Upper mantle structure beneath the Canadian Shield derived from higher modes of surface waves.

Abstract

An analysis of surface waves including the fundamental and first higher modes, and Sa phase was made to determine the upper mantle structure beneath the Canadian Shield. Four earthquakes which occurred beneath the Canadian Shield were used for this purpose. By comparing synthetic and observed seismograms, the shear velocity structure of the upper mantle was determined. The shear velocity structure of the shallower part was determined mainly using the fundamental and first higher modes. The structure in the deeper part was mainly constrained by the Sa phase. Phase of Love type Sa wave was successfully reproduced by a uniform shear velocity structure over the entire area of the Canadian Shield. The average shear velocity of the upper mantle above the depth of 400km obtained in this study is 4.67km/sec, which is higher than velocities of any other models presented before. The low velocity layer of the upper mantle is poorly developed. Since the polarization anomaly of surface waves was not observed for the fundamental and first higher modes and Sa phase, anisotropic velocity structure does not exist in the upper mantle beneath the Canadian Shield.