1. Model Validation
RegSEM waveform fit:
model 3 + Baffin Bay 6.0 event

2. Model Validation
RegSEM waveform fit:
model 3 + Oklahoma 5.6 event

4. Isotropic Vs
Craton and WUS separated by the Rocky Mountain Front (RMF)
Transition zone depth: slow craton; fast WUS; subducted JdF slab

5. Radial Anisotropy (ξ)
Sutures: VSv>VSh

6. Shear Wave Splitting
Shear wave splits to the fast and slow symmetry axis in the anisotropic medium

7. Integrated effect of the medium (δt and ψ)
Spread sensitivity along ray path ψ

8. Region
Long and Becker, EPSL, 2010

10. Shear Wave Splitting

11. Shear Wave Splitting

12. Laminated Mantle Don L. Anderson’s nomenclature (2011) Region B:
Seismic Lid & Low Velocity Layer
G-discontinuity
L-discontinuity
“the most heterogeneous &
anisotropic region of the mantel”
Anderson, J. of Petrology 2011

13. Laminated Mantle Local Tomography Regional Tomography
Crust + Shallow upper Mantle A+B
Regional Tomography
Upper mantle B+C
Global Tomography
Full mantle B+C+D and core
Anderson, J. of Petrology 2011

14. Laminated Mantle Mantle discontinuities Regional Tomography Region B:
Moho;
G-discon. (ocean);
Hales discon.(continents);
L- discon. (220 km);
Lithosphere-asthenosphere-boundary
Region C:
410-km discon.; 520-km discon.; 660-km discon.
Region D:
D’’-discon.
Classic mantle subdivision
Regional Tomography
Anderson, J. of Petrology 2011

15. “2-Layer” SKS Model Lithosphere or asthenosphere origin of the SKS
Silver and Chan 1996;
Vinnik et al 1989
Apparent SKS fitting from 3D model
2-layer model prediction: lithosphere layer and asthenosphere layer
Second application

16. “2-Layer” Model at HRV
Surface wave and local SKS modeling results (Levin et al. 1999) agree well:
Top: east-west direction (N100E)
Bottom: plate motion direction (N50E)
N100E
N50E

17. Western US upper mantle (2010 Model)
Sharp transition from craton to WUS along the RMF
Subducted JdF in the transition zone
Depth dependent anisotropy field explains the “swirl” SKS splitting pattern
Deep anisotropy associated with the stagnant slab?

18. Depth Dependent Anisotropy in WUS
Excellent ray azimuthal coverage from the TA

19. Isotropic Vs Fast Craton and its Boundary wrt. the western US
Fast Vs km under Great Plains
Slow Ridges (<100 km)
“Neutral” CP in Vs
“Neutral” CA Coast
Ranges
Slow B&R (~250 km) N
This 3D Vs plot summarizes some of the key features. The plot shows the isosurfaces for 3% positive and 1.5% negative variation, down to 400 km. We are viewing from south west. CA is here. Wyoming is here.
Unsolved features like beneath Texas, downgoing instability

20. Isotropic Vs Fast Vs (250-400 km) under Oregon: Correlating with the
subducted JdF slab:
van der Lee and Nolet 1997;
Burdick et al. 2009;
Obrebski et al. 2010;
Schmandt and Humphreys 2010 N
One specific feature I want you to keep in mind is the high velocity feature under Oregon. Now we are viewing the model from north west, washington, texas.

21. Need Berkeley Logo to show different “wavelength” of the study so that the story here might not be sold

22. Shallow depth azimuthal anisotropy
Rapid anisotropy pattern change across the RMF
Large amplitude, shallow depth (<150 km), NA/JdF Absolute Plate Motion (APM) parallel

23. Intermediate Depth Azimuthal Anisotropy
Intermediate depth (~150), E-W under B&R, N-S along the RMF, E-W under SRP
dichotamy

24. Deep Azimuthal Anisotropy
Large amplitude, Pacific APM parallel
Deep (>250 km) E-W direction under Washington and Oregon
dichotamy

25. Anisotropy and SKS in WUS
Circular pattern of the SKS splitting (Savage and Sheehan, 2000)
This 3D depth dependent anisotropy model can explain some of the hot topics in SKS measurements in the western US.
More than one-layer of the anisotropic domain needed.

26. Circular pattern of the SKS splitting
Schutt and Humphreys, Pure and Applied Geophysics, 1998
Savage and Sheehan, JGR, 2000

27. Circular pattern of the SKS splitting
NA-SWS-1.1, Liu, G-Cubed 2009
Eakin et al., EPSL, 2010

28. Proposed Interpretations from others
Savage and Sheehan (2000): Active upwelling
Zandt and Humphreys (2008): Passive edge/toroidal flow
West et al (2009): Lithospheric drip + associated mantle flow

29. Savage and Sheehan Models
Plume model
Model 3
Mantle Upwelling
Savage and Sheehan 2000

30. Zandt and Humphreys model
Slab Rollback + Slab window
Passive edge/toroidal Flow
Zandt and Humphreys, Geology, 2008

31. West et al. model Center of the circular pattern Low Volcanism
Low Heat flow
Large Vp variation
Small Splitting Time
They first observed a high velocity body from ~100 down to below the transition zone.
Downward Drip + mantle flow 
West et al., Nature Geo., 2009

32. Berkeley Model Circular Pattern Predicted by 3D model
Not good along the Wasatch front/western border of the CP where extremely large change of Velocity and fast axis directions over 50km distance have been reported from the LA Ristra array studies, which is beyond our current resolution.

