The Radial Anisotropy of the Continental Lithosphere From Analysis of Love and Rayleigh Wave Phase Velocities in Fennoscandia

• Published in: Journal of geophysical research. Solid earth 2022-10, Vol.127 (10), p.n/a
• Main Authors: Maupin, Valérie, Mauerberger, Alexandra, Tilmann, Frederik
• Format: Article
• Language: English
• Subjects: Anisotropy; Beamforming; Compression; continental lithosphere; Crystal structure; Crystallography; Crystals; Dipping; Earth mantle; Earth surface; Earthquakes; Fennoscandia; Geophysics; Lithosphere; Olivine; P-waves; Penetration depth; Preferred orientation; Rayleigh waves; Seismic activity; seismic anisotropy; Seismic waves; Seismology; SH waves; Shear flow; Surface waves; Tectonics; Travel; Upper mantle; Velocity; Wave phase; Wave velocity
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2022JB024445

Radial anisotropy (RA) in the upper mantle of the Fennoscandian Shield is analyzed by joint inversion of Love and Rayleigh wave phase velocities measured from recordings of teleseismic events at the ScanArray network. The phase velocities are measured by beamforming using three geographical subsets of the network as well as the full network. We analyze how different procedures for determining the phase velocities influence the final result and uncertainty. Joint inversion of the phase velocities in the period range 22–100 s reveals the presence of similar RA in the three subregions, with an average ξ value of about 1.05 in the subcrustal lithosphere down to at least 200 km depth. This corresponds to SH waves faster than SV by 2%–3%, a value very similar to those found in other continental regions. Considering this anisotropy together with other observables pertaining to seismic anisotropy in the area, we cannot propose a unique model satisfying all data. We can show, however, in which conditions different types of olivine crystallographic preferred orientations (CPOs) commonly observed in natural samples are compatible with the observations. CPO types associated with the preferred orientation of the a‐axis, in particular the common A‐type CPO, require a‐axes dipping not more than 25° from the horizontal plane to explain our observations. AG‐type CPO, characterized by preferred orientation of the b‐axis and occurring in particular in compressional settings, can be considered as an interesting alternative interpretation of continental lithospheric anisotropy, provided the olivine b‐axis is dipping by at least 60°. Plain Language Summary Large earthquakes generate seismic waves which travel all over the globe. Two types of waves, called surface waves, travel along the Earth's surface with varying penetration depths and enable us to study the Earth's structure at depth. Using waves generated by distant earthquakes, we have analyzed the velocity of these two types of surface waves when they travel across a 1,500 × 600 km2 network of seismological stations located across Sweden, Norway, and Finland, in a geologically old tectonic province called the Fennoscandian Shield. We observe a discrepancy between the velocities of these two types of surface waves and analyze the possible causes for this discrepancy. We conclude that seismic anisotropy of the upper mantle from about 50 to at least 200 km depth is the most likely explanation for our observations. Seismic an