Imaging Radially Anisotropic Crustal Velocity Structure in NW Canada

Imaging Radially Anisotropic Crustal Velocity Structure in NW Canada (a,b) Measurements of Rayleigh and Love wave group velocity, plotted as color-coded line segments along the great-circle path connecting each station pair. Color scale ranges from 2.75-3.25 km/s and 3.0-3.6 km/s for the Rayleigh and Love waves, respectively. (c,d) Isotropic velocity perturbation in the 3-D seismic model of the study area, shown here at 15- and 30-km depth. The white and black lines indicate the Tintina fault and Great Slave Lake Shear Zone, respectively. The white barbed line shows the eastern limit of Cordilleran deformation.
The tectonic evolution of Northwestern Canada spans several billion years of Earth history. As such, it presents an ideal environment for studying the processes of continental accretion and growth. We use ambient- noise cross-correlation to image anisotropic crustal seismic-velocity structure in NW Canada. Our focus area surrounds the CANOE (CAnadian NOrthwest Experiment) array, a 16-month IRIS PASSCAL deployment of 59 broadband seismic stations. We also include 42 broadband stations from the Canadian National Seismograph Network and the POLARIS network. We estimate the Green's function for each pair of stations by cross-correlating day-long time series of ambient noise in the time period July 2004 - June 2005. We observe fundamental-mode Rayleigh waves on cross-correlated vertical-component records and Love waves on the transverse components. We measure group velocities for the surface waves in the period range 5-30 s. Laterally, group velocities vary by as much as ±15% at the shortest periods and ±6% at longer periods, with the fastest velocities found within the Slave province and very slow velocities associated with thick sedimentary layers at short periods.
</p><p>We investigate 3-D shear-velocity structure using two approaches. We use a Monte Carlo approach to test whether the data are consistent with isotropic velocity and find that the Love wave data require faster velocities in the middle--lower crust than the Rayleigh waves do; i.e., VSH>VSV. We also invert the group-velocity values (>2500 interstation paths) for 3-D radially anisotropic shear-wave velocity within the crust. Since the sensitivity kernels depend strongly on the assumed elastic structure, we use local kernels to account for the effects of laterally variable sedimentary structure. The resulting model correlates with several known geologic structures, including sedimentary basins at shallow depths and possibly the Cordillera-craton transition in the lower crust. The crustal model will also be useful for future studies of the upper mantle in this area.</p>


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