Azimuthally anisotropic Rayleigh-wave phase-velocity maps of Europe

Abstract

The European lithosphere is highly heterogeneous, with significant velocity contrasts across the continent. Despite its complexity, much of its detailed structure remains unexplored. We present azimuthally anisotropic phase-velocity maps of Rayleigh waves in the period range 4–40 s for Europe, derived from two years (2011−2012) of continuous waveform data from the Virtual European Broadband Seismic Network (VEBSN) and various temporary arrays. Using ambient-noise cross-correlation, we compute two-station dispersion measurements and integrate them into a tomographic inversion, simultaneously solving for isotropic and anisotropic structures through a least-squares approach. Our thorough suite of tests optimizes regularization and evaluates the resolution and trade-offs between isotropic and anisotropic anomalies. The phase-velocity maps at shorter periods reveal detailed images of major sedimentary basins, with the lowest velocities detected beneath the North German Basin, North Sea Basin, Rhone Basin, Po Plain, Pannonian Basin, and the Bay of Biscay. At longer periods, low-velocity anomalies are prominent beneath the Alps, Northern Apennines, Dinarides, and Anatolian Peninsula. Azimuthal anisotropy is also mapped in the shallow lithosphere, with fast axes in Southern Europe aligned parallel to mountain ranges such as the Pyrenees and Alpine-Apennine system. In Central Europe, the Tornquist-Teisseyre suture zone marks a transition between two domains of different anisotropic structures, suggesting “frozen-in” fabrics originated before the continental collision. The level of detail of these new phase-velocity maps makes them suitable for joint inversion with different types of geophysical data and as starting models for other imaging methods such as full-waveform inversion.