New developments in the study of seismic anisotropy from shear-wave splitting analyses

1. New developments in the study of seismic anisotropy from shear-wave splitting analyses G. Rümpker Goethe-University Frankfurt AlpArray – swath-meeting Potsdam, 18.7.2013

2. Alps – previousstudiesandinterpretations • SKS splitting < seismicanisotropy> mantleflow • hypothesis: • fast polarization || mantleflow • thepossibleroleofcrustalanisotropy • - howtodiscriminatebetweencrustalandmantleanisotropy? • recentobservations: • - fast polarization|| topography • examples/casestudies: South America (Andes), Iran • - anisotropicwaveformmodeling • - receiver-functionsplitting

3. Seismic Anisotropy - SKS-Splitting • “2 splitting parameters“ • : polarisation direction of fast shear wave  LP orientation of olivine or magmatic lenses • t: delay time  extent/strength of anisotropy

4. datacoverage – whatcanweexpect? Walther 2012: 1979-2009: 6,320 potential SKS events (M> 5.5, D>90° ) 240 useful SKS events – 8 per year !

5. Global shear-wave splitting database http://www.gm.univ-montp2.fr/splitting/DB/ • Wüstefeld, A.; Bokelmann, G. H. R.; Barruol, G.; Montagner, J.-P. (2009)

6. Latestsummary: Bianchi & Bokelmann

7. Alps: shear-wave splitting results • Barruol, G., Deschamps, A. and Coutant, O. (2004), Tectonophysics • Kummerow, J.; Kind, R. (2006), Tectonophysics • Barruol, G., Bonnin, M., Pedersen, H., Bokelmann, G.H.R. & Tiberi, C. (2011), EPSL

8. Kummerow & Kind (2006) • Delay times: 0.8 s - 2.0 s • Fast-axis directions: 60° - 70° N (coincide with the trend of the Eastern Alps) • Interpretation: Orogen-parallel flow in the upper mantle; escape movement triggered by Adriatic indenter DX=6 km

9. Barruol et al. (2004) • Delay times: 1.0 - 1.5 s, 2.0 s • Fast-axis directions: WNW–ESE in the Nice area and progressively rotating to NW–SE and to NS for stations located further North • Interpretation: large asthenospheric flow induced by the rotation of the Corsica–Sardinia lithospheric block and the retreat of the Apenninicslab

10. Barruol et al. (2011) • Delay times: maximum anisotropy magnitude is not located beneath the internal zones of the belt but instead beneath external units. • Fast-axis directions: characterized by fast split directions that closely follow the trend of the belt. • Interpretation: All suggests that the anisotropy is likely dominated by sublithospheric mantle deformation. They propose that the observed anisotropy pattern can be explained by recent or active mantle flow around the Eurasian slab.

11. Barruol et al. (2011)

12. Russo & Silver (Science, 1994) Trench-Parallel Flow Beneath the Nazca Plate from Seismic Anisotropy

13. Shear-wavesplittingobervations (Wölbern et al., EPSL submitted) ReFuCa project DX=20 km (!)

14. Localshear-wavesplittingobervations andshear-zones

15. Waveformmodelling comparsionofsyntheticandobservedwaveformeffects • 2D Finite-differencemethod • anisotropicelastictensor • finite-frequencyeffects

18. Receiver functions – convertedphases

19. Ps – splitting analysis

20. Multiple layers of anisotropy

21. Ps – splitting - crust

22. Preliminaryconclusions • Alps: currentanisotropy/mantle-flowmodelsarebased on • insufficientdepthresolution • simplifiedassumptions on therelationshipbetweenflowand fast directions • New toolstodiscriminatebetweencrustalandmantleanisotropyareavailable (Waveformmodeling, Ps-splitting) • Andes: Waveformmodellingprovidesevidenceforsignificantcontributionofcrustalanisotropyto SKS splittingobservations • The relationbetweenanisotropyandmantleflowcanbeclarifiedbygeodynamicmodeling(e.g. Kaminski & Ribe 2002)