1. Messages From the Deep: Reviewing Seismic Evidence for
Deep Mantle Slabs
Thermochemical Piles
Post-Perovskite Phase Transition
Ultra-Low Velocity Zones
and Uncertainties
Deep = below lithosphere to Earth’s center
Ed Garnero
Arizona State Univ. Dept. of Geological Sciences
June 21, th Annual COMPRES Meeting

2. Multidisciplinary research conducted in collaboration with:
Avants, Megan (UCSC)
Ford, Sean (UCB)
Hernlund, John (IPGP)
Hutko, Alex (UCSC)
Igel, Heiner (U Munich)
Lay, Thorne (UCSC)
Manga, Michael (UCB)
McNamara, Allen (ASU)
Rokosky, Juliana (UCSC)
Rost, Sebastian (U Leeds)
Schmerr, Nick (ASU)
Thomas, Christine (U Liverpool)
Thorne, Mike (U Alaska)
Williams, Quentin (UCSC)
Deep = below lithosphere to Earth’s center

3. -- Today -- Some Seismo Truths:
 Important modeling uncertainties/trade-offs
 Outlook: possible things to come
Recent results and interpretations
 Focus on deep mantle ‘high resolution’ work
 Draw connections to global scales/processes
inferred from long wavelength studies
Deep = below lithosphere to Earth’s center

4. Why care? …We’d like to better understand:
Isaacs, Oliver, Sykes [1969]
Why care? …We’d like to better understand:
Mode/style of mantle convection
Depth extent, nature of subduction
Source of hot spot magma
Mantle H2O budget/cycle
Transition zone structure, dynamics
Nature, structure of fluid and solid cores
Thermal evolution/budget of deep interior
Today I’ll predominantly focus on the lower mantle, some TZ

5. Seismology Report Card
Structural feature
Evidence
Constrained?
Transition zone layering/topography
Deep mantle heterogeneity
Deep mantle “piles”
D” Vs discontinuity/layering
D” Vp discontinuity/layering
D” anisotropy
Ultra-low velocity zone
CMB topography B+ C A+ B- C C- B A+ D-
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture B C- A C F

6. to Seismology Report Card ao Amplitude Time
Five reasons for bad “Constraints” grades
1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc) ao to
Amplitude
Time
inner
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture

7. to t’ Seismology Report Card a’ ao Amplitude Time
Five reasons for bad “Constraints” grades
1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc) a’ ao
Amplitude to t’
Time
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture
inner

8. to t’ Seismology Report Card a’ ao Amplitude Time
Five reasons for bad “Constraints” grades
1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc) a’ ao
Amplitude to t’
Time
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture
inner

9. Seismology Report Card
Five reasons for bad “Constraints” grades
1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)
2) Modeling trade-off between discontinuity location & isotropic heterogeneity
Amplitude
Time
New arrival!
inner
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture

10. Seismology Report Card
Five reasons for bad “Constraints” grades
1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)
2) Modeling trade-off between discontinuity location & isotropic heterogeneity
3) Globally, only very long wavelength structure is retrievable, which involves significant smearing, and may not accurately depict physics of interior
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture

11. Seismology Report Card
Five reasons for bad “Constraints” grades
1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)
2) Modeling trade-off between discontinuity location & isotropic heterogeneity
3) Globally, only very long wavelength structure is retrievable, which involves significant smearing, and may not accurately depict physics of interior
4) Only fraction of a percent of the globe has been probed at “high resolution”, and hence projection of results to global scales is conjecture
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture
~ 2502 km
~ 200 x 700 km
~ 1002 km

12. Seismology Report Card
Five reasons for bad “Constraints” grades
1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)
2) Modeling trade-off between discontinuity location & isotropic heterogeneity
3) Globally, only very long wavelength structure is retrievable, which involves significant smearing, and may not accurately depict physics of interior
4) Only fraction of a percent of the globe has been probed at “high resolution”, and hence projection of results to global scales is conjecture
5) Still using 1-D techniques to get 3-D answers….
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture

13. A Mineral Physicist’s Guide to Disbelieving Seismologists
Messages From the Deep: Reviewing Seismic Evidence for Deep Mantle Slabs, Thermochemical Piles, Post-Perovskite Phase Transition, Ultra-Low Velocity Zones
A Mineral Physicist’s Guide to Disbelieving Seismologists
Ignoring
Embracing
Ostracizing
Canonizing
Enslaving :
Deep = below lithosphere to Earth’s center

14. Seismology Report Card: Grades Rapidly Improving !
Unparalleled seismic network seismometer populations
(e.g., NSF-funded EarthScope’s USArray)
Enables technique refinement/development
Permits structural retrieval at smaller scale lengths
Better computational capabilities
2- and 3-D wave propagation computations doable
We are approaching capabilities of benchmarking solution
structures
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture
Advances in mineral physics, geodynamics, & geochemistry
Provides significant guidance of our research targets
and goals

