No abstract

In October 2002, a surface displacement episode of 20–30 mm magnitude was observed over a ∼10 day period on two continuous Global Positioning System (GPS) instruments near Gisborne, North Island, New Zealand. We interpret this to result from slow slip on the northern Hikurangi subduction interface. Using ten years of regional campaign GPS (1995–2004) and recent continuous GPS data, we estimate the recurrence interval for similar events to be 2–3 yrs. In November 2004, a similar slow slip event occurred within this recurrence period. The 2002 event can be modeled by ∼18 cm of slow slip near the down‐dip end of the seismogenic zone on the subduction interface offshore of Gisborne. The campaign GPS data show that the 2002 slow slip event had little effect on regional strain patterns.

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.

Body (P) and Rayleigh wave seismograms are computed for explosions and earthquakes using the expressions developed by Hudson; many of the features of observed seismograms are noted. For the earthquake source models used average Rayleigh wave magnitudes relative to P wave magnitudes are larger than for the explosion source models. This is true even for point sources; increasing the dimensions of the earthquake source only accentuates this difference in the relative excitation of Rayleigh waves. However the relative amplitudes are strongly dependent on the orientation of the observer relative to the fault plane of the earthquake owing to the radiation pattern of the P and Rayleigh waves. Where the observer is at an anti-node of the P radiation the relative amplitudes of the Rayleigh waves are not significantly greater than for explosions. A detailed study of observed P and Rayleigh amplitudes is now needed to see if the same effects of the P radiation pattern can also explain the observed differences in the relative excitation of P and Rayleigh waves by earthquakes and explosions.

The evaluation of P-wave travel times using the Joint Epicentre Method has recently been briefly described.Here the method, which computes epicentres and travel-time corrections simultaneously, is described in full. Results indicate that there exist regional variations in travel times not revealed fully by standard methods. Use of the new travel times leads to a considerable improvement in the location of epicentres whose true positions are known, Reading RG7 4RS, Birmingham 15. Berkshire.