ON THE SEISMIC DISCONTINUITIES IN THE UPPER-MANTLE

Mode conversions and reflections at upper-mantle seismic discontinuiti es may be contained in earthquake seismograms as weak secondary phases that often become visible only after special signal processing techni ques are applied to the data. To extract fully the information these s econdary phases carry about the three-dimensional structure of the Ear th, new observational and interpretational methods have to be develope d. However, new sources of possible systematic errors may lead to conf licting 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 discontin uity are examples where such discrepancies did arise. Although there i s a general consensus that the depths to the 410 km and 660 km discont inuities vary by a few tens of kilometres at most, the question of whe ther the depth variations of the 410 km and 660 km discontinuities are correlated or anticorrelated is still unresolved. Similarly, differen t data sets and methods yielded different answers on the sharpness of the upper-mantle discontinuities at 410 km and 660 ion depth. Finally, data apparently supporting the global existence of a seismic disconti nuity at 520 km depth can be equally well explained by models that do not contain this discontinuity.