Squiggly-line-land view of the Earth
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Transcript of Squiggly-line-land view of the Earth
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?
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)
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)
USArray Permanent Array
Big Foot Array
Flexible Arraysexample from recent
experiments
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
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
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
Chuck Langston, after igniting 50 pounds of explosives in sand
Chuck Ammon’s notes
40-80° distance range best
Ray paths contributingto receiver functions
Chuck Ammon
Radial componentof receiver functionJust useful for finding the Moho
Lat
eral
var
iati
ons
Adam
Alan
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
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
Mike Wysession Keith Koper
• Missouri to Massachusetts transect
• 19 stations placed every 100 km
• Chosen for nice graphics
MOMA
MOMAdiscontinuity imaging
KarenFischer
MikeWysession
Receiver functionsfrom events to thenorth
Eventsto the south
Stereo vision
East!? West
Tomography plus receiver functions
Farallon depression?
T < 150° C
Disagreement with individual profiles
Steve Gao
Shows trend of smaller time separation with more vertical incidence
Gao, GRL, 2002
Again, well-resolved reflections from near 410 and 660
Note the presence of clear 410 conversions at short-period
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
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
Resolution with70 km spacing
T= 15s
Resolution with10 km spacing
T= 2s
Alan
A test model
Recovery of the test model
MOMA Array: Depth Migration LP10sMOMA migration
Cheyenne Belt Receiver Functions
CB
Moho Moho
SLAB
GFSS XD
ModifiedProterozoicMantle
ArcheanMantle
From Ken Dueker
Imbricated Moho
Fast from tomography
Mantle layered
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
Flat discontinuities
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
Field areaAnne Sheehan
Average receiver function structure
Seymour Hersh
410
660
410 topography
660 topography
+/- 10 km
+/- 15 km
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
Science 6 June 2003 - van der Meijde et al.
1000 ppm water broadening the 410-km-discontinuity?
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.
~1 s period 6 s period
9 stations
The general trend is consistent, and statistics can be constructed to support the significance of the trend.
X
Earthquake
Station P'P'df P'P'ab
Mantle
OuterCore
InnerCore
Figure 1
The phase P’P’
Jim Whitomb
DLA
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
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
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
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
-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
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
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
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
SS precursors as a probe of layering near their bounce point
Peter Shearer
(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.
Transects that indicate lateral continuity of structure
Transectsof the 520
Lateral continuity of
structure
A global map, where there is
coverage
JohnWoodhouse
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
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
Seismic characteristics of the inner core
• A large Poisson’s ratio, close to that of a liquid
• High attenuation (Qs~100-200)
• Strong anisotropy
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?
Niu and Wen, 2001Red - western hemisphereBlack - eastern hemisphere
Comparing polar and equatorial data
Ouzounis and Creager, GRL, 2001
Beghein and TrampertScience, 2003 Adam and
Miaki Ishii
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)
Proposed mechanisms of inner core anisotropy
Jeanloz & Wenk, GRL, 1988
Convective flow due to high Rayleigh number aligns crystals (most effective near surface)
Yoshida et al., JGR, 1996
Inhomogeneous growth of inner core drives convective flow that restores isostatic equilibrium
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)
Modified from Annie Souriau, Science, 1998
Strong heterogeneities, various crystal alignment orientations
Bruce Buffett, Nature, 2001
Rotationally wrapped magnetic field around inner core causes Maxwell stresses that align crystals (c-axes cylindrically radially out)
Modified from Shun-Ichiro Karato, Nature, 1999
Lorentz forces produce axisymmetric, sustained flow that aligns crystals
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.]
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
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
25 years of data
Xiao-Dong Song and Paul Richards
Li and Richards, submittedSouth Sandwich Islands Doublet
Song, AGU Monograph, 2002
More Sandwich doublets
Laske and MastersNormal mode analysis
AGU Monograph, 2002
Geometry
Explosions LASA array
ICS
View from Equator
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)
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)
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)
Bottom line:Inner core maylap Earth every
2000 years
Wild card - Does the outer corechange over time?
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