PROPERTIES OF THE MANTLE TRANSITION ZONE IN NORTHERN EURASIA

Short-period, three-component recordings of peaceful nuclear explosion s (PNEs) in northern Eurasia are used to constrain the P wave velocity structure of the mantle transition zone. The properties of the upper mantle discontinuities play an important role in understanding the nat ure of mantle processes. Data from several PNE seismic sounding profil es reveal reflections and refractions from upper mantle discontinuitie s at 410 and 660 km depth. The amplitude and the sharpness of these ve locity discontinuities contain important information to assess models of upper mantle phase changes and chemical layering. The absence of st rong critical and precritical reflections from the 660 km discontinuit y is characteristic for all the data in northern Eurasia. By studying the P wave reflections and refractions from the 660 km discontinuity, several velocity models were derived. To construct a generalized model , is shots observed on seven profiles in Russia were stacked to elimin ate local effects. Synthetic seismograms were calculated and compared with the observations to test different velocity models. A comparison of the observed data with synthetic data predicted by a standard refer ence model (e.g. International Association of Seismology and Physics o f the Earth's Interior 1991, (IASP91)) shows a systematic difference, between the mantle phases. While the observed phases related to the 41 0 km discontinuity are consistent with the phases predicted by the IAS P91 model, models with a smaller velocity step (similar to 50%) across the 661 km discontinuity or with a transitional 660 km discontinuity of several tens of kilometers thickness provide the best fit to the ob served data for the 660 km phases. As long-period data predict a sharp 660 km discontinuity, we have to postulate that under northern Eurasi a the velocity increase across the bottom of the mantle transition zon e is only about 50% of the value predicted by the IASP91 model. We pre sent a generalized P wave velocity model, called Generalized Northern Eurasia Model (GNEM), for the upper mantle beneath northern Eurasia ba sed on all available long-range PNE data.