THE UPPER-MANTLE DISCONTINUITIES IN WESTERN CANADA FROM PS CONVERSIONS

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.