15. Recent Seismic Results
Some short seismic modeling vignettes relating to:
Slabs in the lower mantle
Post-perovskite phase transition
Ultra-low velocity zone
Deep mantle piles?
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture

16. Recent Seismic Results
km depth
Upper mantle, transition zone
structure in the central Pacific
km depth
D” discontinuity
topography
km depth
Slab detection from
seismic reflections
between
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture
km depth
D” fine-scale
layering
km depth
Ultra-low velocity
zone structure

17. Recent Seismic Results
km depth
Upper mantle, transition zone
structure in the central Pacific
km depth
D” discontinuity
topography
km depth
Slab detection from
seismic reflections
between
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture
km depth
D” fine-scale
layering
km depth
Ultra-low velocity
zone structure

18. Long wavelength suggestion: Some slabs continue to CMB.
The ‘best’ evidence for old oceanic lithosphere sinking through the lower mantle is from seismic tomography for the region beneath the Caribbean. Here cross-sections are shown through the 2002 mantle shear wave model of Steve Grand. One can infer a contorted, thickened slab structure extending to D” then deflecting and piling up there, maybe…..
dVs: Grand [2002]
Hutko, Lay, Garnero, Revenaugh [Nature, 2006]

19. Fine-Scale D” Structure Beneath the Cocos Plate
Thomas, Garnero, Lay [JGR, 2004]

20. A few % discontinuous dVs increase is consistent
with the post-perovskite phase transition
Observations: km
above CMB
0.5-3%
velocity
increase
consistent
with onset
of post-
perovskite
phase
e.g., Murakami et al. [Science, 2004]
Lay et al. [EOS, 2005]
Lay et al [PEPI, 2004]

21. Results: Vertical Step in D” Discontinuity:
Fine-Scale D” Structure Beneath the Cocos Plate
Results: Vertical Step in D” Discontinuity:
Height of D” increases by 100 km over >200 km horizontally
Hutko, Lay, Garnero, Revenaugh [Nature, 2006]

22. Fine-Scale D” Structure Beneath the Cocos Plate
Hutko, Lay, Garnero, Revenaugh [Nature, 2006]

23. D” anisotropy Seismic anisotropy and shear wave splitting
After Crampin [1981]

24. D” anisotropy
After Crampin [1981]

25. D” anisotropy
The apparent step in the D” layer coincides with a change in D” anisotropy parameters
Hutko et al. [Nature, 2006, in press]
S, Sdiff
ScS
Garnero, Maupin, Lay, Fouch [Science, 2004]
Maupin,Garnero,Lay,Fouch [JGR, 2005]
Rokosky, Lay, Garnero [EPSL, 2006, in press]

26. Recent Seismic Results
km depth
Upper mantle, transition zone
structure in the central Pacific
km depth
D” discontinuity
topography
km depth
Slab detection from
seismic reflections
between
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture
km depth
D” fine-scale
layering
km depth
Ultra-low velocity
zone structure

27. Raw array data: Fiji EQ recorded on the Canadian Yellowknife Array
Rost,Garnero,Williams [in prep., 2006]

28. Each trace = stack at a different Incoming angle to the YKA array
Array processed data:
Each trace = stack at a different
Incoming angle to the YKA array
Rost,Garnero,Williams [in prep., 2006]

29. Back projecting along determined back azimuth and slowness of each
precursor permits estimation of
reflection location that matches
differential time between precursor
and the direct PP wave
Rost,Garnero,Williams [in prep., 2006]

30. Rost,Garnero,Williams [in prep., 2006]

31. Recent Seismic Results
km depth
Upper mantle, transition zone
Structure in the central Pacific
km depth
D” discontinuity
topography
km depth
Slab detection from
seismic reflections
between
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture
km depth
D” fine-scale
layering
km depth
Ultra-low velocity
zone structure

32. Ultra-low velocity zones at Earth’s core mantle boundary
This model type is appropriate for partial melt of the deepest mantle: dVs=3*dVp
Williams and Garnero [Science, 1996]

33. Best attempt at global coverage ( ~40 % )
Ultra-low velocity zones at Earth’s core mantle boundary
Not detected globally
Isolated anomalous zones
Best attempt at global coverage ( ~40 % )
Thickness depends of several assumptions
Uncertainties in global ULVZ details quite large
This model type is appropriate for partial melt of the deepest mantle: dVs=3*dVp
ULVZ
Thickness
(km)
Thorne and Garnero [JGR, 2004]

34. Ultra-low velocity zones at Earth’s core mantle boundary
Best-fit model properties:
Thickness : (1) km
DVP : (2.5) %
DVS : (4) %
Dr : (5) %
Issues: this is the only spot on Earth, at present, sampled with this degree of sampling density
Rost,,Garnero,Williams,Manga [Nature, 2005]

