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Seismic anisotropy provides a unique constraint on the past and present dynamics of the lithosphere and sublithospheric mantle. To contribute to studies of large-scale tectonic fabric, we have developed code AniTomo for regional anisotropic tomography. AniTomo allows us to invert simultaneously relative traveltime residuals of teleseismic P waves for 3-D distribution of isotropic-velocity perturbations and velocity anisotropy in the upper mantle. Weak hexagonal anisotropy with the symmetry axis oriented generally in 3-D is considered. The first application of novel code AniTomo to data from passive seismic experiment LAPNET results in a model of anisotropic velocities of the upper mantle beneath northern Fennoscandia. We have opted for northern Fennoscandia for the first application because it is a tectonically stable Precambrian region with a thick anisotropic mantle lithosphere without significant thermal heterogeneities. We carefully analyse the distribution of the rays to limit the fully anisotropic inversion only to the volume with the sufficient directional ray coverage. Capability of the given inversion setup to reveal large-scale anisotropic structures in the upper mantle is documented by a series of synthetic tests. The strongest anisotropy and the largest velocity perturbations concentrate at depths corresponding to the mantle lithosphere, while in deeper parts of the tomographic model, the lateral variations are insignificant. We delimit regions of laterally and vertically consistent anisotropy in the mantle–lithospheric part of the model. We attribute the retrieved domain-like anisotropic structure of the mantle lithosphere in northern Fennoscandia to preserved fossil fabrics of the Archean microplates, accreted during the Precambrian orogenic processes. [ABSTRACT FROM AUTHOR]

Considering only isotropic wave propagation and neglecting anisotropy in teleseismic tomography studies is a simplification obviously incongruous with current understanding of the mantle–lithosphere plate dynamics. Therefore, we have developed a code for anisotropic–teleseismic tomography (AniTomo), which allows to invert relative traveltime residuals of teleseismic P waves simultaneously for coupled anisotropic–isotropic P -wave velocity models of the upper mantle. Due to a more complex anisotropic propagation of S waves, the AniTomo is applicable only to P -wave data. Weak hexagonal anisotropy together with isotropic velocity heterogeneities are interpreted as a cause of the observed P -wave traveltime residuals. Moreover, the axis of the hexagonal symmetry can be oriented freely in all directions, which represents a unique approach among recent approaches that usually incorporate only azimuthal or radial anisotropy into the body-wave tomography. Apart from outlining the theoretical background of AniTomo, we examine various aspects coming along with anisotropic tomography such as choice of a set of initial anisotropic models and setup of parameters controlling the inversion. Synthetic testing furthermore allows investigation of the well-known trade-off between effects of P -wave anisotropy and lateral variations of isotropic velocity. The target synthetic models are designed to schematically represent different heterogeneous anisotropic structures of the upper mantle. Considering realistic distributions of stations and events at teleseismic distances, a separation of seismic anisotropy and isotropic velocity heterogeneities is plausible and a stable output model can be achieved within a few iterations. Careful testing of the new code on synthetics, concentrating on its functionality, strength and weaknesses, is a necessary step before AniTomo is applied to real data sets. [ABSTRACT FROM AUTHOR]

Teleseismic body waves recorded during passive seismic experiments allow us to investigate isotropic velocities of the Earth's upper mantle in a great detail, on scales of tens of kilometres, by means of the standard high-resolution travel-time tomography. Nowadays, the quality of data enables exploration of anisotropic properties of the mantle by the means of travel-time tomography as well. Nevertheless, most of the tomography studies neglect the body-wave anisotropy completely or they are limited to either azimuthal or radial anisotropy.Therefore, we have developed a code called AniTomo for coupled anisotropic-isotropic travel-time tomography of the upper mantle (Munzarová et al., Geophys. J. Int. 2018 – Part I). The novel code allows inversion of relative travel-time residuals of teleseismic P waves simultaneously for 3D distribution of P-wave isotropic-velocity perturbations and anisotropy of the upper mantle. We assume weak anisotropy of hexagonal symmetry with either the 'high-velocity' axis or the 'low-velocity' axis that is oriented generally in 3D. Such an approach of searching for orientation of the symmetry axes freely in any direction is unique and more general in comparison with the published methods that usually assume only horizontal or vertical orientation of the high-velocity symmetry axis. The code represents a step further from modelling homogeneously anisotropic blocks of the mantle lithosphere (e.g., Vecsey et al., Tectonophysics 2007; Plomerová et al., Solid Earth 2011) towards modelling anisotropy arbitrarily varying in 3D.We tested the new code thoroughly, involving simple methodological tests to find out basic characteristics of the method and tests mimicking real tomographic inversions as to the target structures and the station-event distribution. Regarding the well-known trade-off between P-wave anisotropy and isotropic heterogeneities, the inversion with code AniTomo can successfully distinguish the isotropic and the anisotropic components of the velocity, depending, of course, on data quality.For a first application of the novel code (Munzarová et al., Geophys. J. Int. 2018 – Part II), we opted for data from international passive seismic experiment LAPNET (2007 – 2009) deployed in a tectonically stable region of northern Fennoscandia. The resulting tomographic model shows that the strongest anisotropy and the largest isotropic-velocity perturbations concentrate at the mantle-lithospheric depths while in the deeper parts their amplitudes decrease significantly. It is possible to delimit regions of laterally and vertically consistent anisotropy in the mantle-lithospheric part of the model. These regions are compatible with domains inferred from a joint interpretation of directional variations of P-wave travel-time residuals and SKS-wave splitting parameters. We associate the domain-like anisotropy with fossil fabrics of blocks of the Archean mantle lithosphere, preserved probably from the time of the lithosphere origin. [ABSTRACT FROM AUTHOR]

The AlpArray seismic experiment provides an unique opportunity to study mountain-building processes in the European Alps. By further constraining the 3D geometries of foreland basins, of the complex Europe-Adria plate boundary and of other important structures such as the Ivrea body in the western Alps and the Alps-Apennines and Alps-Dinarides transitions we aim to improve our understanding of past and ongoing tectonic processes in the region. Ambient-noise tomography, when applied to the high-quality AlpArray datab provides high-resolution information throughout the crust and over the whole region thus enabling to link near-surface geologic information with deep crustal structures and mantle processUsing two years of data collected by the AlpArray seismic network, we are able to obtain more than 150,000 station-station cross-correlations from which we extract phase- and group-velocity measurements. Compared to previous studies (e.g. 25,000 measurements, Kästle et al., 2018), this is an order of magnitude increase in measurement density, giving us the opportunity to study the crustal shear-velocity structure in the entire Alpine arc with unprecedented resolution. We present first results of our work-in-progress, including the technical aspects of handling such a large dataset and first results of the phase-velocity maps, for both Love- and Rayleigh waves. [ABSTRACT FROM AUTHOR]