squiggly-line-land view of the earth what’s going on in the upper mantle? –receiver function,...
TRANSCRIPT
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Squiggly-line-land view of the Earth
• What’s going on in the upper mantle?– Receiver function, powerful seismic tool
• What in the world does the structure of the inner core mean?
• Is it still rotating, like it was in 1996?
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Outline of mantle discussion• USArray
• Receiver function analysis
• MOMA
• Africa
• RISTRA
• The upper mantle discontinuities– Water at 410-km-depth– A double “520”
(I just got a digital camera)
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EarthScope Components
• EarthScope's facilities include the following four coupled components:
– USArray (United States Seismic Array) – SAFOD (San Andreas Fault Observatory at
Depth) – PBO (Plate Boundary Observatory) – InSAR (Interferometric Synthetic Aperture
Radar)
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USArray Permanent Array
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Big Foot Array
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Flexible Arraysexample from recent
experiments
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Why look at the upper mantle?
• Mapping seismic structure– P & S velocity, density, anisotropy
• To deduce physical characteristics– Chemical and thermal heterogeneity
• To deduce what’s going on– Stagnant or moving continental keels– Dynamics of upper thermal
boundary layer of the mantle– Mantle circulation
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Seismic-style study
• Reflection for crustal structure• S-wave splitting for anisotropy
– Flow direction - aesthenosphere– Relic fabric - lithosphere
• Surface and body wave tomography– Absolute velocities in upper few 100 km
• Body wave tomography (deeper)• Receiver functions
– Best resolution of radial velocity gradients
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The receiver function
• Pioneered by seismologists including Bob Phinney and Chuck Langston
• Examines echoes of the P wave to determine zones of high radial gradient in seismic velocity
• It is proving to be a very useful companion to seismic tomography, providing detailed pictures of near-receiver structure
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Chuck Langston, after igniting 50 pounds of explosives in sand
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Chuck Ammon’s notes
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40-80° distance range best
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Ray paths contributingto receiver functions
Chuck Ammon
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Radial componentof receiver functionJust useful for finding the Moho
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Lat
eral
var
iati
ons
Adam
Alan
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Mechanics of a receiver function
• Extract the P wave from the vertical component
• Deconvolve it from the horizontal component
• This should leave a spike at the P arrival time and a string of P-S conversions
• Convert the conversions (as a fcn of time and ground motion) to structure (impedance as a function of depth)
• Average together the records from many distances and azimuths
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Some limitations
• Assumes no lateral variations in structure– Migration can overcome this limitation
• Only works in a frequency pass band– Cannot recover baseline, trends, or really much
beyond about 100-200 km wavelength velocity structure
– Generally falls apart shorter than 5-10 km wavelengths
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Mike Wysession Keith Koper
• Missouri to Massachusetts transect
• 19 stations placed every 100 km
• Chosen for nice graphics
MOMA
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MOMAdiscontinuity imaging
KarenFischer
MikeWysession
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Receiver functionsfrom events to thenorth
Eventsto the south
Stereo vision
East!? West
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Tomography plus receiver functions
Farallon depression?
T < 150° C
Disagreement with individual profiles
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Steve Gao
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Shows trend of smaller time separation with more vertical incidence
Gao, GRL, 2002
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Again, well-resolved reflections from near 410 and 660
Note the presence of clear 410 conversions at short-period
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Thicker transition zone to NETransition thickness near global average of 245 km, so not cold under region, 10 km of relief may correspond to ~60° temperature difference
cooler
warmer
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Receiver function migration
• Just like migrating seismic reflection data
• Benefits from adequate spatial sampling
• Ability to image structure depends on– Depth of structure– Frequency of waves recorded
• Of course, more events with more back-azimuths, and more distances are helpful
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Resolution with70 km spacing
T= 15s
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Resolution with10 km spacing
T= 2s
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Alan
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A test model
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Recovery of the test model
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MOMA Array: Depth Migration LP10sMOMA migration
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Cheyenne Belt Receiver Functions
CB
Moho Moho
SLAB
GFSS XD
ModifiedProterozoicMantle
ArcheanMantle
From Ken Dueker
Imbricated Moho
Fast from tomography
Mantle layered
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RISTRARio Grande RiftRan from Texas into Utah
Receiver functions across the 1000-km line give a good picture of the shallow structure, and show little topography on the 410 and 660.
