Evidence for a sharp 670-km discontinuity as inferred from P-to-S converted waves

The fine structure of the upper mantle discontinuities is investigated using observations of converted waves on short-period and broadband seismograms. Using a stacking technique to analyze the P wave coda of teleseismic records, evidence is found for coherent near-receiver P-to-S converted phases generated by the 400- and 670-km discontinuity beneath a number of the stations used in this study. Variations in travel time, slowness, and amplitude of these phases as observed among the stacks for stations of the Regional Seismic Test Network in the US and of the Network of Autonomously Recording Seismographs in western Europe are very likely the expression of upper mantle heterogeneity. Observations of coherent converted phases from the 400-km discontinuity are fewer in number than of phases converted at the 670-km discontinuity, suggesting that the latter is more pronounced. Some of the seismograms, especially of station RSCP, show extremely high-amplitude P-to-S converted phases from the 670-km discontinuity. These seismograms allow a detailed waveform comparison of the converted phase with the direct P phase and present evidence for a sharp 670-km discontinuity. -Author