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 (Vs) 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 Vs is relatively low. Prominent vertical Vs 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.

CAN‐HK: An a Priori Crustal Model for the Canadian Shield

“…Comparison with two parameters from the Kao13 model are shown in Figure 9 (the 50% and 85% increase in V S between the lower crust and mantle, Z 50% and Z 85% , respectively; Kao et al, 2013). Almost all the values in Figure 9a lie above 0 km, indicating that Z 50% lies consistently shallower than our crustal thickness estimates.…”

“…New crustal models, herein referred to as the Kao13 and NACr14 models, respectively, for the North American continent have been recently presented by Kao et al (2013) and Tesauro et al (2014). The Kao13 model is an S-wave velocity model produced using ambient noise observations, whereas the NACr14 model is a P-wave velocity model that uses the U.S. Geological Survey crustal structure database.…”

“…The skewness suggests slightly larger CAN-HK crustal thicknesses. Kao et al (2013) uses to justify Z 85% as representing the depth to the ambient noise Moho. Also evident in Figure 9b is a subset of stations that are centered at −15 km (i.e., Z 85% is 15 km deeper than the CAN-HK crustal thickness).…”

“…Data from the temporary Hudson Bay Lithospheric Experiment (HuBLE) ▴ Figure 9. Differences between crustal thickness from the CAN-HK model and (a) depth to 50% of the velocity increase from the lower crust to mantle (Z 50% ) and (b) depth to 85% of the velocity increase from the lower crust to mantle (Z 85% ) from Kao et al (2013). The uniformly thicker CAN-HK predictions are shown by the ∼5 km offsets in the Z 50% comparison, whereas bimodality in the Z 85% comparison suggests coherent small-scale upper-mantle architectures beneath the southeast Superior/Grenville orogen region.…”

Methodology of Seismic Tomography

“…Many workers have used ambient-noise tomography to study shear-wave velocity structure of the crust and shallow mantle (e.g., Campillo and Paul 2003;Shapiro et al 2005;Lin et al 2008;Zheng et al 2011;Kao et al 2013). This method, however, requires a dense, modern seismic network.…”