A COMPARISON OF THE RECEIVER STRUCTURE BENEATH STATIONS OF THE CANADIAN-NATIONAL-SEISMOGRAPH-NETWORK

Three-component broadband data from the recently deployed Canadian Nat ional Seismograph Network provide a new opportunity to examine the str ucture of the crust and upper mantle beneath the Canadian landmass. Re ceiver function analysis is an ideal method to use with this data set, as it can provide constraints on the S-velocity structure beneath eac h station of this seismograph network. This analysis method is particu larly useful in that it provides site-specific information (i.e., with in 5-15 km of the station), low-velocity layers can be identified, and it is possible to examine structure to upper mantle depths. In this s tudy, receiver functions were computed for each of the 19 stations tha t made up the seismograph network in June 1994. Five stations, samplin g a variety of tectonic environments, including the Appalachian Orogen , the Canadian Shield, the Western Canadian Sedimentary Basin, and the Cascadia subduction zone, were chosen for detailed modelling. The res ults presented here are the first estimates of the S-velocity structur e beneath these five stations. For those stations where comparisons ca n be made with seismic reflection and refraction results, there is exc ellent agreement. In eastern Canada, simple receiver functions and cle ar Moho Ps conversions at most stations indicate a relatively transpar ent crust and a Moho depth of 40-45 km. In northwestern Canada, Moho P s phases indicate a crustal thickness of 33-38 km. Beneath Inuvik, Nor thwest Territories, the Moho is interpreted as two velocity steps sepa rated in depth by 5 km, and an upper mantle low-velocity zone is near 47 km depth. In western Canada, the data indicate a mid-crustal low-ve locity zone beneath Edmonton. The Moho of the subducting Juan de Fuca plate is interpreted at 52 km depth beneath southern Vancouver Island. Several stations exhibiting complex receiver functions warrant furthe r study. They include stations at Schefferville, Quebec, in the Canadi an Shield; Deer Lake, Newfoundland, on the boundary of the Grenville P rovince and the Appalachian Orogen; and Yellowknife, Northwest Territo ries, at the intersection of the Churchill and Slave provinces and the Western Plains.