HR: 11:20h
AN: S42A-05    [Abstracts]
TI: Velocity Structure and Poisson's Ratio of the Crust and Upper Mantle Beneath Canada and Their Tectonic Implications
AU: Gangopadhyay, A
EM: abhijit@ig.utexas.edu
AF: Institute for Geophysics, University of Texas at Austin, J.J. Pickle Research Campus, 10100 Burnet Road, Bldg. 196, Austin, TX 78758, United States
AU: * Pulliam, J
EM: jay@ig.utexas.edu
AF: Institute for Geophysics, University of Texas at Austin, J.J. Pickle Research Campus, 10100 Burnet Road, Bldg. 196, Austin, TX 78758, United States
AU: Sen, M K
EM: mrinal@ig.utexas.edu
AF: Institute for Geophysics, University of Texas at Austin, J.J. Pickle Research Campus, 10100 Burnet Road, Bldg. 196, Austin, TX 78758, United States
AB: We apply a global-search waveform modeling method to fit S, Sp, SsPmP, and shear-coupled PL phases recorded from 132 earthquakes at eleven permanent broadband seismic stations in Canada. Our objective is to determine the one-dimensional azimuthally dependent crust and uppermost mantle velocity structure and Poisson's ratio beneath individual stations for the purpose of tectonic interpretations. Our technique uses the reflectivity method to compute synthetic seismograms and applies a global optimization algorithm called Very Fast Simulated Annealing (VFSA) to obtain the best possible velocity models. We also use built-in statistical tools such as the Posterior Probability Density (PPD) distributions and parameter correlation matrices to evaluate the reliability of resulting parameter estimates. Our method complements traditional receiver function techniques in that it models a different part of the seismogram and obtains P- and S-wave velocities directly. The Canadian seismic stations each recorded between 3 and 25 earthquakes suitable for analysis. The stations sample major tectonic provinces within the Canadian landmass such as the Cordilleran orogen, western plains and Slave province in western Canada, Grenville province and Appalachian orogen in eastern Canada, and the Canadian Arctic. Preliminary results from our study are consistent with the results from earlier studies using receiver functions and active source data collected through LITHOPROBE, and regional tectonics. Crustal thicknesses beneath stations in the northern Cordilleran orogen, western plains, and Slave province range between 35 and 37 km, whereas their average Poisson's ratio range between 0.21 and 0.24. The Moho appears to be slightly shallower (31-35 km) beneath stations in the southern Cordilleran orogen. However, the average crustal Poisson's ratio is similar (0.20–0.24). In eastern Canada, the crust beneath stations of the Grenville province and Appalachian orogen is generally thick (approximately 44 km) with the exception of that beneath station DRLN, in the northeast, where it is approximately 33 km. The average crustal Poisson's ratio is also higher (0.24-0.29). Moho depths beneath stations in the Canadian Arctic range between approximately 30 and 41 km, whereas the average crustal Poisson's ratio range between 0.22-0.26. Most of the crustal Poisson's ratio values beneath Canada are below the global average for continental crust (approximately 0.265). The higher Poisson's ratio values that are well-constrained tend to be associated with crust of older age, typically Archaean. The spectrum of Poisson's ratio values is suggestive of a more felsic crustal composition in the western parts of Canada and a more mafic composition in the eastern parts. We also observe low-velocity zones (LVZ) in the crust and uppermost mantle in the vicinity of a few stations, however the constraints on these LVZs are poor.
DE: 7200 SEISMOLOGY
SC: Seismology [S]
MN: 2007 Fall Meeting