2-D anisotropicPn-velocity tomography underneath Germany using regional traveltimes

Abstract

This paper, complementing the recent 1-D Pn time-term analysis of Song et al. (2001a), presents tomographic inversions of regional Pn traveltimes to image lateral variations of seismic velocity and anisotropy within the uppermost mantle beneath Germany. As such our study fills a gap of previous regional 2-D anisotropic Pn studies for France and Italy by expanding anisotropic Pn tomography to a continental scale covering much of Europe. In a novel way, the resolving power of the used Pn tomography system is investigated through a series of checkerboard tests combined with tradeoff analyses between resolution and covariance of the models. Methodologically important and commonly interesting problems of seismic tomography are addressed with the data set, such as model parametrization for a given source-receiver configuration, effects of time terms on the model parameters, and ambiguity between lateral velocity perturbations and anisotropy. The reliability and significance of various inversion models (1-D, 2-D, isotropic, anisotropic) is investigated in detail using statistical F tests. The latter indicate that, while both 2-D isotropic and anisotropic models provide better fits of the Pn data than a standard homogeneous, isotropic upper mantle, the most significant improvement arises from a large-scale 1-D anisotropy ellipsoid with the fast Pn-velocity axis in 25°NE direction. This result correlates well with a number of trends related to upper mantle and plate dynamics, for example, major plate motion. Besides this anisotropic dominant traveltime pattern, we observe a number of lateral velocity anomalies. Lower than average velocities were detected in the southwestern Rhenish massif (but not in the area of the Eifel plume), similar to findings for the adjacent region in France. In addition, lower velocities are seen in parts of the Alps and in a region along the upper Main river, south of the Rhön. However, the most striking Pn anomalies are associated with prominent high velocity anomalies in the Ingolstadt region and an area centred between the Harz and the Vogelsberg, where perturbations of up to +5 per cent are found. As a possible mechanism for these features we consider underplating within ancient plume activity. In addition, results for the time-term station delays are consistent with findings of crustal structure in other studies with different methodologies and data, and indicate, for example, crustal updoming of the Moho in the southern Rhine graben around the Kaiserstuhl area, but crustal thickening towards the Alps.