moho
Rick Aster
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Flat discontinuities
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Hot off the JGR “press”
• Hersh, Dueker, Sheehan, and Molnar, JGR• 410 and 660 topography under western US• 20-30 km topography, with 500 km scale length• No relation to surface tectonics• Sharpness not easily related to depth• Conclusions:
– Either transition zone has smaller scale convection than deep mantle
– Or there is a lot of compositional variation down there
Ken Dueker
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Field areaAnne Sheehan
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Average receiver function structure
Seymour Hersh
410
660
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410 topography
660 topography
+/- 10 km
+/- 15 km
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Science 6 June 2003
Seismic evidence for waterdeep in the Earth’s upper mantle
Mark van der Meijde
Suzanne van der Lee
Federica Marone
Domenico
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Science 6 June 2003 - van der Meijde et al.
1000 ppm water broadening the 410-km-discontinuity?
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Main points of van der Meijde • Conversion from “410” stronger at low
frequency than high, but conversion from “660” is steady
• So “410” must be broader, in fact very broad, 20-40 km wide
• Subduction has been pervasive, so water might be common near 410-km-depth
• Entire story is consistent if about 1000 ppm water is present.
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~1 s period 6 s period
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9 stations
The general trend is consistent, and statistics can be constructed to support the significance of the trend.
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X
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Earthquake
Station P'P'df P'P'ab
Mantle
OuterCore
InnerCore
Figure 1
The phase P’P’
Jim Whitomb
DLA
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JGR, Fei, Vidale and Earle
• Rounded 3 good datasets of P’P’– California networks
– LASA recordings
– Highly selected GSN seismograms
• We’ll see– Sharp 660-km-depth discontinuity
– Somewhat less sharp 410, sometimes
– (but MUCH sharper 410 than claimed for Europe)
– No 520
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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
Time relative to P'P'(ab) (sec)
P'P' onset
P'660P' P'410P'
Several minute envelope stack
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0.00
0.05
0.10
0.15
-200 -150 -100 -50
P'P' precursory interval
Amplitude relative to P'P'
Time relative to P'P' (sec)
P'660P' P'410P'Raw stack
Noise-corrected
Figure 4b
The 660 and 410 corrected for steady noise
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0
0.2
0.4
0.6
0.8
1
-200 -150 -100 -50 0 50 100
Stack of best 91 GSN traces
Raw stack
Noise-corrected
Time relative to P'P' (sec)
P'P' onset
P'660P'onset P'410P'
onset?
Figure 8
A global average
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-200 -150 -100 -50 0 50
Summary of envelope stacks
Time relative to P'P' (sec)
P'P'
P'660P' P'410P'CSN
LASA
GSN
LASA + CSN
Figure 9
More 660than 410 energy,Nothing else
Fei Xu
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0
0.01
0.02
0.03
0.04
0.05
-200 -150 -100 -50
Precursors 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
Time Figure 11
"660"
"410"
No visible 410 at higher frequencies
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This means• 660 sharp enough to efficiently
reflect 1 Hz waves - less than 2 km thick transition
• 410 not so sharp - our data is fit by half a sharp jump, half spread over 7 km
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SS precursors as a probe of layering near their bounce point
Peter Shearer
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(Also has claims to see PKJKPand a “250”)
ArwenDeuss
Science, 2001. Sees 520 sometime simple, sometimes split.
Interprets this as the 520 having phase changes in two components, olivine and garnet, whose depths don’t always coincide.
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Transects that indicate lateral continuity of structure
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Transectsof the 520
Lateral continuity of
structure
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A global map, where there is
coverage
JohnWoodhouse
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Some high points• “410”
– Why is it’s brightness variable?– Can we map the pattern globally to learn more?– Is topography real?
• “520”– Why does it flicker?
• “660”– Is topography a thermometer?
