Methods and applications of shear wave splitting An example of the East European Craton

1. Methods and applications of shear wave splitting An example of the East European Craton Soutenance de Thèse Andreas Wüstefeld 27 Sept. 2007

2. Outline • Introduction • Part 1: Splitlab A graphical interface for the splitting process • Part 2: Null criterion Synthetic test reveals characteristic differences of two splitting techniques • Part 3: Splitting Database Access splitting measurements publications online • Part 4: The East European Craton Application to stations on the old EEC

3. Geodynamics: study of deformation [Illustration by Jose F. Vigil. USGS]

4. Causes of seismic anisotropy Horizontal layering Upper and lower crust, transition zone, D ’’ Vertically alignedcracks Crust Alignment of minerals Lower crust, upper mantle, inner core

5. What causes mineral alignment? “Vertically coherent deformation” The last tectonic deformation is frozen-in into the lithosphere “Simple asthenospheric flow” Mainly present day mantle flow causes anisotropy

6. Shear-wave splitting: the phenomenon If initial polarisation coincides with a anisotropy axis, the shear wave is not split (Null case) Anisotropic layer Incoming SKS phase Invert the splitting by grid-searching for combination of fast axis and delay time which best removes the splitting

7. 1. Minimum Energy on Transverse:Remove transverse Energy 2. Rotation-Correlation:Searching for maximum correlation 3. Eigenvalue criteria:Searching for most linear particle motion Shear-wave splitting:the techniques Remove splitting: Radial Transversal

8. European Anisotropy Tomography of Europe at 150km depth (Debayle et al., Nature, 2005) % velocity perturbation Splitting results of various authors Is there mantle flow around the East European Craton? How does the anisotropy continue beneath the Craton?

9. Part I Shear-wave splitting in Matlab

10. Configuration - A shear wave splitting environment in Matlab

11. Seismogram Viewer Select splitting window and filter

12. Diagnostic Viewer SKS Rotation Correlation Minimum Energy

13. ResultViewer www.gm.univ-montp2.fr/splitting Splitlab efficiently compares different techniques [Wüstefeld et al., in press]

14. Part II Synthetic test Null Criterion

15. Null Criterion Rotation correlation method Minimum energy method 90 90 45 45 0 0 fast axis -45 -45 -90 -90 -90 -45 0 45 90 -90 -45 0 45 90 4 4 3 3 delay time 2 2 1 1 0 0 -90 -45 0 45 90 -90 -45 0 45 90 Backazimuth Backazimuth Synthetic test • Comparison of two splitting techniques Model parameters: Fast axis: 0° Delay time: 1.3sec SNR: 15

16. Null Criterion Why is there a 45° difference? • The Rotation-Correlations seeks for maximum wave-form similarity • If the initial energy on Transverse is small (Null case), the maximum correlation is found for a test system 45° rotated: • This also results in small delay time estimates

17. Null Criterion Rotation correlation method Minimum energy method 90 90 45 45 0 0 fast axis -45 -45 -90 -90 -90 -45 0 45 90 -90 -45 0 45 90 4 4 3 3 delay time 2 2 1 1 0 0 -90 -45 0 45 90 -90 -45 0 45 90 Backazimuth Backazimuth Synthetic test • Comparison of two splitting techniques Model parameters: Fast axis: 0° Delay time: 1.3sec SNR: 15 Is this a common feature? 5 SNR between 3 and 30 7 delay times between 0 and 2 sec

18. Null Criterion Null criterion 3185 measurements: 5 SNR between 3 and 30 7 delay times between 0 and 2 sec NULL: |ΦSC - ΦRC| > 22.5º dtSC/dtRC ≤ 0.3 • The comparison of two techniques objectively and automatically • Detect Nulls • Assign a quality to the measurement [Wüstefeld & Bokelmann, BSSA, 2007]

19. Automated splitting? • Perform splitting to a set of test windows around theoretical SKS arrival => No manual phase picking needed! • Skip Null measurements • Stack (non-normalized) energy map [Wolfe & Silver, 1998] • Repeat for different filters! • Determine global energy minimum (of each event)

20. Barruol & Hofmann [1999] Φ = 48°; dt = 1.59sec Automatically detected global minimum Φ = 42°; dt = 1.6sec 330 earthquakes 9 start times 6 end times max = 162 3 filter sets Example station ATD }

21. Automated splitting • Possible with SplitLab • Reduced processing time • Objective and repeatable • Uniform database

22. Part III Shear wave splitting database

23. Shear wave splitting database http://www.gm.univ-montp2.fr/splitting/DB

24. Barruol, G., Hoffman, R. Upper mantle anisotropy beneath the Geoscope stations J. Geophys. Res. 1999 104 10757-10773 http://www.gm.univ-montp2.fr/PERSO/barruol/ Silver & Chan method Shear wave splitting database http://www.gm.univ-montp2.fr/splitting/DB ECH 48.216 7.158 85 0.88

