Complex deep seismic anisotropy below the Scandinavian Mountains

• Published in: Journal of seismology Vol. 17; no. 2; pp. 361 - 384
• Main Authors: Roy, Corinna, Ritter, Joachim R. R.
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01-04-2013; Springer; Springer Nature B.V
• Subjects: Anisotropy; Correlation; Earth and Environmental Science; Earth Sciences; Earth, ocean, space; Earthquakes, seismology; Engineering and environment geology. Geothermics; Exact sciences and technology; Geophysics/Geodesy; Geotechnical Engineering & Applied Earth Sciences; Hydrogeology; Internal geophysics; Lithosphere; Mountains; Natural hazards: prediction, damages, etc; Original Article; Phases; Rock; Seismic phenomena; Seismology; Signal to noise ratio; Spatial distribution; Stations; Structural Geology; Tectonics; Upper mantle; Sweden; Southern Norway; Norway; Scandinavia; Europe; SKS splitting; Anisotropy; Scandinavia; Lithosphere; models; nappes; mountains; tectonic units; projects; anisotropy; seismology; lithosphere; upper mantle; S-waves; preferred orientation; spatial distribution; crust; frequency; depth; fabric; signal-to-noise ratio; birefringence; tectonics; waveforms; direction
• ISSN: 1383-4649; 1573-157X
• DOI: 10.1007/s10950-012-9325-4

Several seismological projects focused on the deep structure of the Scandinavian Mountains, in Norway and neighbouring Sweden. We use these recordings to study seismic anisotropy by analysing the birefringence of SKS and SKKS phases. These phases, which should be polarised radially, are split into an additional transverse component if they propagate through an anisotropic medium. Our results are directions
Φ
of the apparent fast shear wave polarisation and delay times δ
t
between the split phases. For station KONO in Southern Norway, we find frequency-dependent
Φ
and δ
t
values, indicating a depth-dependent anisotropy. Additionally,
Φ
and δ
t
values vary with epicentre backazimuths in Norway, indicating a complex anisotropic structure in the crust and upper mantle. Stacking of the SKS/SKKS waveforms improves the signal-to-noise ratio along one station line and allows us to better determine the splitting parameters. A unique and complete model of the complex anisotropy cannot be obtained due to the limited observed backazimuth range. Near-surface tectonic structures correlate with the splitting pattern and thus the crust is one anisotropic layer in the region. Partly preferred orientations in the rock fabric at the surface can be correlated with
Φ
. Below one or more anisotropic layers must exist to explain the backazimuth- and frequency-dependent observations, as well as the long δ
t
values (>2 s) which cannot be explained with crustal anisotropy alone. The spatial distribution of the splitting results indicates that different tectonics units, e.g. the Sveconorwegian, the Central and Northern Svecofennian and the Caledonian nappes, are each characterised by specific anisotropic signatures.