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.

The Lithospheric Seismic Profile in Britain (LISPB), shot in 1974, included a 310 km profile LISPB DELTA, crossing the Palaeozoic Welsh Basin, the western extent of the Midland microcraton and the Cornubian zone of southern England. This first comprehensive analysis of these data has produced a sub-horizontally layered seismic and associated gravity model that correlates well with surface geology. A north-south decrease in crustal velocity and density corresponds to the change from Avalonian crust into the Rheno-Hercynian zone at the south end of the profile. High velocities and densities in the lowest crustal layer beneath north Wales are proposed to result from Cenozoic and possibly Ordovician igneous intrusive rocks, the former derived from an upwelling plume associated with the opening of the North Atlantic. Examination of the load distribution throughout the model shows that it is strongly correlated with the earthquake distribution along LISPB DELTA. Earthquake focal depth also correlates with heat flow. A simple heat-flow profile has been derived, and the seismic velocity model used to constrain crustal heat production values. A long-wavelength excursion from published data can be explained in terms of an increase in mantle heat flow resulting from a previously identified deep thermal anomaly beneath the Irish Sea.

A regional model of the 3-D variation in seismic P-wave velocity structure in the crust of NW Europe has been compiled from wide-angle reflection/refraction profiles. Along each 2-D profile a velocity-depth function has been digitised at 5 km intervals. These 1-D velocity functions were mapped into three dimensions using ordinary kriging with weights determined to minimise the difference between digitised and interpolated values. An analysis of variograms of the digitised data suggested a radial isotropic weighting scheme was most appropriate. Horizontal dimensions of the model cells are optimised at 40 x 40 km and the vertical dimension at 1 km. The resulting model provides a higher resolution image of the 3-D variation in seismic velocity structure of the UK, Ireland and surrounding areas than existing models. The construction of the model through kriging allows the uncertainty in the velocity structure to be assessed. This uncertainty indicates the high density of data required to confidently interpolate the crustal velocity structure, and shows that for this region the velocity is poorly constrained for large areas away from the input data.

A 100 x 80 km(2) earthquake recording network was operated for three months (January-March 1990) in the Lake Baringo region of the Kenya Rift Valley. Twenty-nine seismic sites were occupied by short-period stations over a region including the Elgeyo escarpment, the Kerio Valley, the Tugen Hills and Lake Baringo itself.Eighty local events of M(L) < 2.0 have been located within 50 km of the network. These events are situated within the central part of the rift, showing some association with the main rift faults but mainly clustering beneath Lake Baringo at a depth of about 5 km and occurring as swarm activity. Ninety percent of the events occurred at depths shallower than 12 km. The brittle-ductile transition zone in this area is determined at a depth of 12-16 km, similar to that in the Lake Bogoria area immediately to the south.A preliminary study of focal mechanisms of suitable events indicates WNW-ESE extension across the Lake Baringo basin and suggests the presence of sub-vertical rupture surfaces beneath the lake. These may be caused by the insertion of basic dykes into the upper crust, and the rupture process assisted by high temperature and pressure geothermal fluids.The inversion of local P-wave arrival time data for the upper crustal velocity structure beneath Lake Baringo identifies a low-velocity zone beneath surface geothermal activity, a relationship previously demonstrated for the Lake Bogoria region.