (YKA) in Canada. We deconvolved the 1974 waveform from the May 11 waveform and found a very high degree of similarity and no evidence for source multiplicity.

The May 13 Explosions

The Indian government announced the origin time of the May 13 test to be 12:21 New Delhi time, which is equivalent to 06:51 GMT. Neither the PIDC nor the USGS detected the explosion, prompting several groups (Khalturin et al., J. Park, F. Vernon) to examine regional seismic records recorded at Nilore (NIL) and the Kyrgyz Network (KNET) for evidence of these events. We analyzed 12 hours of the short-period seismic recording at NIL (6 hours before and 6 hours after the announced origin time). The short-period channel at NIL is derived from a STS-2 seismometer and sampled at 40 sps. NIL is located approximately 740 km north of the Pokharan test site, and the May 11 test was recorded with a signal-to-noise ratio of 800 to 1000 shows the May 11 vertical component waveform). Analysis of the noise spectra on May 11 and May 13 indicates that the operation conditions are essentially the same on both days.

We performed two types of analysis to try and identify a signal from the May 13 events. First, we filtered the data in the frequency band of 5-15 Hz and then passed the data stream through a detection algorithm which triggered at a signal-to-noise ratio of 3 to 1. Although this procedure detected three Hindu Kush earthquakes, nothing was detected that could be have an explosion. Next we used the May 11 explosion as a filter and cross correlated it with the entire 12-hour NIL data stream. There were no windows with correlation coefficients which were large enough to signify a detection. Figure 3 shows a comparison of the time windows on May 11 and 13 which correspond to the expected seismic arrivals at NIL. The net result of the analysis is that there is no evidence of any explosions on May 13.

The null result can be used to place some constraints on the size of the May 13 explosion. We calculated a mb (Lg) of 5.1 for the May 11 event; assuming a signal-to-noise ratio of approximately 1000, the detection capability at NIL for the Pokharan test site is mb (Lg) = 2.5. Using the yield difference equation (Equation 2), the detection capability can be expressed in terms of the yield of the May 11 event: Yield (May 11) / Yield (May 13) = 1800. Conservatively, this implies that the yield of May 13 events were at least three orders of magnitude smaller than the May 11 yield, or 10-15 tons.
It is possible to increase the maximum possible size of the May 13 events by assuming that they were detonated in very porous (and dry) media (recall the press reports that the event was detonated in a sand dune). This could increase the allowable yield by an order of magnitude, or 100-150 tons. This yield is still much smaller than the combined announced yield for the two explosions of 0.8 kt.

The Pakistan Tests

Immediately after the Indian test on May 11, Pakistan denounced the action, the Pakistani Prime Minister announced that his country "had the right to take any steps which are essential for Pakistan's security," and the Foreign Minister said that Pakistan has the technical capability to match any "threats," all but confirming what had long been suspected: Pakistan could produce nuclear weapons. Numerous western nations intensely lobbied Pakistan's government to forego a nuclear test of its own, but on May 26 the CIA reported that Pakistan was ready to conduct a nuclear test. On May 28 Pakistan announced that it had conducted five nuclear tests at its test site in the western part of the country. Subsequently, various press conferences with Foreign Ministry officials and nuclear scientists reported different, and often conflicting, descriptions of the tests (for example, the test was described variously as 2, 3, or 5 separate devices). The reported yield of the largest of these explosions was 30-35 kt, while the remaining were said to be of low yield and tactical in nature. The total yield of the five tests was announced to be 40-45 kt.

Two days later, on May 30, Pakistan announced that it had tested two warheads with yields of approximately 12 kt. The official announcement was later changed to read that Pakistan had tested a single nuclear weapon with a yield of 15-18 kt. Within a week of the May 30 test, the Pakistani government announced that it had completed its test program and no further explosions were planned.

The May 28 Explosions

The May 28 tests were detected and located by the routine operational algorithms of the PIDC and the USGS. As with the May 11 tests, both organizations refined their locations with additional data. Those locations are listed in Table 1 and are shown on In 1986 Pakistan was reported to have conducted a "cold" test . a nuclear implosion device in a region in the southwestern part of the county called the Chagai Hills (Hough, 1995), and it was assumed that any nuclear test would be conducted there. The mountainous region shown on the shaded relief map is sometimes referred to as the Chagai Hills, although the location of the May 28 test is more accurately described as being in the Ras Koh Range.

