A SEISMIC IMAGE OF THE UPPER-MANTLE BENEATH THE NORTH-AMERICAN CRATON

Teleseismic P-waves recorded on the Canadian National Seismograph Netw ork are used to investigate upper mantle structure beneath the North A merican craton. 1090 broadband seismograms are source-normalized and s tacked in 100 bins between 30 degrees and 100 degrees epicentral dista nce. The resulting image reveals a number of P to S converted phases w hich include prominent Ps conversions from the 410- and 660-km discont inuities. Reduction of these signals to single traces via simultaneous deconvolution along theoretical moveout curves permits the analysis o f 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 prev ious suggestions that the phase transformation responsible for the 410 km discontinuity occurs over a narrower transition than that at 660 k m. The image and single trace stacks are examined for evidence of disc ontinuities at other levels, notably at 520 km and 220 km depth. No ob vious expression of P(520)s is evident at high frequencies, and evalua tion 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 interf ace is present in the form of an amplitude maximum at the expected tim e and slowness of a Pp(220)s phase.