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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
0
50
100
150
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]
6
6.5
6.5 6.5
7
7
7
7.5
88
0
50
100
150
DE
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
50
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
0
50
100
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]
0
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˚
146˚
146˚
147˚
147˚
148˚
148˚
-42˚ -42˚
-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
145˚
145˚
146˚
146˚
147˚
147˚
148˚
148˚
-42˚ -42˚
-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
145˚
145˚
146˚
146˚
147˚
147˚
148˚
148˚
-42˚ -42˚
-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
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
144 145 146 147 148 149
144 145 146 147 148 149
-300 -200 -100 0 100 200 300
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
40 41 42
40 41 42
-300 -200 -100 0 100 200 300
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
144 145 146 147 148 149
144 145 146 147 148 149
-300 -200 -100 0 100 200 300
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
40 41 42
40 41 42
-300 -200 -100 0 100 200 300
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
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
144 145 146 147 148 149
144 145 146 147 148 149
-300 -200 -100 0 100 200 300
144˚
144˚
145˚
145˚
146˚
146˚
147˚
147˚
148˚
148˚
149˚
149˚
-42˚ -42˚
-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
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
144 145 146 147 148 149
144 145 146 147 148 149
-300 -200 -100 0 100 200 300
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.
−80
−40
0
−80
−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.
160
120
80
40
0
144 145 146 147 148 149D
epth
(km
)Longitude ( )
160
120
80
40
0
144 145 146 147 148 149
Dep
th (
km)
Longitude ( )
160
120
80
40
0
144 145 146 147 148 149
Dep
th (
km)
Longitude ( )
−500 −250 0 250 500
144˚
144˚
146˚
146˚
148˚
148˚
−42˚ −42˚
−41˚ −41˚
−40˚ −40˚
−500 −250 0 250 500
144˚
144˚
146˚
146˚
148˚
148˚
−42˚ −42˚
−41˚ −41˚
−40˚ −40˚
24 26 28 30 32 34
144˚
144˚
146˚
146˚
148˚
148˚
−42˚ −42˚
−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.
0
5
10
15
20
−200 −150 −100 −50 0 50 100 150 200
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
110˚
110˚
120˚
120˚
130˚
130˚
140˚
140˚
150˚
150˚
160˚
160˚
−40˚ −40˚
−30˚ −30˚
−20˚ −20˚
−10˚ −10˚
Rayleigh w
ave group velocity (km/s)
0.2 Hz