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A Tour of Earth’s Dynamic Mantle: A Synthesis of Seismic Velocity & Quality Factor. Presented by: Jesse Fisher Lawrence Institute of Geophysics and Planetary Physics Scripps Institution of Oceanography University of California, San Diego Presented at: University of Wisconsin, Madison

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a tour of earth s dynamic mantle a synthesis of seismic velocity quality factor

A Tour of Earth’s Dynamic Mantle: A Synthesis of Seismic Velocity & Quality Factor

Presented by: Jesse Fisher Lawrence

Institute of Geophysics and Planetary Physics

Scripps Institution of Oceanography

University of California, San Diego

Presented at: University of Wisconsin, Madison

February 3rd

collaborators
Collaborators:
  • Michael Wysession: Washington University
  • Doug Wiens: Washington University
  • Peter Shearer: Scripps
  • Guy Masters: Scripps
  • Andy Nyblade: Penn State
  • Sridhar Anandakrishnan: Penn State
talk outline
Introduction:

Definitions - e.g. Quality Factor & Attenuation

Motivation

Data Measurement:

Case Studies:

The Caribbean lower mantle thermal anomaly

Antarctic lithospheric study

North American upper mantle

3D model of the mantle

Conclusions:

Talk Outline
some definitions
Seismic Velocity ( V ): The speed at which a seismic wave travels through the Earth.

Seismic Attenuation ( t* ): The amount of energy a seismic wave looses as it travels through the Earth.

Seismic Quality Factor ( Q ): The degree to which the Earth transfers energy without attenuation.

Some Definitions
creep

Attenuation

(high temp, low stress,

& water)

Vacancy

Vacancy

Attenuation

(low temp, high stress,

& water)

Creep:
  • Creep: is a slow, time-dependent strain where energy is not recoverable. Most commonly used in viscous flow.

Diffusion Creep: Reorganization of atoms within a grain or within poor fluids between grains.

Dislocation Creep: Through recrystalization, bonds may be broken, moved, and rebuilt without reorganization of the lattice.

Vacancy Diffusion

Edge Dislocation

rational for studying attenuation
Rational for Studying Attenuation
  • Attenuation & velocity can be used to indicate different sources for anomalies.
    • Water
    • Temperature
    • Chemical

[Lawrence & Wysession., 2005: AGU Monograph in review; Karato, 2003: AGU Mohograph]

rational for studying attenuation1
Rational for Studying Attenuation
  • Anelasticity is Anelasticity:
    • Q(T,H2O)n (T ,H2O)
      • [Karato, 2003; Monograph]
  • Water increases conductivity.
    • Increases heat flow
      • [Li & Jeanloz, 1991; JGR]
  • Hydrous phases are often anisotropic
      • [Wookey & Kendall; 2004 JGR]
  • Water changes melting temperature
      • [Inoue, 1994; PEPI]

[McNamara et al., 2003: JGR]

talk outline1
Introduction:

Definitions - e.g. Quality Factor & Attenuation

Motivation

Data Measurement:

Case Studies:

The Caribbean small-scale lower mantle thermal anomaly

Antarctic lithospheric study

North American upper mantle

3D model of the mantle

Conclusions:

Talk Outline
differential measurements

Radial

Tangential

Differential Measurements
  • Travel-Time Residuals
    • Seismic Velocity
  • Attenuation
    • Quality Factor
  • Anisotropy
    • Velocity
    • Quality Factor
travel time measurement
Travel-Time Measurement

SandScSAlignment

  • Cross- Correlation
attenuation measurement
Attenuation Measurement
  • Cross- Correlation
  • Attenuation Operator

H= ScS/S

corrected travel time measurement
Corrected Travel-Time Measurement

SandScSAlignment

  • Cross-Correlation
  • Attenuation Operator
  • Repeat Cross- Correlation
talk outline2
Introduction:

Definitions - e.g. Quality Factor & Attenuation

Motivation

Data Measurement:

Case Studies:

The Caribbean lower mantle thermal anomaly

Antarctic lithospheric study

North American upper mantle

3D model of the mantle

Conclusions:

Talk Outline
caribbean anomaly
Caribbean Anomaly

Fisher et al., [2003] - GRL

caribbean anomaly1
Caribbean Anomaly

ScS Path

D: 250 km

Outer Core

Fisher et al., [2003] - GRL

the caribbean anomaly
The Caribbean Anomaly
  • Thermal Anomaly is expected at the CMB.

