Ambient-noise Tomography of the European Lithosphere: Preliminary Results

Europe is covered nonuniformly by both seismic instruments and earthquakes. To enhance seismic resolution, it is then essential to exploit the information which comes from cross-correlation of stacked seismic ambient noise. Our group at ETH compiled a dense regional database of European station-station surface-wave dispersion between 8-35 seconds using noise-interferometry, resulting in a dramatic growth in seismic coverage with respect to earthquake-based tomography. It has recently been shown how adjoint techniques can be applied to ambient-noise data, overcoming the often severe nonuniformity in the geographic distribution of noise "sources", and the subsequent discrepancies between the recorded noise cross-correlation and the theoretical Green's function. We focus on using seismic ambient noise generated in oceans as data to map 3D structure beneath Europe. While earlier "noise-tomography" studies have relied on ray-theory (the infinite-frequency approximation), we apply an iterative, gradient-based inversion where Fréchet derivatives are computed using a spectral-element wave propagation (SPECFEM3D) and adjoint method. Our initial model is composed of two contributions: (1) EPcrust, a new crustal model for the European plate, derived from collection of numerous independent previous studies of multiple scale lengths (i.e. EuCRUST07, ESC Moho, Alps Model), (2) a new adaptive-grid surface-wave tomography of the uppermost mantle down to periods of 35 seconds. This model is discretized with irregular meshes (using Cubit) that honor all relevant discontinuities and are adaptive within the inversion procedure. The misfit function between modeled and data-based cross-correlations that defines the adjoint source is based on a multitaper traveltime difference, allowing us to iterative from coarse to fine scale. We address the peculiar issue of non-uniform noise sources by including the noise distribution in the inversion process and show examples of such noise source gradients. Noise sources originate from vastly different regions depending on the frequency content, and important aspect for minimizing misfit functions which is well accommodated by our multi-taper measurements. We present initial results of the iterative inversion process. We assess the relevance of finite-frequency effects in the context of regional-scale ambient-noise tomography, interpreting our tomographic results in terms of crust and lithospheric/asthenospheric structure in the region of interest.