PEAT8002 - SEISMOLOGYLecture 17: Seismic Tomography II

Nick Rawlinson

Research School of Earth SciencesAustralian National University

Seismic tomography IIIntroduction

In this lecture, we will examine several differentapplications of seismic tomography, including localearthquake, teleseismic and wide-anglereflection/refraction tomography.Most of these applications utilize seismic traveltime data,but we will also include one example of surface waveforminversion.Recently, there have been significant advances made inthe field of full waveform tomography, which involve the useof sophisticated codes to directly solve the elastic waveequation. While these techniques show much promise,further development is required if they are to supercedethe more traditional approaches discussed here.

Seismic tomography IILocal earthquake tomography

Local earthquake tomography (LET) is a common tool forimaging subsurface structure in seismically active areas.Apart from the source-receiver geometry, one of thedistinguishing features of LET compared to the other typesof tomography discussed here, is the need to relocateearthquake hypocenters (spatial location and origin time)as part of the image reconstruction.This is because accurate hypocenter location requires anaccurate knowledge of the velocity structure in the regionoccupied by the earthquakes and the recorders whosedata are used in the location process.The following example shows a series of cross-sectionsthrough a 3-D P-wave model of a region in the southerncentral Andes derived using LET.

Seismic tomography IILocal earthquake tomography

6.5

8

8.5

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DE

PT

H [k

m]

70˚W 69˚W 68˚W 67˚W

W E

22.25˚S

4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0

Vp [km/s]

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88

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150

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PT

H [k

m]

70˚W 69˚W 68˚W 67˚W

W E

22.75˚S

4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0

Vp [km/s]

6

6

6.5

6.57

7

7.5

8

9

0

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100

150

DE

PT

H [k

m]

70˚W 69˚W 68˚W 67˚W

W E

23.25˚S

4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0

Vp [km/s]

16,488 P-wave arrivalsfrom 764 events have beenused to constrain this 3-Dmodel, which is describedusing 2496 velocity nodes.It is also quite commonpractice in LET studies toinvert for Vp/Vs.This example has beentaken from the JGR paperof Graeber and Asch(1999).

Seismic tomography IILocal earthquake tomography

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50

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150

DE

PT

H [k

m]

70˚W 69˚W 68˚W 67˚W

W E

23.00˚S

-0.28 -0.21 -0.14 -0.07 0.00 0.07 0.14 0.21 0.28

Vp-PERTURBATION [km/s]

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50

100

150

DE

PT

H [k

m]

70˚W 69˚W 68˚W 67˚W

W E

22.75˚S

-0.28 -0.21 -0.14 -0.07 0.00 0.07 0.14 0.21 0.28

Vp-PERTURBATION [km/s]

0

50

100

150

DE

PT

H [k

m]

70˚W 69˚W 68˚W 67˚W

W E

23.25˚S

-0.28 -0.21 -0.14 -0.07 0.00 0.07 0.14 0.21 0.28

Vp-PERTURBATION [km/s]

(a)

(b)

Checkerboard resolutiontests for the model shownon the previous page.The top plots shows theinput checkerboard, andthe bottom two plots showthe recovered model at twodifferent latitudes.From the JGR paper ofGraeber and Asch (1999).

Seismic tomography IITeleseismic tomography

Like local earthquake tomography, teleseismic tomographyhas been used extensively to map the structure of the crustand lithosphere.Studies are often carried out on a variety of scales rangingfrom 10s of km to 100s of km.Usually, the horizontal extent of the receiver array and thesource distribution determines the depth to which featuresmay be resolved.Most teleseismic studies are carried out in 3-D, partlybecause it is difficult to line up an array of recorders onroughly the same great circle as a set of teleseismicearthquakes with good angular coverage.

Seismic tomography IIThe Tasmanian lithosphere

The TIGGER experiment72 broadband and short period recorders deployed in northernTasmania in 2002 to record distant (teleseismic) earthquakes.

145˚

145˚

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148˚

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-41˚ -41˚

0 100 200

km

A1 B1

A2

E1

E2 F2

O2

L3

C1 D1

F1

G1 H1

I1 K1 L1

M1 N1 O1 P1 Q1 B2

C2 D2

G2 H2 I2 K2 L2 M2

N2 P2

Q2

A3

B3

C3

D3 E3

F3 G3 H3

I3 J3 K3

M3 N3 O3

P3 Q3

A4

B4

C4 D4

E4 F4

H4 I4 J4 K4 L4

M4 N4

O4 P4 Q4

K5 L5 M5 N5

O5 P5

Q5

Seismic tomography IIThe Tasmanian lithosphere

Teleseismic source distribution

Poor coverage from southand west

6,520 paths

P, PP, PcP, ScP, PKiKP

110 phases

101 teleseismic events

Good coverage from northand east

Seismic tomography IIThe Tasmanian lithosphere

Relative arrival time residualsT

rave

ltim

e re

sidu

al

Teleseismic wavefront

Fast lithosphere

Slow lithosphere

ak135 reference

Seismic tomography IIThe Tasmanian lithosphere

Relative arrival time residual patterns

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148˚

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-41˚ -41˚

0 100 200

km

-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 0.50

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146˚

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148˚

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-41˚ -41˚

0 100 200

km

-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 0.50

Mariana Islands (P) South Sandwich Islands (P)

Residual (s) Residual (s)

