Ps conversions from the upper mantle transition zone beneath the Canadian landmass

Broadband teleseismic P waveforms recorded on the Canadian National Seismograph Network have been processed to isolate near-receiver Ps conversions originating from upper mantle discontinuities at approximately 410- and 660-km depth. A simultaneous deconvolution procedure which removes source effects and exploits the move-out characteristics of the weak Ps phases is used to raise signal above the;noise levels in the P coda. Large-amplitude arrivals exhibiting times and slownesses consistent with an interpretation as Ps converted phases from the 410- and 660-km discontinuities are observed at 18 and 20 of the 24 stations examined, respectively. The arrival times, corrected for crustal heterogeneity, show strong correlation with upper mantle structure above 410-km depth as determined in previous studies. Stations in the Cordillera above low-velocity mantle are characterized by residuals up to 3.3 s slow relative to the IASP91 model, whereas sites overlying the high-velocity root below the Canadian Shield are advanced by as much as 1.9 s. The difference between the P660s and P410s times provides information on the structure of the transition zone beneath Canadian stations. The trend of the data indicates that topography on the discontinuities is at or below resolvable levels (+/-5 km) over most of the region, in general accord with a previous study of the transition zone beneath continental stations but in contrast with results for the Pacific basin.