1 Methods and applications of shear wave splitting

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 n Introduction n Part 1: Splitlab A graphical interface for the splitting process n Part 2: Null criterion Synthetic test reveals characteristic differences of two splitting techniques n Part 3: Splitting Database Access splitting measurements publications online n 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 aligned cracks 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 Incoming SKS phase If initial polarisation coincides with a anisotropy axis, the shear wave is not split (Null case) Anisotropic layer Invert the splitting by grid-searching for combination of fast axis and delay time which best removes the splitting

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

8 European Anisotropy % velocity perturbation Tomography of Europe at 150 km depth (Debayle et al. , Nature, 2005) 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

Minimum Energy Rotation Correlation SKS 12 Diagnostic Viewer

13 Result. Viewer www. gm. univ-montp 2. fr/splitting Splitlab efficiently compares different techniques [Wüstefeld et al. , in press]

14 Part II Synthetic test Null Criterion

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

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

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

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

20 Example station ATD 330 earthquakes 9 start times 6 end times max = 162 3 filter sets } Barruol & Hofmann [1999] Automatically detected global minimum Φ = 48°; dt = 1. 59 sec Φ = 42°; dt = 1. 6 sec

21 Automated splitting n Possible with Split. Lab n Reduced processing time n Objective and repeatable n Uniform database

22 Part III Shear wave splitting database

23 Shear wave splitting database http: //www. gm. univ-montp 2. fr/splitting/DB

24 Shear wave splitting database http: //www. gm. univ-montp 2. fr/splitting/DB ECH 48. 216 7. 158 85 0. 88 Barruol, G. , Hoffman, R. Upper mantle anisotropy beneath the Geoscope stations J. Geophys. Res. 1999 104 10757 -10773 http: //www. gm. univ-montp 2. fr/PERSO/barruol/ Silver & Chan method

25 Shear wave splitting database http: //www. gm. univ-montp 2. fr/splitting/DB ECH 48. 216 7. 158 85 0. 88 Barruol, G. , Hoffman, R. Upper mantle anisotropy beneath the Geoscope stations J. Geophys. Res. 1999 104 10757 -10773 http: //www. gm. univ-montp 2. fr/PERSO/barruol/ Silver & Chan method

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

27 Comparison with surface waves Predicted splitting parameters

28 Coherence of predicted and observed splitting n Good global coherence n Splitting in western US occurs above 200 km depth n In Central Europe best coherence at 200 -350 km 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. 4 sec and 1. 1 sec Variable fast orientations, but similar within a block

32 Comparison with other datasets

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

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

35 Excursus: Magnetic structures and seismic anisotropy Magnetic structures reflect tectonic events. Rocks are magnetic up to a temperature of 580° (Currie Temperature) This temperature is generally reached at depths close to the moho The crustal contribution to splitting is presumeably small (<0. 2 sec) Parallelism between magnetic structures and fast orientations indicates that observed anisotropy is in the lithosphere

36 Polish-Lithuanian-Belarus Terrane NE 51 PUL TRTE NE 52 Fast orientations follow magnetic structures SUW NE 53 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 n Weak correlation with plate motion vectors n Variable fast orientations, but consistency within a tectonic block n Short scale variations across the borders of the blocks n Rather good correlation of (crustal) magnetic anomalies and (upper mantle) seismic anisotropy Anisotropy is frozen-in into the lithosphere n Stations in the West align with TTZ Mantle flow around the craton?

42 Conclusions n Splitlab: - User friendly, efficient Simultaneous comparison of methods n Null criterion - Detect Nulls and assign quality Allow for automatic splitting n Splitting database - Central and interactive depository of splitting publications Generally good correlation with surface waves n East European Craton - Weak anisotropy (delay times between 0. 4 - 1. 1 sec) 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? n n Lines: Comparisson with predicted splitting orientations [0° < misfit < 90°] Background: relative predicted splitting [0 < strength < 1]

45 Null Criterion Model Delay time: 0. 7 sec Fast axis comparison Delay time comparison

46 Null Criterion Model Delay time: 1. 3 sec Fast axis comparison Delay time comparison

47 Null Criterion Model Delay time: 2. 0 sec Fast axis comparison Delay time comparison

48 Shear-wave splitting Theory: The resulting radial and transverse components after anisotropic layer are u. R, 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 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 10º; 1. 1 sec Result of LVZ: Rotation-Correlation Minimum Energy 45 0 0 LVZ 90 45 Fast axis 90 -45 -90 -45 0 45 90 135 180 225 270 315 good splitting 360 fair splitting -90 0 90 135 weak 4 fair Null 2 1 good Null 360 3 2 180 225 270 315 4 3 delay time 45 1 0 0 45 90 135 180 225 270 315 Backazimuth 360 0 0 45 90 135 180 225 270 315 360 Backazimuth 44 events

51 Splitting projected to depth of CMB

54 Polish-Lithuanian-Belarus Terrane

55 East European Craton (after Wikipedia) n The East European craton is the core of the Baltica proto-plate and consists of three crustal regions/segments: Fennoscandia to the northwest, Volgo. Uralia to the east, and Sarmatia to the south. Fennoscandia includes the Baltic Shield (also referred to as the Fennoscandian Shield) and has a diversified accretionary Archaean and Early Proterozoic crust, while Sarmatia has an older Archaean crust. The Volgo-Uralia region has a thick sedimentary cover, however deep drillings have revealed mostly Archaean crust. There are two shields in the East European Craton: the Baltic/Fennoscandian shield and the Ukrainian shield. The Ukrainian Shield and the Voronezh Massif consist of 3. 2 -3. 8 Ga Archaean crust in the southwest and east, and 2. 3 -2. 1 Ga Early Proterozoic orogenic belts. n The intervening Late Palaeozoic Donbass Fold Belt, also known as part of the Pripyat-Dniepr-Donets aulacogen, transects Sarmatia, dividing it into the Ukrainian Shield and the Voronezh Massif.

56 The thick & cold EEC % velocity perturbation (fast) (slow) Surface wave tomography after Debayle et al [2005]

57 Excursus: Magnetic field and seismic anisotropy Depth of the 550°C isotherme (after Artemieva [2006])