In the May issue of BARC News Letter, Sikka and Kakodkar (1) had presented preliminary results about the close-in ground motion measurements and seismic records and estimated the combined yield of May 11-13, 1998 explosions (POK2) to be around 60 kt. In a recent paper, Sikka, Roy and Nair (2) analysed the globally measured body-wave magnitudes (mb) reported by International Data Centre (IDC), Arlington, USA for 51 stations. An azimuthal plot of these mb values (see Fig.1) showed that mb values are in general smaller at the azimuths away from the north direction. This has been explained to be because of the time delays introduced by the physical separation of the two large explosions of May 11 (1 km apart in the east-west direction) with the help of synthetic seismograms for various t* values (0.3 s-0.5 s) (Fig.2). In view of this, the resultant amplitude of the two explosions is less in directions other than north-south direction. The average mb estimates of IDC (mb = 5.0) and US Geological Survey (mb = 5.2) are therefore smaller than the true mb values. After taking into account the necessary correction, a value of mb = 5.39 was obtained as the global average. The revised mb estimate gave an average combined yield of 58 ± 5 kt after taking into account the geology of Pokhran test site as calibrated by our POK1 explosion. Since then, we have obtained data from 100 more seismic stations and also carried out analysis of close-in acceleration data for yield determination. We report these results here.
As we have stated above, both the IDC and USGS average values need to be corrected for the source geometry. This is further confirmed by the analysis of mb values from 160 station data of IDC, USGS and Kyrgyz network(KNET). Fig.3 shows a histogram plot of the azimuthal variation of mb values, averaged in 20° intervals, in the northern hemisphere (there are not enough points in the southern hemisphere). The peaking of mb values in the north direction is in agreement with the east-west alignment of our big explosions, with the shaft for the fission device located in the east direction with respect to that of the thermonuclear one. Wallace (3) reports that he did not observe any significant variation in the mb values in an azimuthal plot of USGS data. This is not surprising if one uses only USGS data because there are very few points in the north direction in these data, in fact, about 70 seismic stations lie in the narrow azimuth of 300 - 320° , which distort the plot.
It is well known that although there is lot of uncertainity in the determination of absolute yield from seismic data alone, the relative yield between two tests can be evaluated with a much higher confidence by use of the difference in body wave magnitudes,
D mb @ C2 log (Y1/Y2) (1)
where Y1 and Y2 are the yields of the two explosions and C2 = 0.75 to 0.8. Fig.4 gives a comparison of the seismic wave forms for the 1974 and 1998 Indian explosions as recorded at GBA. The ratio of the amplitudes of the P waves is 4.5. A similar difference is indicated by the common seismic stations for these explosions as shown in Table 1. After making necessary corrections for the source geometry of 1998 Indian explosions, an average D mb » 0.5 is obtained. This, using equation (1), leads to a yield ratio of 4.45 between 1974 and 1998 explosions is obtained, almost the same as reported by Sikka and Kakodkar (1). Taking the yield as 12 to 13 kt for the 1974 explosion, the yield of May 11 explosions is obtained as 53-58 kt. The yield of the POK1 is based on seismic data (4,5) and rock mechanics pheno-menology calculations (6). The latter reproduced the measured cavity radius, spall velocity and the extent of the rock fracturing for this yield of the 1974 event. It may be noted that IDC shows the yield of the 1974 explosion in their data base as 10-15 kt and the same is also quoted by Norris (7).
Fig. 5 shows the measured accelerations along with curves derived from Nevda Test Site (NTS) and from the Rio Blanco and Rulison-Gas Buggy explosions of USA, (these were peaceful nuclear explosions experiments for hydrocarbon gas reservoir simulation, all scaled to a yield of 58 kt and for the relevant depths of detonations(8). The acceleration values of our May 11, 1998 explosion are well bracketed by the Rio-Blanco-Rulison curves. This is interesting as these explosions have been carried out in sedimentary formations (shale-sandstone) away from NTS. Now the observed yields and mb values for these tests are
Mb Y
Rio Blanco 5.4 90 (30x3) kt
Rulison 5.3 43
Gas Buggy 5.1 29
Thus, it is clear that the derived yield estimates of 60 kt for POK2 by us are surely not an over estimate.
It is reported by Sykes and Evernden(9) that the test site dependence is very small for Rayleigh wave magnitude, Ms. This is because for explosions in hard rock at many test sites estimates of yield on using the NTS (Ms versus Y) formula have been close to the actual yields. The four Ms values of May 11 tests as reported by USGS range from 3.4 to 3.8. Our measured ones at Jodhpur, GBA and Bhopal were 3.6, 3.6 and 3.9 respectively. Assuming relation between Ms and yield of Murphy (10) for less than 100 kt
Ms = 2.14 + 0.84 log Y (2)
and an average value of 3.62 for Ms from the above data, a yield value of 58 kt is obtained. This is in perfect match with our reported value.
In two recent papers, Wallace (3) and Barker et al (11) describe the capability of the International monitoring system for estimating the source parameters by fitting their findings of May 1998 nuclear explosions of India and Pakistan. They seem to claim following capabilities :
- It could detect, locate and identify the explosions promptly.
- It could estimate the yield of detected explosions from scaled spectrum even for uncalibrated regions and undetected explosions with no knowledge about source geometry.
3. The system is capable of detecting
multiple tests.
For 2, Wallace used USGS average mb of 5.2 and gave a yield of 10-15 kt and Barker et al gave 9-16 kt based on IDC average mb of 5.0 for our May 11, 1998 explosions. It may be pointed out that the difference of 0.2 units in mb values will give a yield ratio of 1.8.
