Seismic waves and earth structure in the Canadian shield

Abstract

Abstract Careful measurements of phase velocities in the Canadian shield have been made in the period range 3 to 90 sec for Rayleigh waves and 12 to 60 sec for Love waves by phase correlation of wave trains. The continental Love wave phase velocity data are the first to be reported in the literature. The phase and group velocities are higher than yet found in any other continental area, indicating relatively higher shear velocities in the crust and upper mantle. For paths in the Canadian shield a prominent Lg arrival with a velocity of about 3.65 km/sec is observed and an Sn arrival is recorded clearly to distances of about 4000 km with a velocity of about 4.72 km/sec. A theoretical layered model of the crust and upper mantle consistent with the various types of data has been derived by an inversion method employing least-squares curve-fitting of phase velocity data. This model has a three-layered crust 35.2 km thick with shear velocity increasing to about 3.85 km/sec in the lower crust. The upper mantle has a high speed layer with shear velocity 4.72 km/sec down to about 115 km below which the low velocity channel has a shear velocity of about 4.5 km/sec down to a depth of about 315 km. At greater depths the shear velocity closely follows the Gutenberg model. Higher mode phase velocity dispersion curves computed from the theoretical model are used for computing theoretical seismograms for the Canadian shield. These theoretical seismograms possess many of the features of the observed Lg arrivals, showing that Lg can be explained by normal mode wave propagation in a simple, layered crust. This paper shows that new methods of measuring and interpreting surface wave dispersion, combined with travel time data, can provide detailed and reliable information on shear wave velocity distribution down to depths of a few hundred kilometers in regions of horizontally uniform structure.