Unlike the Indian test site, there is a level of background seismicity in the Chagai Hills area. Because of the monitoring interest in the area, several of these earthquakes have been studied; two of the earthquakes (December 4, 1997, and January 5, 1998) were relocated by the PIDC using all available teleseismic arrivals and are listed as calibration events (the locations in Figures 4 and 6 are labeled CEB, or "calibration event bulletin"). We used the teleseismic arrivals for the CEB events and the explosions on May 28 and 30 to perform Joint Hypocenter Determinations (JHD) using the algorithm of Dewey (1983). The method simultaneously calculates all the hypocenters relative to a reference event; we chose the December 4, 1997 CEB event as the reference. Three of the events were recorded at the same 22 seismic stations located at teleseismic distances ; the May 30 explosion was recorded at 21 of the 22 stations. These stations provide good azimuthal coverage . The JHD locations (white stars, Figures 4 and 6) clearly indicate that the May 28 and May 30 explosions were located at distinct test sites separated by about 100 km. Figure 6 shows an enlargement of the May 28 test region. The contours on the map are spaced at 500 m, and it is apparent that the JHD location is on the face of a steep mountain. This observation is consistent with statements by Pakistani officials that indicate that the test occurred in a horizontal rather than a vertical shaft. Subsequent satellite photos shown in various press accounts indicate that the tunnel adit may have been on the south side of the hill within the JHD error ellipse. Perhaps the first recognition that the tunnel was on the south side of the hill was by Frank Pabian: he correlated the time of day with the broadcast images of the tunnel to draw the conclusion that the tunnel entrance was located on the southern side of the Koh Kamaran massif. Subsequently, Pabian provided the following information on the geology of the area: "The mountainous area , identified as the Koh Kambaran massif, rises to a maximum elevation of 2700 meters east and north of the Rayo (seasonal) river valley. It consists of intrusive diorites and syenites of the Post-Paleocene and Pre-middle Eocene Ras Koh formation. These have intruded through the older Cretaceous Kuchakki volcanic group. The Kuchakki formation, in turn lies conformably against the younger Paleocene Rakhshani formation (consisting of shales, limestones, sandstones, and volcanic sediments) on the southeast, and faulted against the same Kuchakki formation. The Kuchakki formation that is situated on this faulted side is also intruded by a wedge of ultabasic rocks of the Bunap formation. A number of old chromite mines are located within the Bunap formation. The geology and the associated steep mountainous terrain of the Ras Koh massive are indicative of very hard rock of substantial thickness."

The USGS reported a mb of 4.8 for the May 30 test. The Chagai Hills region appears to have a slight attenuation bias compared to the Indian test site, and a more appropriate formula for the yield relation is:

mb = 4.10 + 0.75 log Y

This gives a seismic yield of approximately 9 kt. Applying the same uncertainty used in the Indian analysis our estimate of the yield of the May 28 explosion is 9-12 kt.

The seismic waveforms for the May 28 explosion appear much more complex than those from the Indian explosion. Even at teleseismic distances the P wave does not appear simple, and many stations have a significant coda which lasts approximately 25 seconds . This complexity may be the result of source multiplicity, although there are several other possible explanations including scattering from the complex topography in the source region. We attempted to use the waveforms from the May 30 explosion (described below) to isolate source multiplicity within the May 28 waveform. We were able to find a second "event" in the coda, but it was delayed by 22 seconds from the first P wave. This would seem to be a highly unlikely multiple explosion scenario, and so the significance of this event was discounted.

 

The May 30 Explosion

The May 30 explosion was routinely located by the PIDC (see Figure 4) approximately 100 km to the southwest of the May 28 test. The data available for the May 30 event are hampered by the fact that one-half hour prior to the detonation there was a large earthquake in Afghanistan (May 30, 1998, 06:22, MS = 6.9). The bodywaves for the explosions are in the surface wave coda for most of the far-regional and close-teleseismic stations, although it was fairly well recorded at high frequencies. The waveforms for the May 30 event are simpler than those observed for May 28, but still much more complex than for the Indian test. The JHD location puts the event in a region of very subdued topography, especially compared to the May 28 event. This is consistent with remarks by Pakistan's Foreign Minister Gohar Agab Khan about the May 30 test: "The previous ones were in hard rock, but these were conducted in a shaft like a well."

The PIDC reported a magnitude for both the May 28 and May 30 events, which makes it possible to use the magnitude difference to derive a yield. The delta mb = -0.2, which translates to the May 30 event being a factor of ~1.9 smaller than the May 28 explosion (in other words, a yield of 4-6 kt). The difference in emplacement procedures between the two tests may introduce some error. However, experience at NTS, where it is possible to make many comparisons between tunnel and vertical shaft events, indicates that the yield relationship should not change much.

