Seismic P‐waves from nuclear explosions at the Kazakhstan test site in Eastern Kazakhstan (KTS) observed at the Yellowknife‐array (YKA, Canada) are used to analyze the structure at the base of the mantle under Severnaya Zemlya (Arctic Sea). 18 events at the Degelen subsite and 9 events at the Shagan River subsite of KTS are studied. The simple waveforms and well controlled source parameters of nuclear explosions allow the use of the events as source arrays in addition to the usual receiver array configuration. A new method (double‐beamforming) integrating both concepts is presented. This method increases the resolution considerably. For the Degelen⇔YKA path one anomalous arrival (1.9 s before PcP, slowness between P and PcP slowness) is observed. Smaller anomalous arrivals are found 6.5 s (Degelen⇔YKA) and 8.1 s before PcP (Shagan River⇔YKA). These anomalous phases cannot be explained with standard Earth models. They are produced by reflections from an inhomogeneity in the lowermost mantle at (82.3N, 107.5E) under the area north of Severnaya Zemlya. The strong variations between the wavefields for the two source‐receiver combinations indicate a heterogeneous structure in the lower mantle. The anomaly found here is located close to an anomaly in the lowermost mantle beneath the Nansen Basin.

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S U M M A R YSeismic P waves from a total of about 200 nuclear explosions in the USA, the former USSR and China, observed at 10 arrays and four networks in Europe, Canada and the USA, are used to analyse the structure at the base of the mantle and the core-mantle boundary (CMB).The simple waveforms and well-controlled source parameters of nuclear explosions allow one to use the events as source arrays in addition to the usual receiver array configuration. A new array technique (double beamforming; Kruger et al. 1993) integrating both concepts is applied, which increases the slowness resolution considerably.A total of 56 source-receiver combinations (i.e. reflection points in the lower mantle or on the CMB) could be analysed. In five regions, anomalous arrivals ( P d P ) with slowness and arrival times between those of P and PcP are observed. One of these five areas (Svalbard region) shows short-period PcPlP amplitude ratios, which are about three times higher than those predicted by standard earth models. In the Severnaya Zemlya region, where PdP and PcP precursors were observed previously (Kruger et al. 1993), PcP shows azimuth deviations of up to lo". For some other regions, deviations of the PcP waveform from the direct P waveform are also observed.These anomalous phases and the PcP waveform distortions cannot be explained with standard 1-D earth models. They are probably produced by inhomogeneities in the lowermost mantle. The observed variations in the waveforms are strong indications of a laterally heterogeneous structure in two depth ranges. The first is the CMB and its immediate vicinity of a few tens of kilometres; the second region is the depth range between about 200 and 300 km above the CMB. Maps of the North Pole region, giving the distributions of inhomogeneities in the lower mantle and on the CMB, are presented. These maps show evidence of strong heterogeneity of the D" boundary layer and possibly also of the CMB in the same area.

S U M M A R YWe have extended the m b (Lg) method of Nuttli using root-mean-square (rms) amplitudes corrected for noise and a −1 dependence for geometrical spreading. Lg waves recorded on the German Regional Seismic Network (GRSN) for earthquakes in south-central Europe were used to develop an m b (Lg) formula requiring a new calibration constant C rms to keep rms m b (Lg) on the same baseline as Nuttli's traditional formula based on 3rd-peak amplitudes. GRSN stations had to be calibrated for site terms and for Lg attenuation. Lateral variations in Lg Q appear to be significant across the study area, and a regional Q model consisting of constant-Q partitions north, south and in the central Alps was developed using measurements based on interstation and two-event, single-station methods. When plotted against surface wave estimates of M w , rms m b (Lg) measurements in central Europe are found to be consistent with M w -m b (Lg) relationships for north America and southern Asia, thus supporting the transportability of our m b (Lg) formula. Frequency-wavenumber processing of Gräfenberg Array data enabled us to extract Rayleigh waves for small events, and regional M s were measured using the Marshall and Basham formula. Our M s -m b (Lg) relationship extends to M s 2.5 and agrees well with observations in other regions including the western United States. The discrimination potential of M s -m b (Lg) observations was examined under realistic monitoring conditions, where path corrections were inferred from earthquake data and applied uniformly to natural sources and explosions. Under these conditions, m b (Pn, P) are greater than m b (Lg) for large NTS explosions; however, M s -m b scaling slopes are steeper for P waves than they are for Lg, and M s −m b observations for NTS explosions converge near m b 4. Thus, allowing for measurement errors and additional uncertainty in m b (Pn) due to regional bias, there is little difference in the discrimination potential for Pn and Lg waves at small magnitudes. As such, a regional M s −m b discriminant based on Lg might be preferred owing to the better detectability of Lg waves for small earthquakes. These results need to be confirmed for explosions at other test sites. Compared to teleseismic experience, regional M s −m b observations extend the discrimination capability to lower magnitudes by at least one M s unit.Current developments in nuclear test detection are often motivated by the need to improve broad-area monitoring capabilities at low magnitudes. Such is the motivation of this paper, where we follow up recent studies (Patton 2001a; referred to as P01a) investigating the transportability of Nuttli's m b (Lg) and the discrimination potential of M s −m b (Lg) at low magnitudes. Lg waves are well suited for low-magnitude monitoring since they are usually the largest signals on regional seismograms. Establishing the transportability of m b (Lg) would be an important step towards developing M s −m b (Lg) relationships for broad-area discrimination at low magnitudes.The pot...

Summary This paper investigates the uppermost mantle velocity beneath Germany using regional earthquake traveltime data. 2149 Pn traveltimes corresponding to 220 events recorded at 70 stations covering the region of 47°N–52°N latitude, 5°E–15°E longitude result in a satisfactory ray‐path distribution. Three methods with increasing degree of complexity are used to analyse the Pn traveltime data: a straight‐line fit; the classical time‐term method; and a modified time‐term method including azimuthal anisotropy. First, from the straight‐line fit to the data set, an average Pn velocity of 7.98 km s−1 is inferred. Second, the classical time‐term method yields a mean uppermost mantle velocity of 7.99 km s−1. The most important feature in this analysis is the azimuth‐dependent pattern of the residuals, indicating some evidence of velocity anisotropy in the upper mantle. The time‐term method achieves about 55 per cent variance reduction relative to the straight‐line fit. Third, two modified ‘anisotropic’ time‐term methods provide an average Pn velocity of 8.09 km s−1, with a further data variance reduction of 64 and 20 per cent relative to the straight‐line fit and the classical time‐term method, respectively. The estimated anisotropy level is about 3.5–4 per cent, with maximum and minimum velocities of 8.24–8.27 km s−1 and 7.95 km s−1. Our estimated maximum velocity direction of ∼N25°E coincides with those of previous anisotropic studies on the uppermost mantle in this region based on seismic refraction data. The results from the present study thus support the idea that Pn‐wave anisotropy is a large‐scale lithospheric feature over much of central Europe.