The M, Rayleigh wave magnitude formula is revised for purposes of eliminating the heretofore variable effects of near distances and propagation paths on the values computed from standard long-period seismograms. The improved formulation employs a revised distance correction function and a period-dependent path correction that normalizes M, to large teleseismic distance 20-s values. For purposes of earthquake-explosion discrimination, an empirical focal depth correction is derived on the basis of Rayleigh wave frequency content as a function of focal depth, which normalizes M , values to the surface focus equivalent, i.e. aids discrimination when it can be applied by increasing earthquake M , values and moving them away from the equivalent explosion population on M, : m, plots.The revised M , improves on previously achieved discrimination of North American events, and provides reliable discrimination between suites of Eurasian earthquakes and explosions. Having removed the dominant propagation path effects on M,, the residual differences in M , : mb among events are generally attributed to source environment and regional effects on m,,.The 42 Eurasian WWSSN stations employed are shown to have a discrimination threshold at the M,3.2 level. With the improved M , scale now equivalent to first order for North American and Eurasian continental propagation, available Nevada Test Site explosion yields are extrapolated to the Eurasian sites to illustrate that this M,3.2 discrimination threshold is equivalent to an Eurasian explosion of about 20 kt in hard rock. Given improved long-period instrumentation to reduce the Rayleigh wave detection threshold, the principal restriction on further studies of discrimination to lower levels of magnitude and yield will be the availability of earthquake occurrence information at the low magnitudes.

S U M M A R YA study of P-and Lg-based source size estimators has been conducted for 101 presumed underground nuclear explosions at the Shagan River test site near Semipalatinsk, USSR. A systematic variation in the relative patterns across the test site has been observed when comparing the P-wave estimators (world-wide mb and Ulm) and the Lg-based observations (RMS Lg determined from NORSAR and Grafenberg array recordings). These variations correlate well with geological features determined from satellite imagery, and suggest in particular that the northeast and southwest portions of the test site, divided by the Chinrau fault, are characterized by distinct geophysical properties. A difference in m,(P) -mLg bias of as much as 0.15 magnitude units exists between these two subareas. Available data indicate that the mLg observations are very consistent with published yields, and suggest that the magnitude-yield relationship for P-wave based estimators can be significantly improved by introducing regional bias corrections. We have determined such corrections for three subareas of the test site, and used the corrected mh and Ym observations in combination with the Lg data in a weighted averaging scheme to arrive at yield estimates for all events in the database. For the explosion of 1988 September 14, we obtain yield estimates from the three estimators ranging from 106 to 118 kt, which is consistent with the independently derived estimates of 115-122 kt quoted by Sykes & Ekstrom (1989). A further assessment of the validity of our results would require access to independently measured yields of additional explosions.

The m b : M, relation for explosions at the Nevada Test Site (NTS) differs from those for explosions in other parts of the world. There is considerable evidence that this results mostly from high body-wave attenuation in the upper mantle beneath the western US. The authors have developed an empirical magnitude correction for body-wave attenuation and applied it to both source and receiver ends of the teleseismic body-wave path. The results imply that mb values are lower for NTS explosions than for Soviet explosions of comparable yield and seismic coupling. The authors have also developed and applied a source-depth correction to account for pP-P interference in the P-wave arrival.The body-wave magnitude resulting from these corrections is designated WIQ to distinguish it from other definitions of mb. Values of m Q determined for a world-wide set of large explosions show that a single mQ: yield relation is a fair fit to the data for the explosions with high seismic coupling.However, grouping the explosions under two mQ : yield relations gives a better fit to the data. All the studied explosions in salt or granite or below the water table fit a common M, : yield relation. Explosions from North America, Eurasia and Africa have a common m Q : M, relation.

