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SURFACE OF THE EARTH: GLOBAL STUDIES 2006 <strong>IRIS</strong> 5-YEAR PROPOSAL Tectonic Structure and Surface Wave Dispersion in Eurasia and North Africa Michael E. Pasyanos • Lawrence Livermore National Laboratory LLNL has performed a large-scale study of surface wave dispersion across Eurasia and North Africa. The spatial resolution of the inversion has been improved by increasing path density and adopting a variable smoothness conjugate gradient method for the group velocity tomography (Pasyanos, 2005). This technique allows us to achieve higher resolution where the data allow without producing artifacts. The current results include both Love and Rayleigh wave inversions across the region for periods from 7 to 100 s at 1° resolution. While short-period group velocities are sensitive to slow velocities associated with large sedimentary features, and intermediate periods to differences in crustal thickness, at longer periods the group velocities are sensitive to structures in the upper mantle. One would expect hot, upwelling material to have slow group velocities at these periods, while cold material should be fast. Figure 1 shows a map of 80 s group velocities with plate boundaries (solid lines), hot spots (crosses and triangles), and the boundaries of stable platforms and Achaean cratons (single and double hatched lines, respectively). At this period, there are indications that the slow velocities associated with rift zones are deeperseated than in convergence zones, where the slow velocities are confined to the wedge. A significant correspondence can also be found between fast velocities and older crust. For example, while it appears that orogenic zones like the Tethys Belt are slow, stable continental areas are fast. These older areas are underlain by thick, cold, fast lithospheric material. Two exceptions seems to be the Sino-Korean Paraplatform, which has had its lithospheric mantle more recently affected by nearby subduction, and the Benue Trough in Cameroon, which is the remnant of a failed rift system. Figure 2 shows the correlation between lithospheric thickness and 80 s Rayleigh wave group velocities. The thicknesses of the continents are derived from the 1300° C isocontour from Artemieva and Mooney (2001), whereas the thicknesses of the oceanic lithosphere are derived from the oceanic age. The correlations here are quite significant, with older, thicker lithosphere about 0.35 km/s faster than a younger, thinner one. Figure 1. Map of 80 s Rayleigh wave group velocities shown with plate boundaries (thick lines), hot spots (triangles and crosses), and boundaries of platforms and cratons (hatched lines). Figure 2. Correlation between 80 s Rayleigh wave group velocities (in km/s) and lithospheric thickness (in km). Diamonds and bars indicate the mean and standard deviation of group velocity values in that thickness range. Artemieva, I.M., and W.D. Mooney, Thermal structure and evolution of Precambrian lithosphere, J. Geophys. Res., 106, 16387-16414, 2001. Pasyanos, M.E., A Variable-resolution surface wave dispersion study of Eurasia, North Africa and surrounding regions, J. Geophys. Res., in review, 2005. 100
2006 <strong>IRIS</strong> 5-YEAR PROPOSAL SURFACE OF THE EARTH: GLOBAL STUDIES Upper Mantle Q and Thermal Structure Beneath Tanzania, East Africa from Teleseismic P Wave Spectra Anupama Venkataraman • Stanford University Andrew A. Nyblade • Pennsylvania State University Jeroen Ritsema • Institut de Physique du Globe, Paris, France We measure P-wave spectral amplitude ratios from deep-focus earthquakes recorded at broadband seismic stations of the Tanzania network to estimate regional variation of sublithospheric mantle attenuation beneath the Tanzania craton and the eastern branch of the East African Rift. One-dimensional profiles adequately explain the systematic variation of P-wave attenuation in the sublithospheric upper mantle: beneath the cratonic lithosphere, while it is ~ 80 beneath the rifted lithosphere. By combining the values and a model of P-wave velocity perturbations, we estimate that the temperature beneath the rifted lithosphere (100-400 km depth) is higher than ambient mantle temperatures, consistent with the observation that the 410-km discontinuity in this region is depressed by 30-40 km. Venkataraman, A., A. Nyblade, and J. Ritsema, Upper mantle Q and thermal structure beneath Tanzania, East Africa, Geophys. Res. Lett., 31, No. 15, L15611, 10.1029/2004GL020351, 2004. 101
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