Upper mantle P velocity structure of the Canadian Shield

Waveform modeling of the phases P nl , P , and PP is used to determine the P wave upper mantle structure beneath the Canadian shield. Long‐period recordings at World‐Wide Standard Seismograph Network, long‐range seismic measurement, and Canadian network stations in North America are used, with earthquakes within and just outside the shield as sources. The P data set is incomplete due to the scarcity of large events within the shield, and the inclusion of PP provides much more data. Using the WKBJ approach to calculate the mantle response, the PP waveforms can be modeled almost as well as the P . The phase P nl persists to large distances in shield regions, and the interference of P nl with triplication range P helps constrain the structure in the upper 400 km. Important features of our model, which is designated S25, are (1) a thick, high‐ Q , high‐velocity lid to 165 km, (2) a deep low‐velocity zone, and (3) discontinuities of 5% at 405 km and 4% at 660 km. The lid and low‐velocity zone create an unusual B branch arrival, not seen in other regions, which persists well beyond 30°. Only slight lateral variation is noted within the shield region. The major features of the model are similar to those of Grand and Helmberger's S wave model for the Canadian shield.