E. R. Engdahl

Shear velocity structure of central Eurasia from inversion of surface wave velocities

We present a shear velocity model of the crust and upper mantle beneath central Eurasia by simultaneous inversion of broadband group and phase velocity maps of fundamental-mode Love and Rayleigh waves. The model is parameterized in terms of velocity depth profiles on a discrete 2 2 grid. The model is isotropic for the crust and for the upper mantle below 220 km but, to fit simultaneously long period Love and Rayleigh waves, the model is transversely isotropic in the uppermost mantle, from the Moho discontinuity to 220 km depth. We have used newly available a priori models for the crust and sedimentary cover as starting models for the inversion. Therefore, the crustal part of the estimated model shows good correlation with known surface features such as sedimentary basins and mountain ranges. The velocity anomalies in the upper mantle are related to differences between tectonic and stable regions. Old, stable regions such as the East European, Siberian, and Indian cratons are characterized by high upper-mantle shear velocities. Other large high velocity anomalies occur beneath the Persian Gulf and the Tarim block. Slow shear velocity anomalies are related to regions of current extension (Red Sea and Andaman ridges) and are also found beneath the Tibetan and Turkish‐Iranian Plateaus, structures originated by continent‐continent collision. A large low velocity anomaly beneath western Mongolia corresponds to the location of a hypothesized mantle plume. A clear low velocity zone in vSH between Moho and 220 km exists across most of Eurasia, but is absent for vSV. The character and magnitude of anisotropy in the model is on average similar to PREM, with the most prominent anisotropic region occurring beneath the Tibetan Plateau.

Important findings expected from Europe's largest seismic array

An international, interdisciplinary project, which 2 years ago deployed the largest dense seismic antenna ever in Europe, expects in the next 2 years to present important findings on the lithosphere and asthenosphere of a portion of the Trans-European Suture Zone (TESZ). Final processing is currently under way of the data from the array of 120 seismographs along a 900-km-long by 100-km-wide strip from Gottingen, Germany, in the south, through Denmark, to Stockholm, Sweden in the north, across the northwestern part of the TESZ (Figure 1). Project Tor is a teleseismic tomography experiment with interdisciplinary data exploitation. It extends across the broad TESZ boundary between two markedly different lithospheric domains.These are (1) Proterozoic Europe, with Precambrian crust in Sweden and eastern Europe, and (2) Phanerozoic central Europe, with most of the crust influenced by the Caledonian and Variscan orogenies and only small areas of relic Precambrian crust. The project is designed to investigate the deep lithosphere traces of the broad-scale geology of the TESZ area, including the Tornquist Zone, from which Project Tor has its name. It is part of EUROPROBE, a major Earth science program of the European Science Foundation, which is run by a regional committee of the International Lithosphere Program.

Crust and upper mantle P- and S-wave delay times at Eurasian seismic stations

Median crust and upper mantle P- and S-wave delay times, based on residuals for teleseismic P- and S-wave arrival times included in the groomed ISC/NEIC database of Engdahl et al. [Bull. Seism. Soc. Am. 88 (1998) 722] are estimated as functions of time and azimuth for Eurasian seismic stations. The effects of source and lower mantle 3-D structure on the station residuals are corrected by ray tracing all phase data (to a depth of 400 km below the station) through the 3-D P- and S-wave models of Bijwaard et al. [J. Geophys. Res. 103 (1998) 30055] and Bijwaard [Seismic travel-time tomography for detailed global mantle structure. University of Utrecht, Utrecht, The Netherlands, 1999, 179 pp.], respectively. In general, crust and upper mantle P- and S-station delays based on medians of azimuthally binned residuals are spatially coherent and can be qualitatively associated with Eurasian tectonic features such as orogens and cratons, as well as with structural elements such as sediment and crustal thickness, and average uppermost mantle velocities. Paired P- and S-station delays are correlated with a rather poorly determined slope (S/P delay time ratio) of about 1.9.