Wallace (3) also uses the NTS relation for the scaled depth of burial
d = 122 y1/3 (3)
to estimate the yield of our 1974 explosion. The above relationship is for containment of radioactive gases. He erroneously uses it as a scaling law for formation of a subsidence crater. US publications list that subsidence craters may form from scaled depth of burial of 60 w1/3 m onwards e.g. see reference 12. His assumption that Pokhran I formed a subsidence crater is also not tenable. In fact, Pokhran I created a shallow crater. The difference between a shallow crater and a subsidence crater is that the crater radius in the latter is very nearly the same as cavity radius. For Pokhran I, the crater radius was 47 m compared to the cavity radius of 30 m. (This should be 27 m according to the formula of Terhune (13)). Our design for optimum depths for nuclear device emplacement is based on careful computer simulations taking into account the geo-physical properties of the concerned rock medium. All the five explosions of 1998 - and in fact the earlier one of 1974 - have confirmed the correctness of our emplacement design procedure.
The assumption of Wallace (3) and Barker et al (8) that the geology of the Pokhran site was similar to that of former Soviet Shagan River site is scientifically not correct. It may be noted that Indian plate is different from the Eurasian plate The close in rock formations of an explosion determine the seismic coupling and the value of the constant C1 in the relation
mb = C1 + C2 log Y (4)
This was realised by the scientific community after the threshold test ban treaty, which set the maximum yield of a permitted nuclear test explosion to be 150 kt. When the former Soviet Union conducted these tests, higher mb values were measured by western seismic networks. From these, using NTS values for C1 and C2 , some American Seismologists estimated the Soviet yields to be about 300 - 500 kt. Later on joint USSR-USA calibration experiments supported the Soviet yields to be near 150 kt. This led to a procedure (called correction of site bias) of estimating true Soviet yields. This involved substracting a value of a few tenths from the observed mb values or increasing the value of C1 constant by the same amount (Stevens et al, 14). Such site biases have now been found to exist for different testing locations e.g. between eastern and western United States and between Shagan River and another former Soviet site Deglen, less than 70 kilometers away from the former. In view of the above, the value of C1 @ 4.05 derived by Sikka et al., by making the NTS mb versus Y curve pass through the POK1 point, we feel, is the correct one, which for a magnitude of 5.4 for POK2 gives a yield of 58 kt.
It is well established that the yield of a nuclear explosion can be determined with more certainity by close-in ground motion measurements, radiochemical methods and hydrodynamic shock measurements. Without the availability of such data and surroundings of the device, it will be not just highly subjective but erroneous , as explained above, to draw firm conclusions on yields. Such data is unlikely to be available to investigators other than those involved in the test . In the context of the May 13 explosions, Barker et al have given a detection threshold of mb(Lg) of 2.5 at Nilore (NIL) in Pakistan at a distance of 740 km from the Pokhran site. Based on the yield ratio derived from equation (1) and using the yield of May 11 tests as 9-16 kt, they give the yield of May 13 explosions between 30 to 300 tons. With the actual yield of ~ 60 kt for May 11 tests, this detection limit will be ~0.1 to 1 kt very close to our announced values. Thus, while the CTBT monitoring mechanism does appear to detect high yield tests, the claims on its capability to correctly estimate the yield, to detect multiple tests and to detect low yield tests appear doubtful. To infer about the type of device itself is stretching the perception of the system capability too far, particularly if one remembers that PNE thermonuclear devices have been tested down to a yield of at least 2.3 kt (Cabriolot test, referred to in the paper by Siddons(15)).
Authors are grateful to Dr. R. Chidambaram for many illuminating discussions.
References :
- Sikka, S.K. and Kakodkar, Anil, BARC News Letter (1998), 172, 1-4.
- Sikka, S.K., Roy, F. and Nair, G.J. (1998), Current Science, 75, 486-491.
- Wallace, T.C., (1998) Seismic Research Letters.
- Chidambaram, R., Ramanna, R., (1975), Proc. Tech. Committee on
- Nair, G.J. (1974), AG 224, Procurement executive, MOD, UK.
- Chidambaram, R., Sikka, S.K. and
Peaceful Nucl. Explosions IV (Vienna,
IAEA) p.421.
8. Holzer L. (1977) Proc. Tech. Committee on Peaceful Nucl. Explosions V (Vienna, IAEA), p.27.
9. Sykes, L.R. and Evernden, J.F. (1982), Sci. Am. 247, 29.
10. Murphy, J.R. (1977) , Bull. Seismol.Soc. Am. 67, 135.
11. Barker, B. et al (1998), Science 281, 5385.
- Teller, E., Tally, E.K., Higgins, G.H. and
uses of nuclear explosives".
14. Stevens, J.I., Murphy, J.R. and Rimer, N. (1991), Bull. Seismol. Soc. Am.
|
Station |
Mb2-mbl (a) |
Azimuthal
corretion
for mb (b) |
D mb (a) + (b) |
| EKA (UK) | 0.3 | 0.1 | 0.4 |
| YKA (Canada) | 0.5 | 0.0 | 0.5 |
| GBA (India) | 0.5 | 0.0 | 0.5 |
| NUR (Finland) | 0.2 | 0.2 | 0.4 |
| KEV (Ukraine) | 0.4 | 0.0 | 0.4 |
| NB2 (Norway) | 0.4 | 0.1 | 0.5 |
| COLA (USA) | 0.6 | 0.0 | 0.6 |
| PMR (USA) | 0.5 | 0.0 | 0.5 |
The value of mbl corresponds to body wave magnitude for 1974 Indian explosion and mb2 corresponds to that of 1998 Indian explosions. The average D mb value comes to 0.5, which corresponds to a yield ratio of 4.45 between the POK2 (11 May 1998) total yield and the POK1 (18 May 1974) yield.
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