Seismic tomography IIThe Tasmanian lithosphere

Tomographic scheme

δmn mn+1= mn +

Model parameterizationCubic B−splines

Inversion methodSubspace inversion

Traveltime predictionFast marching method

δm = −A[AT(G TCd−1G +εCm

−1+ηDTD)A]−1ATγ

Iterative non−linear inversion scheme

velocity (km/s)

Seismic tomography IIThe Tasmanian lithosphere

Synthetic recovery tests

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144 145 146 147 148 149

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144 145 146 147 148 149

144 145 146 147 148 149

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δvp (m/s)

Longitude ( E)

δvp (m/s)

Latitude ( S)

δvp (m/s)

Longitude ( E)

δvp (m/s)

Latitude ( S)

Dep

th (

km)

Dep

th (

km)

Input Input

Recovered Recovered

Dep

th (

km)

Dep

th (

km)

41.4 S 147.6 E

41.4 S 147.6 E

Seismic tomography IIThe Tasmanian lithosphere

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0

144 145 146 147 148 149

144 145 146 147 148 149

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149˚

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-41˚ -41˚

-40˚ -40˚δvp (m/s)

Longitude ( E)

Dep

th (

km)

41.4 S

15 km

Seismic tomography IIThe Tasmanian lithosphere

KingIsland

FlindersIsland

Hobart

Launceston

Middle Cambrian sedimentaryand felsic rocks

Neoproterozoic sedimentaryand volcanic rocks

Cambrian medium to highgrade metamorphic complexes

Cambrian ophiolite Devonian granites

East Tasmania Terrane

West Tasmania Terrane

Inferred structural boundary

WestTasmaniaTerrane

TyennanBlock

Mt ReadVolcanic

Arc

RockyCapeBlock

ArthurLineament

EastTasmaniaTerrane

System

Fracture

Tamar

Bass Strait

100 km

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Elevatedcrustalvelocity

δvp (m/s)

Longitude ( E)

Elevatedcrustal

velocityZone of

relatively lowvelocity

Easterly dipping

structure

Smearing

SmearingDep

th (

km)

TFS

41.4 S

Proposed TFS does notcorrespond to wavespeedtransition zone.Rawlinson et al. (2006),JGR, 111.

Seismic tomography IIWide-angle tomography

Wide-angle tomography exploits refraction and wide-anglereflection data in order to constrain variations in seismicstructure.

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−40

0

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−40

0

138 139 140 141 142

138 139 140 141 142

Seismic tomography IIWide-angle tomography

The example below shows the result of an inversion ofwide-angle data for the structure of the Tasmanian Moho.

TFS

TFS

a b

AL

AL

Seismic tomography IICombined tomography

It is possible to combine multiple datasets in asimultaneous inversion. The example below shows theresult of combining the Tasmanian teleseismic andwide-angle datasets.

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0

144 145 146 147 148 149D

epth

(km

)Longitude ( )

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0

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Dep

th (

km)

Longitude ( )

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120

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40

0

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Dep

th (

km)

Longitude ( )

−500 −250 0 250 500

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146˚

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148˚

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−40˚ −40˚

−500 −250 0 250 500

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146˚

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148˚

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−41˚ −41˚

−40˚ −40˚

24 26 28 30 32 34

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146˚

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148˚

148˚

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−41˚ −41˚

−40˚ −40˚

−8.0% 8.0% −6.3% 6.3%

TFSSmearing SmearingTFS SmearingSmearingTFSSmearing Smearing

(a) (b) (c)

(d)

??

??

??

?? ?

Smearing

Tamar River

??

?

?

??

Smearing

δVp (m/s)

15 km

δVp (m/s)

41.6 S41.4 S41.2 S

Moho depth (km)

52 km

MRV

Seismic tomography IISurface wave tomography

-8 -6 -4 -2 0 2 4 6 8Perturbation (%)

30˚S

20˚S

10˚S

30˚S

20˚S

10˚S

110E 130E 150E

110E 130E 150E

20S

30S

40S

20S

30S

40S

Reference model: ak135 (4.51km/s)

Reference model: average vel. (4.38km/s)

Long period surface wavescan be inverted for 3-Dseismic structure.In this example, waveforminversion has been used toconstruct 1-D path-averagemodels of Rayleigh-wavephase velocity.Seismic tomography isthen applied to produce2-D horizontal velocityslices at different depths.

Seismic tomography IIAmbient noise tomography

It has only recently been shown that it is possible to extractinformation from the ambient seismic noise-field by longterm cross-correlation of waveforms recorded at twoseparate locations.It turns out that the cross-correlation produces an estimateof the Green’s function between two points; that is, thesignal that would arrive at one point if the source waveformwere a delta function located at the other point.A number of papers have been published recently thathave inverted the group traveltimes of short periodRayleigh waves for crustal velocity structure.One advantage of this technique is that path coverage iscontrolled largely by the placement of receivers.

Seismic tomography IIAmbient noise tomography

The plot below shows Rayleigh wave Green’s functionsextracted from the cross-correlation of one month of datafrom a short period seismic array in southern NSW.

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Time (s)

Sta

tion

no.

Sta

tion

sepa

ratio

n (k

m)

91

96

107

140

153

159

244

299

333

347

392

468

480

126

157

178

223

49

58

Seismic tomography IIAmbient noise tomography

Tomographic imaging results using group traveltimes ofRayleigh wave Green’s functions extracted from ambientseismic noise recorded in Australia

2.0

2.4

2.8

3.2

3.6

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−10˚ −10˚

Rayleigh w

ave group velocity (km/s)

0.2 Hz