As discussed earlier, the May 30 explosion shows some seismic waveform complexity at regional distances. The apparent simplicity of the source of the May 30 event (a single announced explosion of 15 to 18 kt) cast doubt on interpreting the waveform complexity of May 28 in terms of multiple sources. At this time it is not known if the May 30 test created a crater, or whether such a crater was of the throw-out or subsidence type.

Discussion

In the days after the May 11 Indian test there was considerable political bluster in the U.S. to the effect that there had been a total failure by the intelligence community in warning that a nuclear test was coming. While it may be true that there was a failure to predict the test, it is clear that the monitoring system developed for assuring compliance with a CTBT worked very well. Further, considering that the monitoring system will significantly improve as more stations are installed, and the fact that there are a large number of high-quality stations which are accessible to the researcher which are not part of the official system, we will have the ability to detect and locate coupled 1 kt explosions globally in the near future. Further, seismology offers a window into the nature of the nuclear tests: forty years of seismological research has built a fundamental understanding of how seismic waves are generated near an explosion, how those waves propagate, and how to interrogate those waves to infer seismic yield and in some cases source configuration.

The May 11 India test had a seismic yield of 10-15 kt. This is a factor of 4 smaller than that announced by the Indian government, and there have been several attempts to explain the discrepancy. In June the Bhabha Atomic Research Center (BARC) released their own analysis. They state that "an elaborate analysis, now completed, gives an mb of 5.4. This means that our earlier quoted yield values perhaps were underestimates. It may be noted that the USGS average estimate is 5.4, which corresponds to a yield of 65 kt." In actuality, the USGS magnitude is 5.2, but the larger issue is how BARC arrived at the yield of 65 kt. It turns out this is what would be expected at NTS for a similar explosion where the yield relation is given by:

mb = 3.95 + 0.75 log Y

However, it is clear from the frequency content of the teleseismic P waves from the Pokharan test site that it is not a region of high attenuation. Further, the explosions appear to be well coupled. The reports of a combined yield of 0.8 kt for the May 13 test is even more perplexing. Even if the yield relationship for NTS is used, the expected mb would be 3.88. This would produce a signal at NIL which would be at least a factor of 50 times larger than the noise. If the test were indeed detonated in a sand dune, seismic waves could be reduced, but the expected mb would still be larger than 3.0.

The Pakistan test of May 28 had a seismic yield determination of 9-12 kt. As with the Indian tests, this yield appears to be much smaller than the official announcement of 40-45 kt. The seismic yield of the May 30 event is 4-6 kt; again this is smaller than the official announcement.

The May nuclear tests are not the first time that seismological results have been at odds with government statements. However, it is the first time that seismologists have said that the yields are smaller than announced by the government conducting the test. Although the seismic analysis presents a consistent picture, it is not definitive. It is always possible to construct arguments about coupling that would reduce the seismic efficiency. On the other hand, the seismic yields provide invaluable constraints on the nature of the weapons tested. For example, conventional wisdom states that 10-15 kt would be too small to have been a full test of a thermonuclear weapon.

In the future the International Monitoring System will grow to a larger number of seismic stations. In addition, plans by various organizations, such as IRIS, and individual nations' networks will lead to hundreds of very high quality seismic stations which will be available on the Internet within the next few years. This global, real-time monitoring system will add a new dimension to discouraging clandestine nuclear testing stations.

Acknowledgments

The information presented in this paper came from discussions with many colleagues who worked on these events. In particular, I want to acknowledge insightful discussions with Paul Richards, Frank Vernon, Steve Taylor, and Greg van der Vink. I want to thank Jon Berger and the IDA group for timely retrieval of the NIL data and the IRIS DMC staff for retrieving the GSN data without an official USGS alert. Meredith Nettles helped with the analysis at the University of Arizona. SASO contribution 110.

References

Dewey, J. W. (1983). Relocation of instrumentally recorded pre-1974 earthquakes in the South Carolina region, in Studies Related to the Charleston, South Carolina Earthquake of 1886, G. S. Gohn (Editor), U.S. Geological Survey Professional Paper 1313, Q1-Q9.
Gupta, V. and F. Pabian (1996). Investigating the allegations of Indian nuclear test preparations in the Rajasthan Desert, located at http://www.ca.sandia.gov/casite/gupta/index.html.
Hough, H. (1995). Pakistan?s nuclear status-confusion or strategy?, Jane?s Intelligence Review 7, 270-272.
Housen, F. N. (1970). USGS Technical Report 474-41.
Ringdal, F., P. D. Marshall and R. W Alewine (1992). Seismic yield determination of Soviet underground nuclear explosions at the Shagan River test site, Geophys. J. Int. 190, 65-77.