35. Conceptual model possibility
Ultra-low velocity zones at Earth’s core mantle boundary
Conceptual model possibility
5 to 30 vol.% melt
no spreading along CMB
trapped intercumulus liquid
incompatible-element enriched liquid
crystals are initially overgrown
and trap residual
requires large overlying thermal anomaly
downward percolation of melt
correlation to dynamic instabilities/upwellings
probably a fixed base for mantle upwellings
Rost, Garnero, Williams, Manga [Nature, 2005]

36. Recent Seismic Results
km depth
Upper mantle, transition zone
structure in the central Pacific
km depth
D” discontinuity
topography
km depth
Slab detection from
seismic reflections
between
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture
km depth
D” fine-scale
layering
km depth
Ultra-low velocity
zone structure

37. Central Pacific D” Layering
dVs [Grand, 2002]
Lay, Hernlund, Garnero, Thorne [Science, 2006, in review]
Avants, M., T. Lay, S. Russell, and E.J. Garnero [JGR, 2006]
Avants, M., T. Lay, and E.J. Garnero [GRL, 2006]

38. Central Pacific D” Layering
Lay, Hernlund, Garnero, Thorne [Science, 2006, in review]

39. Central Pacific D” Layering
~ 500 km
Bin3
Bin2
Bin1
~ 1000 km
Lay, Hernlund, Garnero, Thorne [Science, 2006, in review]

40. Recent Seismic Results
km depth
Upper mantle, transition zone
structure in the central Pacific
km depth
D” discontinuity
topography
km depth
Slab detection from
seismic reflections
between
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture
km depth
D” fine-scale
layering
km depth
Ultra-low velocity
zone structure

41. Probing the Transition Zone Regionally: Central Pacific
SS waves and precursors
Schmerr and Garnero[2006, JGR, in press]

42. Probing the Transition Zone Regionally: Central Pacific
Thinning of the TZ
( ~ 15 km average)
410 disc slightly depressed
- 660 disc upwarped
Schmerr and Garnero[2006, JGR, in press]

43. What about chemically distinct piles in the deep mantle?
Step in D” discontinuity:
consistent w/ slab piling/folding
Slabs: at least
to 1000 km
Thinned
transition
zone
What about chemically distinct piles
in the deep mantle?
This supports the geodynamic calculation: hot spots appear to overly edges of low velocities.
Very localized ULVZ:
dense, partial melt,
base of upwelling
D” stratification:
LLSVP, pPv, ULVZ
Thorne, Garnero, Grand [2004, PEPI]

44. Sharp “edges” to low velocities inferred from seismic waveforms
mantle
Caribbean
anomaly
core
Sharp top
African
anomaly
Pacific
anomaly
Ni and Helmberger [2003,2005, Science, EPSL]
Ford, Garnero,McNamara [2006, JGR]
Sharp “edges” to low velocities inferred from seismic waveforms
South pole

45. Indirect evidence from global tomography
red: lowest velocities for S20RTS
green: strongest lateral VS gradients
Thorne, Garnero, Grand, PEPI., 2004

46. Deep mantle shear velocity and hot spots
This supports the geodynamic calculation: hot spots appear to overly edges of low velocities.
Model: Grand 2002
Iso-velocity contour: - 0.7% Hotspots:
Thorne, Garnero, Grand [2004, PEPI]

47. Summing up Topic More constrained Less constrained
Deep mantle heterogeneity
Long wavelength dVs patterns
Short scale structure, Vp
Deep mantle anisotropy
It exists, it laterally varies
Strength, depth distribution, geometry
D” discontinuity
dVs reflector strength, location
Sharpness, height above CMB, dVp disc, deeper disc assoc. w/ pPv phase
ULVZ
Except for one spot: internal properties, geographical distribution, sharpness
Deep mantle slabs
Existence in upper half of lower mantle
Structure/behavior in lower half of the mantle
Deep mantle piles
Abrupt transition between low velocities and mantle
Density, internal structure
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture

48. “there are known unknowns
Today: lots of seismically imaged short scale details
in just a few spots of the volume of the interior
Point: Where we’re afforded the ability to image
in great detail, richness in complexity is apparent
“there are known unknowns
and unknown unknowns”
What does USArray allow us to do that was not previously possible?
How do we best combine USArray body wave data from different time periods / geographies?
Image short wavelength regional structure
Draw connections to bigger picture

49. garnero@asu.edu http://garnero.asu.edu
Thus, do slabs make it all the way to the CMB? At least in this location, the evidence is pretty good. Note however, in other locations the evidence is not so compelling. This may be a unique location.
Garnero [Ann. Rev. , 2000]
Garnero, Maupin, Lay, Fouch [Science, 2004]