We present a new 1-D P wave seismic velocity model (called MP1-SUW) of the upper mantle structure beneath the western rim of the East European Craton (EEC) based on the analysis of the earthquakes recorded at the Suwałki (SUW) seismic station located in NE Poland which belongs to the Polish Seismological Network (PLSN). Motivation for this study arises from the observation of a group of reflected waves after expected P410P at epicentral distances 2300–2800 km from the SUW station. Although the existing global models represent the first-arrival traveltimes, they do not represent the full wavefield with all reflected waves because they do not take into account the structural features occurring regionally such as 300 km discontinuity. We perform P wave traveltime analysis using 1-D and 2-D forward ray-tracing modelling for the distances of up to 3000 km. We analysed 249 natural seismic events from four azimuthal spans with epicentres in the western Mediterranean Sea region (WMSR), the Greece and Turkey region (GTR), the Caucasus region (CR) and the part of the northern Mid-Atlantic Ridge near the Jan Mayen Island (JMR). For all chosen regions, except the JMR group for which 2-D modelling was performed, we estimate a 1-D average velocity model which will characterize the main seismic discontinuities. It appears that a single 1-D model (MP1-SUW model) explains well the observed traveltimes for the analysed groups of events. Differences resulting from the different azimuth range of earthquakes are close to the assumed picking uncertainty. The MP1-SUW model documents the bottom of the asthenospheric low-velocity zone (LVZ) at the depth of 220 km, 335 km discontinuity and the zone with the reduction of P wave velocity atop 410 km discontinuity which is depressed to 440 km depth. The nature of the regionally occurring 300 km boundary is explained here by tracing the ancient subduction regime related to the closure of the Iapetus Ocean, the Rheic Ocean and the Tornquist Sea.

The upper mantle beneath the Alps and the Variscides of Central Europe has a varied seismic structure as a result of the accretionary and evolutionary processes that have shaped it. Natural earthquake data, the raypaths of which pass beneath these regions, have enabled the calculation of travel times out to 3000km. The source-receiver geometry ensured there was broadband station coverage at offsets between 900 and 3000km, enabling the detection of the appropriate upper mantle phases over an azimuthal span embracing the studied structures. The resulting data allowed us to model first order discontinuities to a depth of about 420km. We have used modelling based on ray-tracing in a cylindrical coordinate system reflecting the geometry of the wave propagation medium. The data allow determination of the thickness of the upper mantle low velocity zone (LVZ) beneath the Alpine orogen. It appears to be thicker than beneath surrounding areas, with its upper ‘lid’ at a depth of about 90km and its base, the ‘Lehmann discontinuity’, being depressed to 220km. Such thickening of the LVZ is typical below young orogens. A seismic discontinuity at a depth of around 300km, with the P-wave velocity increasing to 9km/s, has been observed for all the sub-Alpine raypaths. We consider this discontinuity to have originated as a result of Alpine orogen subduction. This is responsible not only for changes in the thermal field as a result of transporting colder material to depths probably exceeding 200km but also for delivering the necessary amounts of excess silica required for the coesite–stishovite phase transition. For rays passing beneath the Variscides, there is a scatter in travel times over the 8–15° distance range. However, the scattering does not indicate significant thickening of the LVZ as identified beneath the Alpine orogen, and the Lehmann discontinuity at the base of the LVZ does not exceed 200km depth. Also, in contrast to the Alps, no high velocity phases are observed for a discontinuity near 300km depth. This supports our contention that the 300-km discontinuity is a regional feature ascribed to the subduction regime beneath the younger orogen. For all azimuths from which structures beneath the Alps are illuminated, we clearly observe both the refraction as well as the reflection branch from the 410-km discontinuity.

2-D P-wave velocity models of the upper mantle beneath the Variscan units of Central Europe have been derived. Our modelling is based on ray-tracing in a cylindrical coordinate system. Travel time data from shallow crustal earthquakes clustered within epicentral areas small enough to justify superposition of different events, have been derived for distances up to 3000 km. The epicentres were chosen to ensure that there was broadband station coverage at offsets between 900 and 2800 km, enabling the detection of appropriate upper mantle phases over an azimuthal span embracing the studied Variscan structures.

We present the first detailed seismic velocity models of the crust and uppermost mantle around the Mimyi kimberlite field in Yakutia, Siberia. We have digitized vintage seismograms that were acquired in 1981 and 1983 by use of Taiga analogue seismographs along two perpendicular seismic profiles. The 370-km long, northwest striking profile I across the kimberlite pipe was covered by 41 seismographs, which recorded seismic signals from 21 chemical shots along the line, including one off-end shot. The perpendicular, 340-km long profile II across profile I ca. 30km to the south of the Mimyi kimberlite field was covered by 45 seismographs, which recorded seismic signals from 22 chemical shots, including four off-end shots. Each shot involved detonation of between 1.5 and 6.0tons of TNT, distributed in individual charges of 100-200kg in shallow water (< 2m deep). The data is of high quality with high signal/noise ratio to the farthest offsets. We present the results from two-dimensional ray tracing, forward modelling.Both velocity models show normal cratonic structure of the ca. 45-km-thick crust with only slight undulation of the Moho. However, relatively small seismic velocity is detected to 25-km depth in a ca. 60-km wide zone around the kimberlite pipe, surrounded by elevated velocity (> 6.3 km/s) in the upper crust. The lower crust has a relatively constant velocity of 6.8-6.9km/s. It appears relatively unaffected by the presence of the kimberlite field. Extremely large P-wave velocity (> 8.7 km/s) of the sub-Moho mantle is interpreted along profile 1, except for a 70-km wide zone with a "normal" Pn velocity of 8.1 km/s below the kimberlite. Profile 11 mainly shows Pn velocities of 8.0-8.2km/s, with unusually large velocity (> 8.5km/s) in two, ca. 100-km wide zones, at its southwestern end, one zone being close to the kimberlite field. The nature of these exceptionally large, sub-Moho mantle velocities is not yet understood. The difference in velocity in the two profile directions indicates anisotropy, but the effect of unusual rock composition, e.g. from a high concentration of garnet, cannot be excluded. (c) 2006 Elsevier B.V. All rights reserved.