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Methods and applications of shear wave splitting

An example of the East European Craton

Soutenance de Thèse

Andreas Wüstefeld27 Sept. 2007

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Outline

Introduction

Part 1: SplitlabA 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 CratonApplication to stations on the old EEC

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Geodynamics: study of deformation

[Illustration by Jose F. Vigil. USGS]

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Causes of seismic anisotropy

Horizontal layeringUpper and lower crust, transition zone, D ’’

Vertically aligned cracks Crust

Alignment of mineralsLower crust, upper mantle,inner core

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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

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Shear-wave splitting: the phenomenon

Inco

min

g S

KS

pha

se

Anisotropic layer

Invert the splitting by grid-searching for Invert the splitting by grid-searching for combination of combination of fast axisfast axis and and delay timedelay time

which best removes the splittingwhich best removes the splitting

If initial polarisation coincides with a If initial polarisation coincides with a anisotropy axis, the shear wave is anisotropy axis, the shear wave is

not split (not split (Null caseNull case))

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Shear-wave splitting:the techniques

Remove splitting:

3. Eigenvalue criteria: Searching for most linear particle motion

2. Rotation-Correlation: Searching for maximum correlation

1. Minimum Energy on Transverse: Remove transverse EnergyRadial

Transversal

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European Anisotropy

Is there mantle flow around the East European Craton?Is there mantle flow around the East European Craton?

How does the anisotropy continue beneath the Craton?How does the anisotropy continue beneath the Craton?

% v

elo

city

pe

rtu

rba

tion

Tomography of Europe at 150km depth (Debayle et al., Nature, 2005)

Splitting results of various authors

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Part I

Shear-wave splitting in Matlab

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Configuration

- A shear wave splitting environment in Matlab

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Seismogram Viewer

Select splitting window and filter

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Diagnostic Viewer

SK

S

Ro

tatio

n C

orr

elat

ion

Min

imu

m E

nerg

y

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ResultViewerwww.gm.univ-montp2.fr/splitting

Splitlab efficiently compares different

techniques

[Wüstefeld et al., in press]

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Part II

Synthetic test

Null Criterion

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Synthetic test

Comparison of two splitting techniques

-90 -45 0 45 90-90

-45

0

45

90

fast

axis

Rotation correlation method

-90 -45 0 45 90-90

-45

0

45

90Minimum energy method

-90 -45 0 45 900

1

2

3

4

dela

y t

ime

Backazimuth-90 -45 0 45 900

1

2

3

4

Backazimuth

Model parameters:

Fast axis: 0° Delay time: 1.3sec SNR: 15

Null Criterion

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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:

sin

cos

cossin

sincos

'

'

Q

Q

T

Q

T

Q

This also results in small delay time estimates

Null Criterion

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Synthetic test

Comparison of two splitting techniques

-90 -45 0 45 90-90

-45

0

45

90

fast

axis

Rotation correlation method

-90 -45 0 45 90-90

-45

0

45

90Minimum energy method

-90 -45 0 45 900

1

2

3

4

dela

y t

ime

Backazimuth-90 -45 0 45 900

1

2

3

4

Backazimuth

Model parameters:

Fast axis: 0° Delay time: 1.3sec SNR: 15

Null Criterion

Is this a common feature?

5 SNR between 3 and 30

7 delay times between 0 and 2 sec

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Null criterion

Null Criterion

|ΦSC - ΦRC| > 22.5º

dtSC/dtRC ≤ 0.3

NULL:

3185 measurements:

5 SNR between 3 and 30

7 delay times between 0 and 2 sec

[Wüstefeld & Bokelmann, BSSA, 2007]

The comparison of two techniques objectively and automatically The comparison of two techniques objectively and automatically - Detect NullsDetect Nulls- Assign a quality to the measurementAssign a quality to the measurement

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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)

