Radial and Azimuthal Anisotropic Structure of the North American Upper Mantle From Inversion of Surface Waveform Data

Federica Marone and Barbara Romanowicz

Introduction

Seismic anisotropy is required for a correct interpretation of the retrieved S-velocity structure in tomographic studies at least in the first 400 km of the upper mantle (Gung et al., 2003). A detailed knowledge of the seismic anisotropic structure of the earth's mantle also provides insight into paleo and recent deformation processes and therefore mantle dynamics. As a consequence, seismic anisotropy is a very powerful ``tool'' with the potential to shed light onto debated geophysical issues, such as the nature and strength of the lithosphere/asthenosphere coupling, the depth extent of continental sub-regions and the relation of imaged seismic anisotropy to present-day asthenospheric flow and/or past tectonic events recorded in the lithosphere.

To date, our knowledge of the North American upper mantle anisotropic structure arises mainly from global tomographic models (e.g. Ritsema et al., 1999; Gung et al., 2003) or SKS splitting studies (e.g. Fouch et al., 2000; Savage and Sheehan, 2000), which lack horizontal and vertical resolution respectively, and are limited to either radial or azimuthal anisotropy. It is most probably due to these limitations that to date continental anisotropic models derived from surface and body wave data are not in agreement and cannot be reconciled.

Our goal is a new high resolution model for the North American upper mantle incorporating both radial and azimuthal anisotropy. We aim at unprecedented lateral and depth resolution by improving both data coverage and methodology.

Dataset

We consider fundamental and overtone surface waveforms selected from 3 component long period seismograms. Surface wave data for paths relevant to the study region have been extracted from the existing compilation used for global tomography in Panning and Romanowicz (2006). This dataset has been further complemented with waveforms from events at teleseismic and far regional distances (15$^{\circ }$$<\Delta<$165$^{\circ }$) recorded at permanent and temporary broad band seismic stations in North America. The collected dataset includes data for 657 events from 1990 to 2003, with $M_{w}$ between 6.0 and 7.0. Each seismogram is filtered (between 60 s and 220 s to 1 hour, according to the event magnitude) and divided into wavepackets containing individual fundamental and higher mode first orbit energy packets. Each wavepacket is weighted so as to equalize the contribution of large and small amplitude packets in the least-squares inversion. Our final dataset consists of more than 18,000 fundamental mode and 27,000 higher modes high quality surface wave packets, which provides a fairly homogeneous path and azimuthal coverage for North America.

In addition, we compiled station average SKS splitting measurements (delay times dt and fast axis directions $\phi$ and their uncertainties) from published studies for about 300 North American stations.

Methodology

We apply a full waveform tomographic method based on the Non-linear Asymptotic Coupling Theory (NACT - Li and Romanowicz (1995)), which permits the inversion of entire long period seismograms in the time domain (including fundamental mode and overtones portions of the record) for 3D elastic structure. NACT is a normal-mode perturbation approach, which takes into account coupling between modes both along and across dispersion branches. The asymptotic calculation of this coupling allows the computation to 2D broad band sensitivity kernels which more rigorously reproduce the sensitivity of body waveforms to structure along and around the ray geometrical path in the vertical plane containing the source and the receiver.

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 (Montagner and Jobert, 1988).

We followed an iterative inversion approach. In a first step, we inverted waveform data simultaneously for perturbations in the isotropic S-velocity structure and the anisotropic parameter $\xi=v_{SH}^2/v_{SV}^2$. While keeping this obtained radial anisotropic model fixed, in a second step we inverted the waveform dataset jointly with the compiled SKS splitting measurements for two additional parameters related to the dominant 2$\Psi$ azimuthal dependence of the propagation velocity of surface waves (Montagner et al., 2000; Marone and Romanowicz, 1996a).

Results

Our 3D radial anisotropic model (Marone et al., 2006b) shares the large scale features of previous regional studies for North America (e.g. Van der Lee and Nolet, 1997; Grand, 2001). We confirm the pronounced difference in the isotropic velocity structure (Figure 28.3a) 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 radial anisotropic signature (Figure 28.3b), we observe a positive $\xi $ anomaly in correspondence of the cratonic areas down to 300 km depth, while a negative $\xi $ anomaly beneath the Appalachians mapped at 250 km depth, where a low velocity feature is also present, supports the hypothesis of mantle upwelling induced by water released during subduction of the Iapetus ocean or related to the subducted Farallon plate (Van der Lee et al., 2005).

Our 3D azimuthal anisotropic model (Figure 28.3c) indicates the presence of two layers of anisotropy with distinct fast axis directions under the stable part of the North American continent: a deeper layer with the fast axis direction aligned with the absolute plate motion direction suggesting lattice preferred orientation of anisotropic minerals in a present day asthenospheric flow and a shallower lithospheric layer likely showing records of past tectonic events. Under the tectonically active western US, where the lithosphere is thin, the direction of tomographically inferred anisotropy is stable with depth and compatible with both absolute plate motion direction and the dominant direction obtained from SKS splitting measurements. The combined radial and azimuthal anisotropic 3D structure retrieved in our model, resolved throughout the upper mantle, represents the advancement of this study with respect to previous works. These new results seem to suggest a possible reconciliation between anisotropic models derived from surface and body wave data.

Acknowledgements

This research was partially supported through a grant from the Stefano Franscini Foundation (Switzerland) and NSF-EAR (Earthscope) grant #0345481. The digital seismograms used in our work have been distributed by the IRIS-DMC, the Geological Survey of Canada and the Northern California Earthquake Data Center (data contributed by the Berkeley Seismological Laboratory, University of California, Berkeley).

References

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