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Lateral Variations in the Scattering and Viscoelastic Properties of the Inner CoreVernon F. Cormier1 and Anastasia Stroujkova2
1Physics Department, University of Connecticut, Storrs, CT 06269-3046 [email protected] Geophysical Corporation, Lexington, MA 02420 [email protected]
(a) Contours thickness of anomalous lower velocity layer in the uppermost inner core determined in the study by Stroujkova and Cormier [4].
(b) excitation of backscattered PKiKP coda from heterogeneity in the uppermost inner core determined in the study by Leyton and Koper [5.
(c) summary of lateral variations in attenuation and P velocity in the equatorial region of the inner core determined in the study by Yu and Wen [6].
INNER CORE EQUATORIAL STRUCTURAL VARIATIONS
Processed PKiKP coda with numbered events keyed to the map on the right.Note changes in onset and duration of the coda as a function of the direction of the ray with respect to the earth axis
AZIMUTHAL VARIATIONS IN PKiKP CODA BENEATH PACIFIC
CONCLUSIONS
1. The equatorial region of the upper inner core is characterized by lateral variations in the scattering and attenuation of elastic waves. This lateral variation may be correlated with lateral variations in the solidificaiton fabric of the inner core and flow in the liquid oter core.2. Long PKiKP coda is observed almost exclusively in the area of the equatorial and northern pacific [5]. These codas are characterized by long duration (hundreds of seconds in some cases) and unusually high frequencies (1-5 Hz), possibly suggesting low intrinsic attenuation in the upper layer of the inner core in this area.3. The coda of PKiKP in the equatorial Pacific exhibits azimuthal variations in coda length, with equatorial paths having a longer duration high frequency coda compared to polar paths.4. This azimuthal variation in PKiKP coda may be explained by longer time lengths of travel in a region of more intense scattering in the equatorial Pacific or by an anisotropic distribution of heterogeneity scale lengths in the upper inner core.
REFERENCES[1] Cormier, V.F., Earth Planet. Sci. Lett., 258, (2007) 442-453, doi: 10.1016/j.epsl.2007.04.003.[2] Shearer, P.M. and P.S Earle, Geophys. J. Int., 158, (2004), 1103-1117.[3] Cormier, V.F., and X. Li, J. Geophys. Res. 107(B12) (2002), doi: 10.1029/2002JB1796[4] Stroujkova, A., and V.F. Cormier, , J. Geophys. Res., 109(B10) (2004), doi: 10.129/2004JB002976.[5] Leyton, F.., and K.D. Koper, J. Geophys. Res., 112, (B05317) (2007), doi:10.1029/2006JB004370.[6] Yu, W., and L. Wen, Earth Planet. Sci, Lett., 245 (2006), 581-594.
ACKNOWLEDGEMENTS: National Science Foundation Grant EAR 02-29586 supported this work.
AbstractRecent studies [5] find unusual high-frequency coda following the PKiKP phase, reflected from the inner core boundary in the area of the Central Pacific and observed at teleseismic distances between 50∞ to 90∞ range. The majority of the observations of such codas were made at short-period teleseismic arrays in the USA, Eastern Asia and Australia. An array analysis of the coda wavetrains shows the apparent horizontal velocity of the coda to be consistent with scattering from the uppermost inner core. The PKiKP codas have different durations and frequency content depending on the location and the azimuth of the ray reflection point on the inner core boundary, as well as the distance between source and receiver. Radiative transport modeling of PKiKP coda shapes in the 1-4 Hz band is combined with pseudospectral modeling of PKIKP pulses in the 0.2 to 1 Hz band to assess and separate the effects of scattering by small-scale heterogeneity from the effects of viscoelasticity in the uppermost inner core. Observed lateral variations in elastic attenuation and scattering in the uppermost inner core may record lateral variations in the solidification process of the inner core and lateral variations in fluid velocities of the outer core near the inner core boundary.
WAVEFIELD MODELING AND STRUCTURAL SENSITIVITY
0�5� 10�
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-0.10 -0.05 -0.00 0.05 0.10
∆VP/VP
km/s
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∆VP/VP
km/s
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∆VP/VP
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DIS
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(deg
)
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T (sec)
pPcS
p400
p
PcPP
cP
PKiK
P
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T (s
ec)
0 30 60 90 120 150 180D(deg)
Left: Possible texture in the equatorial eastern hemisphere to explain isotropic velocities and high isotropic attenuation of transmitted PKIKP (120o to 140o) and lack of backscattered coda of PKiKP. Middle: possible dominant texture in the equatorial western hemisphere to explain more pronounced equatorial versus polar anisotropy in velocity and attenuation of transmitted PKIKP (120o to 140o) and weaker backscattered coda of PKiKP. Right: possible texture in certain regions of the western hemisphere to explain isotropic velocities, weak isotropic attenuation of transmitted PKIKP (120o to 140o), and high amplitudes of backscattered coda of PKiKP.
