THE LEVEL OF DETERRENCE PROVIDED BY DATA FROM THE SPITS SEISMOMETER ARRAY TO POSSIBLE VIOLATIONS OF THE COMPREHENSIVE TEST BAN IN THE NOVAYA ZEMLYA REGION

Abstract

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.