[after Tan & Gurnis, 2002; Grand et al., 1997]

the story so far
The Story So Far …
  • Small-scale velocity and quality factor anomalies can be isolated in the lower mantle.
  • The Caribbean anomaly is likely thermal due to heat flow perturbations at the CMB.

But we are are at

the surface …

And the anomaly

is deep in the

Earth …

Then we have

to peel back

the layers to

really see what

is inside

transantarctic mountain seismic experiement
Transantarctic Mountain Seismic Experiement
  • 42 stations spanned from the Ross Sea 1300 km into East Antarctica.

Photos

Courtesy of

Jen Curtis

antarctic lithosphere
Antarctic Lithosphere
  • Velocity & Attenuation are inversely correlated
  • 300C difference between East & West Antarctica
  • Measured crustal thickness
  • Modeled TAMs uplift as flexural response to thermal load

Lawrence et al., [2006a -JGR]

closer to home
Closer to Home:
  • New & improved technique
    • 2 weeks from start to finish not 4 months.
  • P- waves correlate with S-waves (R2 > 0.6)
  • Attenuation & travel times are less correlated (R2 < 0.3)

[Lawrence, Shearer, & Masters, 2006: in review at GRL]

the story so far1
The Story So Far …
  • Travel times and attenuation vary significantly for both upper and lower mantle.
  • Quality factor correlates with velocity indicating thermal anomalies
  • So, both upper and lower thermal boundary layers possess lateral thermal variations.

But we are are at

the surface …

And we’ve only

imaged a small

part of the

Earth

So, what about

the Earth as a

whole?

vqm3da
VQM3DA
  • V - Velocity
  • Q - Quality Factor
  • M - Whole Mantle
  • 3D - 3 Dimensional
  • A - Anisotropy

[Lawrence & Wysession, 2006: in review G-cubed]

slide25

Juliana Rokosky

Emily Carter

Tracy Portle

Kurt Solander

DATA

Data used:

†for Radial Component Only

* Uses VanDecar and Crosson, [1990]

tomography
Velocity:

Quality Factor:

Tomography

Ray Tracing:

quality factor velocity
Quality Factor&Velocity
  • Velocity has poor correlation with seismic velocity.
    • A ring of high velocity high quality factor is clear around the Pacific.
    • There is a large, very low Q anomaly between 800 and 1500 km depth.

[Lawrence & Wysession., 2006: in reviewAGU Monograph]

vqm3da v qrlw8

QRLW8:

    • From surface waves
    • Looses resolution with depth

Gung & Romanowicz [2004]

  • VQM3DA:
    • From body waves
    • Gains resolution with depth

Lawrence & Wysession [2006]

VQM3DAv.QRLW8
  • VQM3DA is more accurate in the lower mantle than in the upper mantle.
    • Even in the upper mantle it has excellent resolution.
    • In the upper mantle, the highest attenuation occurs at subduction zones due to dehydration effects.
upper mantle dehydration melt
Upper Mantle Dehydration Melt:
  • Water subducts within hydrous minerals such as serpentine
  • At 100-400 km depth the hydrous minerals become unstable
  • Water is released into the mantle above the slab
  • Peacock [1990] estimates a net flux of ~6.7  1011 kg/year water into the mantle.

+100%

-120%

dln 1/Q

[Roth & Wiens, 1999: JGR]

the transition zone water filter
The Transition Zone Water Filter:
  • Water enters mantle at subduction zones.
  • Upper mantle is generally anhydrous.
  • Transition zone sucks up the water.
  • Lower mantle is generally anhydrous.

[Bercovici et al., 2003: Nature]

hydrous phases b d
Hydrous Phases B & D:
  • Quench-type experiments:
  • Pressure:
  • 20-53 GPa
  • Temperature:
  • 800-1800°C
  • Results:
  • Phase D is stable to 42 GPa
  • or ~1400km depth.

[adapted from Shieh et al., 1998: EPSL]

the east asian anomaly
The East Asian Anomaly

Water Anomaly

Anomaly Volume:

1.8  1011 km3

Water Volume:

> 5.5  108 km3 (0.1 wt% water)

Water in the Oceans:

1.3  109 km3

[Lawrence & Wysession., 2006c: in review AGU Monograph]

consequences
Consequences:
  • Anelasticity is Anelasticity:
    • Q(T,H2O)n (T ,H2O)
      • [Karato, 2003; Monograph]
  • Water increases conductivity.
    • Increases heat flow
      • [Li & Jeanloz, 1991; JGR]
  • Hydrous phases are often anisotropic
      • [Wookey & Kendall; 2004 JGR]
  • Water changes melting temperature
      • [Inoue, 1994; PEPI]

[McNamara et al., 2003: JGR]

megaplumes

?

