Ambient seismic noise tomography of Canada and adjacent regions: Part I. Crustal structures

Abstract

This paper presents the first continental-scale study of the crust and upper mantle shear velocity (V_s) structure of Canada and adjacent regions using ambient noise tomography. Continuous waveform data recorded between 2003 and 2009 with 788 broadband seismograph stations in Canada and adjacent regions were used in the analysis. The higher primary frequency band of the ambient noise provides better resolution of crustal structures than previous tomographic models based on earthquake waveforms. Prominent low velocity anomalies are observed at shallow depths (<20 km) beneath the Gulf of St. Lawrence in east Canada, the sedimentary basins of west Canada, and the Cordillera. In contrast, the Canadian Shield exhibits high crustal velocities. We characterize the crust-mantle transition in terms of not only its depth and velocity but also its sharpness, defined by its thickness and the amount of velocity increase. Considerable variations in the physical properties of the crust-mantle transition are observed across Canada. Positive correlations between the crustal thickness, Moho velocity, and the thickness of the transition are evident throughout most of the craton except near Hudson Bay where the uppermost mantle V_s is relatively low. Prominent vertical V_s gradients are observed in the midcrust beneath the Cordillera and beneath most of the Canadian Shield. The midcrust velocity contrast beneath the Cordillera may correspond to a detachment zone associated with high temperatures immediately beneath, whereas the large midcrust velocity gradient beneath the Canadian Shield probably represents an ancient rheological boundary between the upper and lower crust.