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Example station ATD

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

}

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Automated splitting

Possible with SplitLab

Reduced processing time

Objective and repeatable

Uniform database

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Part III

Shear wave splitting database

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Shear wave splitting database

http://www.gm.univ-montp2.fr/splitting/DB

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Shear wave splitting database

http://www.gm.univ-montp2.fr/splitting/DB

48.216 7.158 85 0.88ECH

Barruol, G., Hoffman, R.Upper mantle anisotropy beneath the Geoscope stations

J. Geophys. Res.

1999

Silver & Chan method

10410757-10773

http://www.gm.univ-montp2.fr/PERSO/barruol/

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Shear wave splitting database

http://www.gm.univ-montp2.fr/splitting/DB

48.216 7.158 85 0.88ECH

Barruol, G., Hoffman, R.Upper mantle anisotropy beneath the Geoscope stations

J. Geophys. Res.

1999

Silver & Chan method

10410757-10773

http://www.gm.univ-montp2.fr/PERSO/barruol/

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2286 measurements

122 references

Global mean: 1sec

SKS database:

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Comparison with surface waves

Predicted splitting parameters

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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

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Part IV- The real world -

Shear wave splitting beneath the East European Craton

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The East European Craton

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Results16 stations analyzed

Delay times between 0.4 sec and 1.1 sec

Variable fast orientations, but similar within a block

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Comparison with other datasets

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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

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Polish-Lithuanian-Belarus Terrane

[after Bogdanova et al., 2006]

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Excursus:Magnetic structures and seismic anisotropy

Magnetic structures reflect tectonic events.

This temperature is generally reached at depths close to the moho

The crustal contribution tosplitting is presumeably small (<0.2sec)

Parallelism between magnetic structures and Parallelism between magnetic structures and fast orientations indicates that observedfast orientations indicates that observed

anisotropy is in the lithosphereanisotropy is in the lithosphere

Rocks are magnetic up to a temperatureof 580° (Currie Temperature)

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Polish-Lithuanian-Belarus Terrane

Fast orientations follow magnetic structures

Lithospheric anisotropyLithospheric anisotropy

Magnetic intensity anomaly

SUWNE53

NE52

TRTE

PULNE51

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Fennoscandia

Results in Fennoscandia are in good agreement with the SVEKALAPKO

experiment

Continous rotation of fast Continous rotation of fast orientations supports single-block orientations supports single-block

hypothesishypothesis [after Vecsey et al., 2007]

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Ural mountains

AKTK

ARU

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Ural mountains

ARU and AKTK show fast orientations ARU and AKTK show fast orientations perpendicular to trend of mountain chain. perpendicular to trend of mountain chain.

Distance to deformation front might Distance to deformation front might indicate out of reach for compressive indicate out of reach for compressive

deformation of orogeny. deformation of orogeny.

Anisotropy possibly related to Anisotropy possibly related to ancient subduction processesancient subduction processes

Magnetic intensity map

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Sarmatia

[modified after Thybo et al., 2003]Lateral erosion due to mantle Lateral erosion due to mantle

flow along western edge of the flow along western edge of the craton?craton?

No clear magnetic structures

Fast orientations in the west align with TTZ

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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 lithosphereAnisotropy is frozen-in into the lithosphere

Mantle flow around the craton? Mantle flow around the craton?

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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

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Thank you …Thank you …

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Can the depth of splitting be constrained?

Lines: Comparisson with predicted splitting orientations [0° < misfit < 90°] Background: relative predicted splitting [0 < strength < 1]

