Selected Seismic Observations of Upper-Mantle...
Transcript of Selected Seismic Observations of Upper-Mantle...
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Selected Seismic Observations of Upper-Mantle Discontinuities
Peter Shearer IGPP/SIO/U.C. San Diego
August 31, 2009 Earthquake Research Institute
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Interface Depth vs. Publication Date
Most depths are sampled at least once
Consistency in depths greatest for 220, 410, 520, 660
Note: plot is not complete, especially in last 15 years
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• Analyze entire dataset whenever possible
• Use simple methods to get sense of data before doing complicated inversions
• Consider reflection seismology methods like stacking and back-projection
• Avoid any hand-processing of seismograms!
Advice on Seismic Data Crunching
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Global Stacking using Automatic Gain Control (AGC) • Calculate average absolute value in 5 s bins • Divide each bin by average of previous 24 bins.
This normalizes the amplitude of each trace. • Stack in 0.5˚ distance bins
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AGC Stack: Long-period vertical
from Shearer (1991) Distance (degrees)
Tim
e (m
inut
es)
90
60
30
0 0 90 180 270 360
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Stacking using a reference phase Unaligned SH waves Aligned SH waves
1 minute Stack
Reference pulse stacks for 20 different range bins
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CD-ROM stacks (1991) P wave (vertical)
S wave (transverse)
P
PP
410-km discontinuity
660-km discontinuity
No global 220-km discontinuity
SS
S
Topside reflections
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CD-ROM stack: SS precursors SS-wave stack (transverse)
SS
S660S
660
410-km discontinuity 520
Sdiff
SS
from Shearer (1991)
80 100 120 140 160 180
4
2
Range (degrees)
Tim
e (m
inut
es)
0
-2
-4
-6
-8
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No coherent reflectors above 410 or below 660
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SS precursors are ideal for global mantle discontinuity studies
Source Receiver Bounce point
Good global distribution of bounce points
from Flanagan & Shearer (1998)
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Depression in ‘660’ in NW Pacific
from Shearer (1991)
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Gu et al. (2002)
Shearer & Masters (1992)
Flanagan & Shearer (1998)
‘660’ topography from SS precursors
blue = depressed (~10–20 km) red = elevated
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CD-ROM stacks (1991) P-wave stack (radial)
P/SV discontinuity conversions (Vinnik, 1977)
SV/P discontinuity conversions (Faber & Muller, 1984)
PcSdiff
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Receiver functions at GSN stations
Shearer (1991) Lawrence & Shearer (2005)
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Transition Zone Thickness Models SS precursors Receiver functions
Gu et al. (1998)
Flanagan & Shearer (1998)
Lawrence & Shearer (2005)
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Slabs in the transition zone
from Karson and van der Hilst (2000)
Flanagan & Shearer (1998)
660 topography
P-wave tomography
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Slabs in the transition zone
Lesser deflection in large region beneath slab
50–100 km deflection in vicinity of slab
Response of 660-km discontinuity to slab:
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figure from Lebedev et al. (2002)
410 and 660 observations are consistent with mineral physics predictions for olivine phase changes
• Absolute depths agree with expected pressures
• Topography consistent with Clapeyron slopes
• Size of velocity and density jumps are about right
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Flanagan & Shearer (1998) Lebedev et al. (2002)
Global, SS precursors Australia region, Receiver functions
• Correlation between TZ thickness and velocity anomalies • Agrees with mineral physics data for olivine phase changes • Permits calibration of dT/dv and Clapeyron slopes
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Analysis of different discontinuity phases can resolve density, P & S velocity jumps across discontinuities
A puzzle: Where is the 660 reflector?
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Shearer & Flanagan (1999)
SS & PP precursors
Kato & Kawakatsu (2001)
ScS reverberations
Tseng & Chen (2004) Triplicated waveforms
Estimated S velocity and density jumps across 660 km
Global Study Northwest Pacific Philippine Sea
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Computing simple ray theoretical synthetics
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Solve for best-fitting model using niching genetic algorithm
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• 660-km discontinuity has small contrasts in density & P velocity • Largest change at 520 km is in density • 410-km discontinuity is thicker than 660-km discontinuity • 410 seems to fit pyrolite model, 660 is more complicated, may be
double discontinuity with more than one phase change
From Lawrence & Shearer (2006)
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Earthquake
Station P'P'df P'P'ab
Mantle
OuterCore
InnerCore
Figure 1
P’P’ phase: seen at short periods, good for sharpness constraints
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0
0.2
0.4
0.6
0.8
1
-200 -150 -100 -50 0 50 100
Envelope stack:1/19/69 earthquake at LASA
Rela
tive
ampl
itude
Time relative to P'P'(ab) (sec)
P'P' onset
P'660P' P'410P'
from Xu et al. (2003)
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0
0.01
0.02
0.03
0.04
0.05
-200 -150 -100 -50
Precursors to P'P'
Am
plitu
de r
elat
ive
to P
'P'
Time relative to P'P' (sec)
P'660P'P'410P'
XXlong-period
reflectionamplitudes
Comparison to long-period reflections
Corrected for attenuation
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0.00
0.02
0.04
0.06
0.08
0.10
2200 2240 2280 2320
LASA stacks at two frequencies
0.7 Hz stack1.0 Hz stack1.3 Hz stack
Am
plitu
de r
elat
ive
to P
'P'
Time Figure 11
"660"
"410"
No visible 410 in P’P’ at higher frequencies
from Xu et al. (2003)
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Conclusions from Xu et al. P’P’ study
410 is not so sharp — results suggest half is sharp jump, half is spread over 7 km
520 is not seen in short-period reflections — jump must occur over 20 km or more
660 is sharp enough to efficiently reflect 1 Hz P-waves — less than 2-km thick transition
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Regional constraints on discontinuity topography
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Dueker & Sheehan (1997)
Snake River Plane Eastern US, MOMA Array
Li et al. (1998)
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Tibet
Tanzania
Kosarev et al. (1999)
Owens et al. (2000)
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Southern Africa
Gao et al. (2002)
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from Niu et al. (2005)
410 P-to-S conversion points
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Future of upper-mantle discontinuity studies
• Continued high-resolution regional analyses using seismic arrays and migration processing methods (USArray, Japan)
• More detailed comparisons to mineral physics (temperature, composition, water content, possible multiple phase changes)
• Analyses of hard-to-image interfaces between the Moho and the 410, e.g., the lithosphere-asthenosphere boundary (LAB).