• Other discontinuities?• Better images on the way from USArray
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The enigmatic inner core• Layering
• Anisotropy
• Rotation
• Possible origins of structure
• Combined my slides with those of Ken Creager and Shun Karato
Some slides lent byKen Creager and Shun Karato
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Seismic characteristics of the inner core
• A large Poisson’s ratio, close to that of a liquid
• High attenuation (Qs~100-200)
• Strong anisotropy
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Anisotropic Lower inner Core
Transition R
egion
Isotropic Upper Inner C
ore
A current working model
IMIC
Upper Inner Core:
Isotropic, finely heterogeneous
West: 0.8% slower
250 km thickQ = 600
East: thickerQ = 250 in east
Middle Inner Core:
Strong anisotropy
Isotropic Voigt average is homogeneous
Innermost Core:
Different anisotropy?
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Niu and Wen, 2001Red - western hemisphereBlack - eastern hemisphere
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Comparing polar and equatorial data
Ouzounis and Creager, GRL, 2001
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Beghein and TrampertScience, 2003 Adam and
Miaki Ishii
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Summed slant stackSlowness (s/km)
predicted for PKiKP
1000 1050 1100 1150 1200 1250
-0.10
-0.05
0.00
0.05
0.10
Time after event (s)
Slowness (s/km)
0.00 0.25 0.50 0.75 1.00
Amplitude
Stack of envelopes of slant stacks13 earthquakes and 4 nuclear tests
X
direct P coda slowness
(Vidale & Earle)
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Proposed mechanisms of inner core anisotropy
Jeanloz & Wenk, GRL, 1988
Convective flow due to high Rayleigh number aligns crystals (most effective near surface)
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Yoshida et al., JGR, 1996
Inhomogeneous growth of inner core drives convective flow that restores isostatic equilibrium
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Michael Bergman, Science, 1997 (modified by Michael Wysession)
Dendritic growth of crystals aligns a-axes radially with heat flow direction (assumes c-axis is fast)
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Modified from Annie Souriau, Science, 1998
Strong heterogeneities, various crystal alignment orientations
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Bruce Buffett, Nature, 2001
Rotationally wrapped magnetic field around inner core causes Maxwell stresses that align crystals (c-axes cylindrically radially out)
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Modified from Shun-Ichiro Karato, Nature, 1999
Lorentz forces produce axisymmetric, sustained flow that aligns crystals
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Hemispherical asymmetrySumita and Olson (1999)
Hemispherical asymmetry might be due to heterogeneous thermal boundary conditions at the inner-core boundary caused by core-mantle interaction.
[Time-scale for anisotropic structure formation must be comparable to or shorter than the time scale for changes in mantle structure.]
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Does the inner core rotate with respect to the mantle?
Song and Richards, 1996 yes 1.1 deg/yr
Creager, 1997, yes 0.2-0.3 deg/yr
Vidale et al., 2000, yes 0.15 deg/yr
Souriau, 2001, no, at least not very fast, <0.1 - 0.2 deg/yr
Laske and Masters , 2002, maybe 0.13±0.11 deg/yr
Song, 2002, yes 0.5-1.0 deg/yr
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Why do we care?
• I think it’s interesting
• Would mean the core has either– Quite low viscosity
• Can deform fast enough to keep moving
– Quite low viscosity• Deforms so little that there is little viscous drag
• Would prevent association of IC structure with mantle structure
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25 years of data
Xiao-Dong Song and Paul Richards
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Li and Richards, submittedSouth Sandwich Islands Doublet
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Song, AGU Monograph, 2002
More Sandwich doublets
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Laske and MastersNormal mode analysis
AGU Monograph, 2002
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Geometry
Explosions LASA array
ICS
View from Equator
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PKKP comparison
-6
-4
-2
0
2
4
6
1870 1875 1880 1885 1890 1895 1900
Stacked PKKP waveforms
9/27/718/29/74
Time after blast (s)
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PKiKP waveform correlation
-6
-4
-2
0
2
4
6
1070 1075 1080 1085 1090 1095 1100
Stacked PKiKP coda waveforms
9/27/718/29/74
Time after blast (s)
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P’660P’ correlation
-3
-2
-1
0
1
2
3
2220 2225 2230 2235 2240 2245 2250
Stacked P'660P' waveforms
9/27/718/29/74
Time after blast (s)
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Bottom line:Inner core maylap Earth every
2000 years
Wild card - Does the outer corechange over time?
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Quick Review• Mantle discontinuities still
remain interesting after 40 years
• Inner core is being mapped but not yet understood
• Inner core is likely turning slowly
• Seismology and mineral physics must progress together