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Model Delay time: 0.7sec

-90 -45 0 45 900

2

4

SN

R =

3

-90 -45 0 45 900

2

4

dela

y tim

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-90 -45 0 45 900

2

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SN

R =

5

-90 -45 0 45 900

2

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dela

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SN

R =

10

-90 -45 0 45 900

2

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dela

y tim

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-90 -45 0 45 900

2

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SN

R =

20

Rotation correlation method-90 -45 0 45 900

2

4

dela

y tim

e

Minimum energy method

Fast axis comparison Delay time comparison

Null Criterion

-90 -45 0 45 90-90-45

04590

SN

R =

3

-90 -45 0 45 90-90-45

04590

Fas

taxi

s

-90 -45 0 45 90-90-45

04590

SN

R =

5

-90 -45 0 45 90-90-45

04590

Fas

taxi

s

-90 -45 0 45 90-90-45

04590

SN

R =

10

-90 -45 0 45 90-90-45

04590

Fas

taxi

s

-90 -45 0 45 90-90-45

04590

SN

R =

20

Rotation correlation method-90 -45 0 45 90

-90-45

04590

Fas

taxi

s

Minimum energy method

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Model Delay time: 1.3sec

-90 -45 0 45 90-90-45

04590

SN

R =

3

-90 -45 0 45 90-90-45

04590

Fas

taxi

s

-90 -45 0 45 90-90-45

04590

SN

R =

5

-90 -45 0 45 90-90-45

04590

Fas

taxi

s

-90 -45 0 45 90-90-45

04590

SN

R =

10

-90 -45 0 45 90-90-45

04590

Fas

taxi

s

-90 -45 0 45 90-90-45

04590

SN

R =

20

Rotation correlation method-90 -45 0 45 90

-90-45

04590

Fas

taxi

s

Minimum energy method

-90 -45 0 45 900

2

4

SN

R =

3

-90 -45 0 45 900

2

4

dela

y tim

e

-90 -45 0 45 900

2

4

SN

R =

5

-90 -45 0 45 900

2

4

dela

y tim

e

-90 -45 0 45 900

2

4

SN

R =

10

-90 -45 0 45 900

2

4

dela

y tim

e

-90 -45 0 45 900

2

4S

NR

= 2

0

Rotation correlation method-90 -45 0 45 900

2

4

dela

y tim

e

Minimum energy method

Fast axis comparison Delay time comparison

Null Criterion

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Model Delay time: 2.0sec

-90 -45 0 45 900

2

4

SN

R =

3

-90 -45 0 45 900

2

4

dela

y tim

e

-90 -45 0 45 900

2

4

SN

R =

5

-90 -45 0 45 900

2

4

dela

y tim

e

-90 -45 0 45 900

2

4

SN

R =

10

-90 -45 0 45 900

2

4

dela

y tim

e

-90 -45 0 45 900

2

4

SN

R =

20

Rotation correlation method-90 -45 0 45 900

2

4

dela

y tim

e

Minimum energy method

-90 -45 0 45 90-90-45

04590

SN

R =

3

-90 -45 0 45 90-90-45

04590

Fas

taxi

s

-90 -45 0 45 90-90-45

04590

SN

R =

5

-90 -45 0 45 90-90-45

04590

Fas

taxi

s

-90 -45 0 45 90-90-45

04590

SN

R =

10

-90 -45 0 45 90-90-45

04590

Fas

taxi

s

-90 -45 0 45 90-90-45

04590

SN

R =

20

Rotation correlation method-90 -45 0 45 90

-90-45

04590

Fas

taxi

s

Minimum energy method

Fast axis comparison Delay time comparison

Null Criterion

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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

)]2/()2/([2sin)(~sin)2/(cos)2/()(~

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ttuttutu

ttuttutu

RRlTransversa

RRRadial

TransverseRadialjidtttututC jiij ,,;),(~),(~),(

uR,T = initial radial and transverse particle motion

lTransversaRadialu ,~

= particle motion after splitting

α = angle between fast direction and backazimuth

δt = delay time between fast and slow component

Search for combination of Search for combination of fast axisfast axis and and delay timedelay time which gives most singular Covariance matrix to which gives most singular Covariance matrix to

remove the splitting of the shear wave remove the splitting of the shear wave

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Data example LVZ

Null Criterion

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0 45 90 135 180 225 270 315 360-90

-45

0

45

90

LVZ

Minimum Energy

0 45 90 135 180 225 270 315 360-90

-45

0

45

90

Fas

t axi

s

Rotation-Correlation

Backazimuth

dela

y tim

e

0 45 90 135 180 225 270 315 3600

1

2

3

4

Backazimuth

0 45 90 135 180 225 270 315 3600

1

2

3

4

good splitting fair splitting weak good Null fair Null

Result of LVZ: 10º; 1.1sec

44 events

Null Criterion

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Splitting projected to depth of CMB

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Polish-Lithuanian-Belarus Terrane

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East European Craton (after Wikipedia)

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.

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.

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The thick & cold EEC

% velocity perturbation

(fast)

(slow)

Surface wave tomography after Debayle et al [2005]

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Excursus:Magnetic field and seismic anisotropy

Depth of the 550°C isotherme(after Artemieva [2006])