Short-period three-component recordings of the seismic wave field of peaceful nuclear explosions (PNEs) along several long-range profiles in northern Eurasia are analyzed to study the structure of the upper mantle. We use the data to search for evidence of the disputed 520 km discontinuity in the mantle transition zone. Here we present short-period evidence for the existence of this proposed mantle discontinuity. On three long-range profiles in Siberia we observe in the distance range from 2400 to 2600 km a typical change of the apparent velocity of the first arrivals and the appearance of two characteristic travel time branches. We interpret these phenomena as being caused by the proposed upper mantle discontinuity. To determine the optimum one-dimensional P wave velocity model, synthetic seismograms were calculated with the reflectivity method. By comparing them with the observed data, we estimate the P wave velocity contrast across the 520 km discontinuity to be 0.25¿0.05 km/s, while the velocity gradient below the boundary is 0.001¿0.0005 1/s. This result indicates that in the mantle transition zone both phase transitions and variations in chemical composition can occur.¿ 1997 American Geophysical Union