A tour of earth s dynamic mantle a synthesis of seismic velocity quality factor
This presentation is the property of its rightful owner.
Sponsored Links
1 / 56

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


  • 35 Views
  • Uploaded on
  • Presentation posted in: General

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

Download Presentation

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

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


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


Purely elastic harmonic velocity

Purely Elastic (Harmonic Velocity)


Elastic and anelastic attenuation anharmonic velocity

Elastic and Anelastic(Attenuation & Anharmonic Velocity)


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]


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

    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


    Anisotropic quality factor

    Anisotropic Quality Factor

     = QSH2/QSV2


    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?


    Other research

    Other Research


  • Login