Three-dimensional radial anisotropic structure of the North American upper mantle from inversion of surface waveform data

Abstract

S U M M A R YSeismic anisotropy provides insight into palaeo and recent deformation processes and, therefore, mantle dynamics. In a first step towards a model for the North American upper mantle with anisotropy characterized by a symmetry axis of arbitrary orientation, aimed at filling the gap between global tomography and SKS splitting studies, we inverted long period waveform data simultaneously for perturbations in the isotropic S-velocity structure and the anisotropic, in the framework of normal mode asymptotic coupling theory (NACT). The resulting 2-D broad-band sensitivity kernels allow us to exploit the information contained in long period seismograms for 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 data set consists of more than 40 000 high quality three 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. The isotropic part of our tomographic 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. The new regional 3-D radial anisotropic model indicates the presence of two distinct anisotropic layers beneath the cratonic part of the North American continent: a deep asthenospheric layer, consistent with present day mantle flow, and a shallower lithospheric layer, possibly a record of ancient tectonic events.

The seismic mid-lithosphere discontinuity

“…Lebedev et al, 2009). Radial anisotropy in lithospheric mantle across North America (Marone et al, 2007;Nettles and Dziewoński, 2008;Yuan et al, 2011), Australia (Debayle and Kennett, 2000;Fichtner et al, 2010), Africa (Saltzer, 2002;Sebai et al, 2006) and globally (Lebedev et al, 2009;Nettles and Dziewoński, 2008) generally shows faster horizontal wavespeeds than vertical wavespeeds (V SV < V SH ) with a magnitude that decreases with depth from the Moho. However, the magnitude of the anisotropy is generally <4% (Lebedev et al, 2009;Nettles and Dziewoński, 2008), which is too small to account for the MLD phase.…”

Anisotropic shear‐wave velocity structure of the Earth's mantle: A global model

“…Panning and Romanowicz [2006] also found a strong v SH > v SV anomaly in the Pacific at 150 km, which, in contrast to the results of Ekström and Dziewonski [1998], does not change significantly at shallower depths. Gung et al [2003] reported a strong v SH > v SV trend not only in the suboceanic mantle but also beneath the continental lithosphere, and Marone et al [2007] also found lateral variations in anisotropy under the North America. Significant anisotropic variations within continental plates were observed by Nettles and Dziewoński [2008].…”

Interstation phase speed and amplitude measurements of surface waves with nonlinear waveform fitting: application to USArray

“…The single-station method has been primarily used to constrain the large-scale S-wave structure (e.g. van der Lee & Nolet 1997;Godey et al 2003;Marone et al 2007;Nettles & Dziewonski 2008), employing permanent global networks and local stations in the United States. The deployment of the Transportable Array (TA; USArray) in the last decade provides us with high-quality broad-band seismic waveform data, which have facilitated a variety of tomographic studies in this region; for example, ambient noise tomography (e.g.…”