Properties of the mantle transition zone in northern Eurasia

Abstract

Short‐period, three‐component recordings of peaceful nuclear explosions (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 nature of mantle processes. Data from several PNE seismic sounding profiles reveal reflections and refractions from upper mantle discontinuities at 410 and 660 km depth. The amplitude and the sharpness of these velocity discontinuities contain important information to assess models of upper mantle phase changes and chemical layering. The absence of strong critical and precritical reflections from the 660 km discontinuity 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, 18 shots observed on seven profiles in Russia were stacked to eliminate 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 reference model (e.g. International Association of Seismology and Physics of the Earth's Interior 1991, (IASP91)) shows a systematic difference between the mantle phases. While the observed phases related to the 410 km discontinuity are consistent with the phases predicted by the IASP91 model, models with a smaller velocity step (∼50%) across the 660 km discontinuity or with a transitional 660 km discontinuity of several tens of kilometers thickness provide the best fit to the observed data for the 660 km phases. As long‐period data predict a sharp 660 km discontinuity, we have to postulate that under northern Eurasia the velocity increase across the bottom of the mantle transition zone is only about 50% of the value predicted by the IASP91 model. We present a generalized P wave velocity model, called Generalized Northern Eurasia Model (GNEM), for the upper mantle beneath northern Eurasia based on all available long‐range PNE data.