The upper mantle beneath the north‐east Pacific rim: a comparison with the Gulf of California

Summary Seismograms from 22 earthquakes along the north‐east Pacific rim recorded in southern California form the data set for investigation of the upper mantle beneath the Cascade Range‐Juan de Fuca region, a transitional area encompassing both very young ocean floor and a continental margin. These data consist of 853 seismograms (6d̀ < Δ < 42d̀) which produce 1068 travel times and 40 ray parameter estimates. We compare these data directly to another large suite of records representative of structure beneath the Gulf of California, an active spreading centre. We use the spreading centre model, GCA, as a starting point in WKBJ synthetic seismogram modelling and perturb GCA until the NE Pacific data are matched. Application of wavefield continuation to these two groups of data provides checks on the model's consistency with the data as well as an estimate of the resolvability of differences between the two areas. The resulting NE Pacific model, CJF, is very similar to GCA. Disparities in travel times and waveforms from 16 to 22d̀, however, are interpretable in terms of lateral structural variations between the two regions from 200 to 410 km depth. CJF features velocities from 200 to 350 km that are intermediate between the very low values of GCA and the more moderate values of T7, a model for the inland western United States. Models of continental shield regions have even higher velocities in this depth range, but all four model types are similar below 450 km. This regular pattern observed in the rate of velocity increase with tectonic regime suggests an inverse relationship between velocity gradient and lithospheric age above 400 km depth.