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DDC:
550 - Earth sciences
Abstract:
Mode conversions and reflections at upper-mantle seismic discontinuities may be contained in earthquake seismograms as weak secondary phases that often become visible only after special signal processing techniques are applied to the data. To extract fully the information these secondary phases carry about the three-dimensional structure of the Earth, new observational and interpretational methods have to be developed. However, new sources of possible systematic errors may lead to conflicting results. Studies carried out by various research groups on the thickness of the upper-mantle transition zone, the sharpness of upper-mantle discontinuities and the global existence of a 520 km discontinuity are examples where such discrepancies did arise. Although there is a general consensus that the depths to the 410 km and 660 km discontinuities vary by a few tens of kilometres at most, the question of wither the depth variations of the 410 km and 660 km discontinuities are correlated or ant correlated is still unreso Similarly, different data sets and methods yielded different answers on the sharpness of the upper-mantle discontinuities at 410 km and 660 km depth. Finally, data apparently supporting the global existence of a seismic discontinuity at 520 km depth can be equally well explained by models that do not contain this discontinuity.