?

Megaplumes:

[Masters et al., 2001:

AGU Monograph]

Shear Velocity

Quality Factor

Pacific

Africa

conclusions
Conclusions
  • Quality factor & velocity are highly heterogeneous throughout the mantle on very large and very small scales.
  • Velocity and Quality Factor are often positively correlated indicating thermal anomalies.
  • Velocity and Quality Factor are often poorly correlated, requiring other sources for the anomalies.
  • Water likely plays a key role in shaping the anelastic Earth.
    • There is a second dehydration cycle in the mantle related to subduction.
  • Vertical profiles through VQM3DA are consistent with core-to-surface communications for spreading ridges and subduction zones.
so what is the anomaly
So, What is the Anomaly?
  • Temperature:
    • V & Q change a lot
    • High T is unlikely above a slab.
  • Grain Size:
    • Hard to reconcile with size & magnitude of anomaly
  • Composition:
    • Velocity variation?
    • dln Q is less than observed
  • Water:
    • V doesn’t change much.

[Lawrence & Wysession., 2006:

in review AGU Monograph;

Karato, 2003: AGU Mohograph]

the east asian anomaly1
The East Asian Anomaly

Anomaly

[Lawrence & Wysession., 2006: in review AGU Monograph]

3d rendering of vqm3da
3D Rendering of VQM3DA

Model: Isotropic Velocity

View: South:  = 30

Contour:  0.5 %

compare q r to temperature
Compare Q(r) to Temperature

Q(z) =  e[/TH(z)]

 = 2.14  1

 = 3.45  0.6

  •  and  are controlled by chemical composition and may have some pressure dependence

Lawrence and Wysession [2005a]

anisotropic velocity
Anisotropic Velocity

 = VSH2/VSV2

tomography1
Velocity:

Quality Factor:

Tomography

Inversion:

: No net perturbation

: Smoothing constraint

what is anelasticity
What is Anelasticity?
  • Anelasticity: The property of a solid indicating that deformation depends on the time and stress.

Elastic

Anelastic

Spring

Silly Putty

purely elastic anisotropy
Purely Elastic Anisotropy

Slow Direction

Fast Direction

anelastic anisotropy
Anelastic Anisotropy

Anelastic Direction

Elastic Direction

saw16b16an v vqm3da
SAW16B16AN v. VQM3DA
  • SAW16B16AN
    • Uses surface & body waves
    • Spherical Harmonics

Gung et al., [2003]

  • VQM3DA:
    • Uses only body waves
    • Gains resolution with depth

Lawrence & Wysession [2006b]

VSV-Fast

VSH-Fast

1d quality factor structure
1D Quality Factor Structure
  • S and ScS reach greater depth with distance
  • Measured 30,000 dt*ScS-S
  • Excellent global data coverage

Lawrence and Wysession [2006 EPSL]

attenuation vs distance
Attenuation vs. Distance

~27,000 Data Points

~3,000 Data Points

Lawrence and Wysession [2006 EPSL]

previous work
Previous Work

Lawrence and Wysession [2006 EPSL]

1d models
1D Models

9-Layer Model: QLM9

21-Layer Model

Lawrence and Wysession [2006 EPSL]

dt scs s r
dt*ScS-S(r)

Lawrence and Wysession [2006 EPSL]

compare q r to viscosity
Compare Q(r) to Viscosity

Lawrence and Wysession [2006 EPSL]

temperature constraints
Temperature Constraints
  • Using theoretical calculations of Karato [1993], Jackson et al., [1992, 2002] we compute a theoretical temperature profile.
  • Our model fits the rough expectation

Lawrence and Wysession [2006 EPSL]

the story so far2
The Story So Far …
  • Upper and lower mantle quality factor varies significantly laterally as well as with depth.
  • Quality factor correlates with both observed viscosity and velocity.
  • So far, the Earth appears to be largely thermally driven.

But we are are at

the surface …

And the Earth is

not 1D …

So, what does

the Earth

look like?

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