TEXTURAL MODELS TO EXPLAIN AZIMUTHAL VARIATIONS
Longitude
Latit
ude N-S
Dep
th
Upwelling Outer Core Flow(Driven by Compositional Convection at ICB)
Figure 7
Slow + High Q
Fast + Low Q
Fast
+ Lo
w Q
E-WWeak Backscattering
Longitude E-W
Latitutd
e N-S
Dep
th
Tangential Outer Core Flow at ICB (Driven by Compositional Convection)
Figure 8
Fast
+ Lo
w Q
Slow + High Q
Slow + High Q
Weak Backscattering
Longitude
Latit
ude N-S
Dep
th
Externally Driven Outer Core Downwelling(Tangential/no preferred direction at ICB)
Figure 9
Fast + Low Q
Slow + High Q
Slow
+ H
igh Q
E-W
Strong Backscattering
2-D pseudospectral modeling of PKiKP and its coda in textures of the uppermost inner core [1].
Radiative transport modeling of coda energy envelopes [2] in a model having small scale heterogeity in the crust, mantle and inner core.
Array processing isolates scattered high frequency coda of PKiKP, removing noise and contaiminating phases arriving at different azimuths and slownesses.This example shows that the PKiKP coda has higher frequencies than other phases, and sometimes can be only observed after high-pass filtering.
ARRAY DATA
1030 1040 1050 1060 1070 1080 1090 1100
1030 1040 1050 1060 1070 1080 1090 1100
Raw data
t, sec
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Bandpassed between 1-5 Hz
t, sec
Bandpassed between 2-5 Hz
1030 1040 1050 1060 1070 1080 1090 1100
1030 1040 1050 1060 1070 1080 1090 1100
t, sec
PKiKP
1040 1045 1050 1055 1060 1065 1070t, sec
PKiKP
t, sec
f, H
z
1040 1045 1050 1055 1060 1065 10700246
Back azimuth
Vp, m
/s
0 50 100 150 200 250 300 3501020304050
Continuous wavelet transform
MUSIC estimator
Processing window
Enhanced f-k processoing (MUSIC)
t, sec
Apparent velocity and backazimuthestimate using MUSIC estimator.
Mapped PKiKP reflection points on ICB and orientation of ray paths
∞ 160∞ 180∞ 200∞ 220∞ 240∞ 260∞-20∞
0∞
20∞
40∞
60∞
ILAR PDAR TXAR
140
CMAR NVAR
1
4
5
3
2
Seismic data observed at following arrays:
1020 1040 1060 1080 1100 1120 1140 1160 1180 1200
1020 1040 1060 1080 1100 1120 1140 1160 1180 1200
PKiKP
1000 1020 1040 1060 1080 1100 1120 1140 1160
1000 1020 1040 1060 1080 1100 1120 1140 1160
PKiKP
2002.10.04 19:05 ILARDistance 89∞ Azimuth at the ICB 13∞
1000 1020 1040 1060 1080 1100 1120 1140 1160
1000 1020 1040 1060 1080 1100 1120 1140 1160
PKiKP
Time, s
Time, s
2002.10.04 19:05 PDARDistance 90∞ Azimuth at the ICB 41∞
Time, s
2004.04.09 15:23 NVARDistance 87∞ Azimuth at the ICB 50∞
980 1000 1020 1040 1060 1080 1100 1120 1140 1160
980 1000 1020 1040 1060 1080 1100 1120 1140 1160
PKiKP
2002.01.31 16:27 ILARDistance 84∞ Azimuth at the ICB 19∞
Time, s
1020 1040 1060 1080 1100 1120 1140 1160 1180
1020 1040 1060 1080 1100 1120 1140 1160 1180
PKiKP
Time, s
2002.01.31 16:27 PDARDistance 88∞ Azimuth at the ICB 74∞
0200
400600
8001000
00.2
0.40.6
0.81
2
4
6
8
10
12
14
16
x 10 3
Radius (km)
Global
Orientation w.r.t. pole
Q1
at 1
Hz
1/QP versus radius and angle w.r.t. rotation axis measured from PKIKP waveform modeling [3].
Event 1998.02.28 ILAR
Ray pathsof PKiKP and PKIKP
0.0 0.2 0.4 0.6 0.8 1.0
coda/PKiKP
Equatorial PKIKP Observations
High QαSlow
Low QαFast
Transition Layer (a)
(b)
(c)Figure 6
Radiative transport modeling [2] of coda energy envelopes in a model having small heterogeneity scale lengths in the crust, mantle, and inner core.
90o W 90o E
180o E180o W
0o
PKIKP
PKiKP
Mantle
OuterCore
InnerCore
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