Long period (16 s) and short period (1 s) P-wave amplitudes from shallow earthquakes recorded in the distance range 0"-114" are analysed to separate the effects of earthquake size, epicentral distance and station structure. The LP and SP vertical component amplitudes are analysed separately so that any period dependent effects can be determined.The results show that distance effects on both LP and SP amplitudes are significant when the whole range 0-114" is considered but between about 30" and 90" the distance effects are not significant. Further the distance effects do depend on period in the range 0-114" but again between 30" and 90" the distance effects are independent of period. In determining magnitude then a single constant correction for distance can be applied to both LP and SP recordings in the range 30"-90". Outside this range the correction depends on both distance and period.Station effects are significant for both the LP and SP analyses with little correlation between the LP and SP effects for individual stations. The SP effects can be attributed to variations in Q in the upper mantle; stations recording below average amplitude being underlain by lower Q than those recording amplitudes above average. The SP effects for a particular station will thus also be source effects for shallow events with epicentres near the station. The value of applying corrections for such source effects to magnitude estimates is demonstrated using data from two explosions of the same yield fired at two widely different test sites. Without corrections for source effects the magnitudes differ by 0.6 units, with source effects the difference is 0.02 units.The LP station effects are more difficult to interpret, but for one group of stations on the coastal plains of the Gulf of Mexico and Eastern USA which show large positive effects (above average amplitudes) it is demonstrated by modelling that these effects could be explained by the amplification effects of the large thicknesses of sediment underlying the stations.Comparing the estimates of magnitude (earthquake size) obtained from the LP and SP analyses shows that on average the LP and SP magnitudes are equal but correlation of LP and SP magnitudes for individual earthquakes is poor. This lack of correlation may simply be due to lack of data since only a small number of readings (1-4) contribute to each magnitude estimate, but this needs further study.

SUMMARY The yield threshold at which a fully decoupled explosion can be identified has been a recurring issue in the debate on whether the Comprehensive Nuclear Test Ban (CTB) can be adequately verified. Here, we assess this yield threshold for the Novaya Zemlya (NZ) and Kola Peninsula regions by analysing seismograms from six small body wave magnitude (mb≤3.5) seismic disturbances recorded at regional distances (1050<Δ<1300 km) by the seismometer array at Spitsbergen (SPITS). Multiple filter analysis of the seismograms shows clear high‐frequency Pn (f≥14 Hz), except from a calibration explosion on the Kola Peninsula. From four of the disturbances studied we observe clear high‐frequency Sn; the explosion showed no clear high‐frequency Sn and the data from the remaining disturbance was potentially contaminated by a data glitch. Frequency‐domain analysis indicates that the Pn and Sn attenuation across the Barents Sea is similar to that observed across stable tectonic regions (shields). We define a spectral magnitude for the 2.5–3.5 Hz passband that is tied to teleseismic mb from NZ explosions; the six disturbances considered have 2.3≤mb≤3.5. Three‐component data are available from SPITS for four of the disturbances considered (including the explosion). From the explosion the S/P ratios on the vertical (Z), radial (R) and tangential (T) components (in the 3.0–6.0 Hz passband) are all less than unity. The S/P ratios for the same passband on the Z component from the remaining three disturbances are less than unity, but the ratios on the R and T components are significantly greater than unity. We argue that S/P ratios (3.0–6.0 Hz passband) of less than unity on all of the Z, R and T components at SPITS may indicate a potential treaty violation in the Kola Peninsula and NZ regions. The temporal variation of seismic noise, in the 3.0–6.0 Hz passband, at SPITS suggests that our three‐component S/P criterion will be effective 95 per cent of the time for disturbances with mb≥2.8. We suggest that mb=4.25+b log10W, where W is the explosive yield in kilotons (kt), with b=0.75 for W≥1, and b=1.0 for W<1, is suitable for conservatively estimating the yield threshold of a potential violation of the CTB in the NZ region. From this we infer that a 35 ton fully coupled explosion in the NZ region is likely to be identified as suspicious under the CTB using the three‐component S/P criterion. Simulations show that the low‐frequency decoupling factor (DF) for a fully decoupled nuclear explosion in hard rock is about 40, suggesting that such an explosion with a yield of 1.6 kt in the NZ region is likely to be identified using data from SPITS. The conservatism likely to be employed by a potential violator and uncertainties in the DFs for nuclear explosions in hard rock cavities, together with data from stations other than SPITS within 2000 km of the NZ region, suggest that the yield at which a potential violator of the CTB could confidently escape detection (using decoupling) in the NZ region is in reality probably less than 0.5 kt.