HR: 08:30h
AN: T21D-03    [Abstracts]
TI: Radial and Azimuthal Anisotropic Structure of the North American Upper Mantle From Inversion of Surface Waveform Data
AU: * Marone, F
EM: federica@seismo.berkeley.edu
AF: Berkeley Seismological Laboratory, University of California, 215 McCone Hall #4760, Berkeley, CA 94720-4760 United States
AU: Romanowicz, B
EM: barbara@seismo.berkeley.edu
AF: Berkeley Seismological Laboratory, University of California, 215 McCone Hall #4760, Berkeley, CA 94720-4760 United States
AB: Seismic anisotropy provides insight into paleo and recent deformation processes and therefore mantle dynamics. To date, our knowledge of the North American anisotropic structure arises mainly from global tomographic models or SKS splitting studies which lack horizontal and vertical resolution respectively, and are limited to either radial or azimuthal anisotropy. Our goal is a high resolution model for the North American upper mantle incorporating both radial and azimuthal anisotropy. We hope to achieve unprecedented lateral and depth resolution by improving both current methodology and data coverage. In a first step, we inverted long period waveform data simultaneously for perturbations in the isotropic S-velocity structure and the anisotropic parameter ξ=v_SH²/v_SV², in the framework of normal mode asymptotic coupling theory (NACT). The resulting 2D broad band sensitivity kernels allow us to exploit the information contained in long period seismograms for body, fundamental and higher mode surface waves at the same time. To ensure high quality of the retrieved regional upper mantle structure, accurate crustal corrections are essential. Here, we follow an approach which goes beyond the linear perturbation approximation and split the correction into a linear and non-linear part. The inverted dataset consists of more than 40,000 high quality 3 component fundamental and overtone surface waveforms, recorded at broad band seismic stations in North America from teleseismic events and provides a fairly homogeneous path and azimuthal coverage. Our 3D radial anisotropic model shares the large scale features of previous regional studies for North America. We confirm the pronounced difference in the isotropic velocity structure between the western active tectonic region and the central/eastern stable shield, as well as the presence of subducted material (Juan de Fuca and Farallon plate) at transition zone depths. Concerning the anisotropic signature, we observe a positive ξ anomaly in correspondence of the cratonic areas between 200 and 300 km depth, while a negative ξ anomaly beneath the Basin and Range province suggests possible mantle upwelling. In a second step, starting from our 3D radial anisotropic model, we have begun addressing the distribution of azimuthal anisotropy, for which we have extended our NACT formalism. Resolving azimuthal anisotropy's four different components will only be possible with the improved coverage we expect from the USArray data. Meanwhile, using appropriate scaling relationships for the upper mantle, we are developing a preliminary 3D model, which, in addition to the isotropic S-velocity structure and the anisotropic parameter ξ, also incorporates the dominant 2-Ψ variations of anisotropy. We hope to obtain constraints on the depth distribution of azimuthal anisotropy and in particular to discriminate between a lithospheric and an asthenospheric origin of the observed SKS splitting.
DE: 7255 Surface waves and free oscillations
DE: 8103 Continental cratons
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8180 Tomography (6982, 7270)
DE: 9350 North America
SC: Tectonophysics [T]
MN: Fall Meeting 2005