A seismic image of the upper mantle beneath the North American craton

Teleseismic P-waves recorded on the Canadian National Seismograph Network are used to investigate upper mantle structure beneath the North American craton. 1090 broadband seismograms are source-normalized and stacked in 100 bins between 30 degrees and 100 degrees epicentral distance. The resulting image reveals a number of P to S converted phases which include prominent Ps conversions from the 410- and 660-km discontinuities. Reduction of these signals to single traces via simultaneous deconvolution along theoretical moveout curves permits the analysis of P(410)s and P(660)s over a range of frequencies. Relative amplitudes vary with P(410)s dominating at higher frequencies. This results in a greater pulse width for P(660)s in broadband stacks and supports previous suggestions that the phase transformation responsible for the 410 km discontinuity occurs over a narrower transition than that at 660 km. The image and single trace stacks are examined for evidence of discontinuities at other levels, notably at 520 km and 220 km depth. No obvious expression of P(520)s is evident at high frequencies, and evaluation of lower frequency stacks is complicated by sidelobe interference from P(410)s and P(660)s. Evidence in support of a weak 220-km interface is present in the form of an amplitude maximum at the expected time and slowness of a Pp(220)s phase.