33. Anisotropy and SKS in WUS
More than “one-layer” of anisotropic domain is needed.
Deep east-west direction beneath Oregon
More than one-layer of the anisotropic domain needed.

34. Deep anisotropy & Subducted slab
Fast Vs ( km) under Oregon:

35. Origin of the deep anisotropy
Isotropic Vs
So what causes the deep anisotropy?

36. Origin of the deep anisotropy
Anisotropy direction
Isotropic Vs
Fast velocity correlates with the east-west fast axis direction, indicating there is contribution from the slab!
There are a lot of slab segments in the WUS transition zone.

37. Origin of the deep anisotropy
Anisotropy direction
Isotropic Vs
Slab
Fast velocity correlates with the east-west fast axis direction, indicating there is contribution from the slab!
There are a lot of slab segments in the WUS transition zone.

38. Stagnant Slab Frozen-in/structural anisotropy
in the stagnant/flattened slab
Slab rollback caused stagnant segments above the 660-km and slab flattening above the transition zone.
Also the rollback may cause east-west flow around 150-km depth
Schmid et al., EPSL, 2002;
Fukao, Annu. Rev. Earth Planet. Sci. 2009

39. Depth dependent anisotropy in the WUS
Shallower than 150 km NE-SW plate shear
At 150 km circular flow due to slab rollback
At > 350 km east-west frozen-in/structural anisotropy in the stagnant/flattened slab
Slab Rollback
Plate shear
Pacific Plate
NA Plate
Plate shear
Slab rollback caused stagnant segments above the 660-km and slab flattening above the transition zone.
Also the rollback may cause east-west flow around 150-km depth
Stagnant
slab
Frozen-in fabric
660 km
Modified from Fukao, Annu. Rev. Earth Planet. Sci. 2009

40. Global Stagnant Slab in Transition Zone
SEMum2.2 Vs model (French et al 2011 AGU)
Expect to see

41. Future Directions
Higher frequencies: better vertical resolution in the lithosphere
Numerical approach: Spectral Element Method and Adjoint methods
Other regions: East Asia and Middle East
Global detection of the LAB and MLD
Plunging symmetry axis: geodynamic implications
Anisotropy detecting with SKS and receiver functions

42. Laminated Mantle Mantle discontinuities Region B: Moho; G-discon.;
Hales discon.;
L- discon. (or 220-km);
Lithosphere-asthenosphere-boundary (LAB)
Region C:
410-km discon.; 520-km discon.; 660-km discon.
Region D:
D’’-discon.
Classic mantle subdivision
Anderson, J. of Petrology 2011

43. Region B in the continents
Xenolith data suggest depleted
(i.e. high velocity) layer goes
down to km range:
Lee et al. 2011;
Fischer et al. 2010;
Eaton et al. 2008
Sheared peridotites or refertilized by the melts from asthenosphere near the base of the lithosphere
Fischer et al., Annu. Rev. Earth Planet. Sci. 2010;
Lee et al., Annu. Rev. Earth Planet. Sci. 2011
Romanowicz, Science 2009

44. North American Regional Inversion
Rich tectonic history of the continent
USArray Transportable Array (TA) of EarthScope
Better inversion technique inherited from global inversion

45. NA Continent Formation
Hoffman, P. F. (1988)
United Plates of America, The Birth of a Craton: Early Proterozoic Assembly and Growth of Laurentia, Annu. Rev. Earth Planet. Sci., 16(1),
Karlstrom 1988
Condie et al. 1992
Thomas 2006 …
Whitmeyer & Karlstrom 2007
Big picture

46. NA Assembly Processes Whitmeyer & Karlstrom 2007 Collision of
Archean blocks
Accretion of
juvenile terranes
Rifting along
margins

47. NA Continent Assembly & Evolution
Whitmeyer & Karlstrom 2007
Collision of
Archean blocks
Accretion of
juvenile terranes
Rifting along
margins

48. NA Continent Assembly & Evolution
Whitmeyer & Karlstrom 2007
Collision of
Archean blocks
Accretion of
juvenile terranes
Rifting along
margins
Rich history makes it ideal for studying continental tectonics through various means e.g. seismology

49. USArray of EarthScope: Ubiquitous Coverage of the Continent
A network of seismometers deployed across the U.S. to record earthquakes and provide high-resolution images of the continent's structure and the Earth's deep interior.
“A network of seismometers deployed across the U.S. to record earthquakes and provide high-resolution images of the continent's structure and the Earth's deep interior.”

50. NA Station Coverage IRIS DMC Canadian Geological Survey; GEOSCOPE
>20years
>2000 Stations
Most dense
broadband coverage

51. Anisotropy layering: craton wide feature
All over the craton
Continuous lines: % Fo (Mg) from
Griffin et al. 2004
Grey: Fo%93
black: Fo%92

52. Surface wave and receiver function MLD
LAB
MLD
LAB in the WUS
MLD in the craton
Nearly same depth (gray bar)!
People feel confused when

53. LAB or MLD Kumar et al SRL in press

54. Velocity matters In WUS: LAB on top of asthenosphere
MLD
In WUS: LAB on top of asthenosphere
In Craton: MLD in the middle of high Vs lid
Need to consider velocity!
People feel confused when
Surface wave model Yuan et al 2011