25. Barruol, G., Hoffman, R. Upper mantle anisotropy beneath the Geoscope stations J. Geophys. Res. 1999 104 10757-10773 http://www.gm.univ-montp2.fr/PERSO/barruol/ Silver & Chan method Shear wave splitting database http://www.gm.univ-montp2.fr/splitting/DB ECH 48.216 7.158 85 0.88

26. Global mean: 1sec • 2286 measurements • 122 references SKS database:

27. Comparison with surface waves Predicted splitting parameters

28. Coherence of predicted and observed splitting • Good global coherence • Splitting in western US occurs above 200km depth • In Central Europe best coherence at 200-350km km depth interval

29. Part IV- The real world - Shear wave splitting beneath the East European Craton

30. The East European Craton

31. Results 16 stations analyzed Delay times between 0.4sec and 1.1 sec Variable fast orientations, but similar within a block

32. Comparison with other datasets

33. Comparison with other datasets • Weak correlation with plate motion vectors • Anisotropy not related to present day asthenospheric processes • Regionally good correlation with predicted splitting • Short scale variations, but consistent within a block • Anisotropy within the lithospheric blocks

34. Polish-Lithuanian-Belarus Terrane [after Bogdanova et al., 2006]

35. This temperature is generally reached at depths close to the moho Rocks are magnetic up to a temperature of 580° (Currie Temperature) The crustal contribution to splitting is presumeably small (<0.2sec) Parallelism between magnetic structures and fast orientations indicates that observed anisotropy is in the lithosphere Excursus:Magnetic structures and seismic anisotropy Magnetic structures reflect tectonic events.

36. TRTE NE52 NE53 SUW Polish-Lithuanian-Belarus Terrane NE51 PUL Fast orientations follow magnetic structures Lithospheric anisotropy Magnetic intensity anomaly

37. Fennoscandia Results in Fennoscandia are in good agreement with the SVEKALAPKO experiment Continous rotation of fast orientations supports single-block hypothesis [after Vecsey et al., 2007]

38. Ural mountains ARU AKTK

39. Ural mountains Magnetic intensity map ARU and AKTK show fast orientations perpendicular to trend of mountain chain. Distance to deformation front might indicate out of reach for compressive deformation of orogeny. Anisotropy possibly related to ancient subduction processes

40. Sarmatia No clear magnetic structures Fast orientations in the west align with TTZ Lateral erosion due to mantle flow along western edge of the craton? [modified after Thybo et al., 2003]

41. The EEC shows • Weak correlation with plate motion vectors • Variable fast orientations, but consistency within a tectonic block • Short scale variations across the borders of the blocks • Rather good correlation of (crustal) magnetic anomalies and (upper mantle) seismic anisotropy • Stations in the West align with TTZ Anisotropy is frozen-in into the lithosphere Mantle flow around the craton?

42. Conclusions • Splitlab: • User friendly, efficient • Simultaneous comparison of methods • Null criterion • Detect Nulls and assign quality • Allow for automatic splitting • Splitting database • Central and interactive depository of splitting publications • Generally good correlation with surface waves • East European Craton • Weak anisotropy (delay times between 0.4 - 1.1sec) • Comparison of splitting with magnetic structures possible • Lithospheric frozen-in anisotropy • Possible mantle flow around the craton

43. Thank you …

44. Can the depth of splitting be constrained? • Lines: Comparisson with predicted splitting orientations [0° < misfit < 90°] • Background: relative predicted splitting [0 < strength < 1]

45. Null Criterion Model Delay time: 0.7sec Fast axis comparison Delay time comparison

46. Null Criterion Model Delay time: 1.3sec Fast axis comparison Delay time comparison

47. Null Criterion Model Delay time: 2.0sec Fast axis comparison Delay time comparison

48. uR,T = initial radial and transverse particle motion = particle motion after splitting α = angle between fast direction and backazimuth δt = delay time between fast and slow component Shear-wave splitting Theory: The resulting radial and transverse components after anisotropic layer are The splitting can be inverted by a search for a singular covariance matrix Search for combination of fast axis and delay time which gives most singular Covariance matrix to remove the splitting of the shear wave

49. Null Criterion Data example LVZ

50. Null Criterion Rotation-Correlation Minimum Energy 90 90 45 45 0 0 LVZ Fast axis -45 -45 -90 -90 0 45 90 135 180 225 270 315 360 0 45 90 135 180 225 270 315 360 good splitting fair splitting weak good Null fair Null 4 4 3 3 2 2 delay time 1 1 0 0 0 45 90 135 180 225 270 315 360 0 45 90 135 180 225 270 315 360 Backazimuth Backazimuth Result of LVZ: 10º; 1.1sec 44 events