Mg. Bostock et Jf. Cassidy, THE UPPER-MANTLE DISCONTINUITIES IN WESTERN CANADA FROM PS CONVERSIONS, Pure and Applied Geophysics, 145(2), 1995, pp. 219-233
We have investigated variations in the travel times of Ps converted ph
ases from the upper mantle 410 and 660 km discontinuities recorded on
the western stations of the Canadian National Seismograph Network usin
g a variant of the technique introduced by VINNIK (1977). Clear and un
ambiguous signals for both discontinuities are observed at 8 of the 11
stations considered and exhibit variations which correlate well with
regional tectonic setting. Stations located in regions which are curre
ntly tectonically active are characterized by larger Ps arrival times
relative to direct P than those situated on the North American craton.
In addition the difference in arrival times between the 410 and 660 P
a phases suggest that most of the variation is the result of structure
above the transition zone but below the Moho. Stations located in the
Cascadia subduction zone generally exhibit poorer signal quality than
those elsewhere, a feature that may result from upper mantle velocity
heterogeneity, discontinuity topography or a combination of both. A d
etailed investigation of possible lateral variations in discontinuity
topography associated with subduction awaits the compilation of a more
comprehensive data set which will permit the monitoring of an azimuth
al dependence in signal.