P-WAVE MANTLE VELOCITY STRUCTURE BENEATH NORTHERN EURASIA FROM LONG-RANGE RECORDINGS ALONG THE PROFILE QUARTZ

High-resolution seismic sounding of the mantle using two Peaceful Nuclear Explosions (PNEs) along the 3950 km long Quartz profile in northern Eurasia reveal major unexpected vertical and lateral mantle inhomogeneities. In contrast to standard seismological earth models such as PREM and IASP91 three, rather than two, mantle discontinuities are found between 400 and 700 km depth. They are located at about 420, 550 and 670 km depth. Lateral variations in depths amount to 10-20 km, and associated P-wave velocities vary by 0.1 km s-1. In the upper mantle, the uppermost layer through which P(n) propagates extends down to about 100 km depth. The mantle below this depth, but above the 410 km discontinuity, is characterized by alternating high and low P-wave velocities in the depth range between 100 and 200 km. The velocity fluctuates between 8.3 and 8.7 km s-1 and shows variations from shotpoint to shotpoint. Average velocities are clearly higher than standard seismological reference models. This part of the mantle overlies a low-velocity zone between about 200 and 300 km depth. This channel also displays substantial lateral variation in depth which could be interpreted as a laterally variable asthenosphere. Beneath the West Siberian basin the asthenosphere seems to be well established whereas it is only poorly defined under the Uralian belt and the tectonic units to the northwest. With the addition of more data from this and other profiles, future aims include two-dimensional and three-dimensional tomographic modelling, as well as modelling of petrological composition, of the mantle down to 850 km depth beneath northern Eurasia.