Imaging anisotropic layering with Bayesian inversion of multiple data types

Abstract

Azimuthal 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.

Layered structure in the upper mantle across North America from joint inversion of long and short period seismic data

“…Layering has also been found in scattering studies (e.g. Bostock, 1998) and the presence of negative discontinuities in the mid-lithosphere was clearly demonstrated in recent RF studies across the NA craton (Abt et al, 2010;Rychert et al, 2010;Kumar et al, 2012;Hansen et al, 2015). Interestingly, an MLD with large topographic variations in the depth range 100-160 km is detected by a rapid change in the fast axis direction of anisotropy (e.g.…”

“…Below, we briefly present the method. For a more detailed description, we refer the reader to Bodin et al (2012Bodin et al ( , 2014Bodin et al ( , 2016. In Appendix 1, we present synthetic tests showing the ability of the method to resolve seismic structures down to 350 km beneath individual stations.…”

“…Still, biases due to limited azimuthal coverage could be present at some stations, especially for anisotropy, a computationally demanding issue we will investigate in the future, when we apply this methodology to a larger dataset and also solve for azimuthal anisotropy (e.g. Bodin et al, 2016).…”

“…In order to image such discontinuities, methods based on the analysis of the scattered wavefield at shorter periods (10-30 s) have been developed. The most frequently considered method uses information contained in phases converted at crust and upper mantle interfaces under single stations, the so-called receiver function method (RF, Vinnik, 1977;Ammon et al, 1990;Bostock, 1998).…”

On the Implications of A Priori Constraints in Transdimensional Bayesian Inversion for Continental Lithospheric Layering

“…Recently, Calò et al () imaged discontinuities in the upper mantle of the North American craton by modeling surface wave dispersion data and converted P s phases using a Markov chain Monte Carlo transdimensional method (Malinverno, ), while Bodin et al () added S K S waveforms to this approach. They presented evidence for strong lithospheric layering in the stable part of the North American craton, marked by at least two midlithospheric discontinuities (MLDs) at intermediate depths.…”