Geomagnetic depth sounding and upper mantle structure in the Cordillera region of western North America

This report describes recent geomagnetic depth-sounding investigations on the inland (Cordillera) anomaly first reported by Schmucker at latitude 32°N. It includes Hyndman's profile at latitude 49.5°N and two subsequent profiles at latitudes 35°N and 51°N. All three profiles crossed the transition between the ‘high-I’ and ‘low-I’ zones. Characteristic parameters have been derived from power spectral analysis of the data. The attenuation in the normalized vertical component at the western stations increases with frequency, from 0.97±0.65 db at 0.025 c/ks (10-hour period) to 8.10±1.25 db at 0.35 c/ks (45-minute period). The attenuation is uniform along all three profiles, confirming that the same conductivity structure exists in the southwest United States and in southern British Columbia. Quantitative analysis indicates a rise of highly conducting material (10−11 emu ± 20%) to within 25–35 km of the surface under the western region.