Imaging anisotropic layering with Bayesian inversion of multiple data types

International audienceAzimuthal anisotropy is a powerful tool to reveal information about both the present structure and past evolution of the mantle. Anisotropic images of the upper mantle are usually obtained by analysing various types of seismic observables, such as surface wave dispersion curves or waveforms, SKS splitting data, or receiver functions. These different data types sample different volumes of the earth, they are sensitive to different length scales, and hence are associated with different levels of uncertainties. They are traditionally interpreted separately, and often result in incompatible models. We present a Bayesian inversion approach to jointly invert these different data types. Seismograms for SKS and P phases are directly inverted using a cross-convolution approach, thus avoiding intermediate processing steps, such as numerical deconvolution or computation of splitting parameters. Probabilistic 1-D profiles are obtained with a transdimensional Markov chain Monte Carlo scheme, in which the number of layers, as well as the presence or absence of anisotropy in each layer, are treated as unknown parameters. In this way, seismic anisotropy is only introduced if required by the data. The algorithm is used to resolve both isotropic and anisotropic layering down to a depth of 350 km beneath two seismic stations in North America in two different tectonic settings: the stable Canadian shield (station FFC) and the tectonically active southern Basin and Range Province (station TA-214A). In both cases, the lithosphere-asthenosphere boundary is clearly visible, and marked by a change in direction of the fast axis of anisotropy. Our study confirms that azimuthal anisotropy is a powerful tool for detecting layering in the upper mantle

3D Radial and Azimuthal Anisotropy Structure in North America

We recently developed a 3D tomographic model of the upper mantle beneath North America that includes both isotropic S velocity structure as well as radial and azimuthal anisotropy (Marone et al., 2007; Marone and Romanowicz, 2007). This model was constructed from a joint inversion of fundamental and higher mode surface waveforms together with constraints on azimuthal anisotropy derived from SKS splitting measurements. This model showed evidence for the presence of two layers of anisotropy beneath the stable part of the North American continent: a deeper layer with Vsh\u3eVsv and 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 the absolute plate motion direction. We here present an updated model, which includes a larger waveform data set (three more years of data including new data from US Array) and a larger SKS splitting dataset. In addition, we are going one step further in that we now combine the results from inversion for radial and azimuthal anisotropy, to recover the distribution of the direction and amplitude of the fast axis in 3D, under the assumptions of hexagonal symmetry. We discuss the results in the light of possible causes of anisotropy in the cratonic lithosphere