Synthetic seismic anisotropy models within a slab impinging on the core-mantle boundary

Sanne Cottaar, Mingming Li, Allen K. McNamara, Barbara Romanowicz, Hans Rudolf Wenk

Research output: Contribution to journalArticle

20 Citations (Scopus)

Abstract

The lowermost few hundreds of kilometres of the Earth's mantle are elastically anisotropic; seismic velocities vary with direction of propagation and polarization. Observations of strong seismic anisotropy correlate with regions where subducted slab material is expected. In this study, we evaluate the hypothesis that crystal preferred orientation (CPO) in a slab, as it impinges on the core-mantle boundary, is the cause of the observed anisotropy. Next, we determine if fast polarization directions seen by shear waves can be mapped to directions of geodynamic flow. This approach is similar to our previous study performed for a 2-D geodynamic model. In this study, we employ a 3-D geodynamic model with temperaturedependent viscosity and kinematic velocity boundary conditions defined at the surface of the Earth to create a broad downwelling slab. Tracers track the deformation that we assume to be accommodated by dislocation creep. We evaluate the models for the presence of perovskite or post-perovskite and for different main slip systems along which dislocation creep may occur in post-perovskite [(100),(010) and (001)]-resulting in four different mineralogical models of CPO. Combining the crystal pole orientations with single crystal elastic constants results in seismically distinguishable models of seismic anisotropy. The models are evaluated against published seismic observations by analysing different anisotropic components: the radial anisotropy, the splitting for (sub-)vertical phases (i.e. azimuthal anisotropy), and the splitting for subhorizontal phases. The patterns in radial anisotropy confirm our earlier results in 2-D. Observations of radial anisotropy and splitting in subhorizontal phases are mostly consistent with our models of post-perovskite with (010)-slip and (001)-slip. Our model of (001)-slip predicts stronger splitting than for (010)-slip for horizontally propagating phases in all directions. The strongest seismic anisotropy in this model occurs where the slab impinges on the core-mantle boundary. The azimuthal anisotropy pattern for (001)-slip shows fast axis directions at the edges of the slab (sub-)parallel to flow directions, suggesting horizontal flows may be mapped out in the lowermost mantle using seismic observations.

Original languageEnglish (US)
Article numberggu244
Pages (from-to)164-177
Number of pages14
JournalGeophysical Journal International
Volume199
Issue number1
DOIs
StatePublished - 2014

Fingerprint

core-mantle boundary
seismic anisotropy
slab
slabs
Anisotropy
anisotropy
slip
perovskite
Geodynamics
geodynamics
Crystal orientation
crystal
dislocation creep
preferred orientation
Crystals
Creep
Earth mantle
polarization
Earth (planet)
Polarization

Keywords

  • Body waves
  • Composition of the mantle
  • Plasticity, diffusion, and creep
  • Rheology: mantle
  • Seismic anisotropy

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics

Cite this

Synthetic seismic anisotropy models within a slab impinging on the core-mantle boundary. / Cottaar, Sanne; Li, Mingming; McNamara, Allen K.; Romanowicz, Barbara; Wenk, Hans Rudolf.

In: Geophysical Journal International, Vol. 199, No. 1, ggu244, 2014, p. 164-177.

Research output: Contribution to journalArticle

Cottaar, Sanne ; Li, Mingming ; McNamara, Allen K. ; Romanowicz, Barbara ; Wenk, Hans Rudolf. / Synthetic seismic anisotropy models within a slab impinging on the core-mantle boundary. In: Geophysical Journal International. 2014 ; Vol. 199, No. 1. pp. 164-177.
@article{ea0599e576c046d8922577ba13a18fcf,
title = "Synthetic seismic anisotropy models within a slab impinging on the core-mantle boundary",
abstract = "The lowermost few hundreds of kilometres of the Earth's mantle are elastically anisotropic; seismic velocities vary with direction of propagation and polarization. Observations of strong seismic anisotropy correlate with regions where subducted slab material is expected. In this study, we evaluate the hypothesis that crystal preferred orientation (CPO) in a slab, as it impinges on the core-mantle boundary, is the cause of the observed anisotropy. Next, we determine if fast polarization directions seen by shear waves can be mapped to directions of geodynamic flow. This approach is similar to our previous study performed for a 2-D geodynamic model. In this study, we employ a 3-D geodynamic model with temperaturedependent viscosity and kinematic velocity boundary conditions defined at the surface of the Earth to create a broad downwelling slab. Tracers track the deformation that we assume to be accommodated by dislocation creep. We evaluate the models for the presence of perovskite or post-perovskite and for different main slip systems along which dislocation creep may occur in post-perovskite [(100),(010) and (001)]-resulting in four different mineralogical models of CPO. Combining the crystal pole orientations with single crystal elastic constants results in seismically distinguishable models of seismic anisotropy. The models are evaluated against published seismic observations by analysing different anisotropic components: the radial anisotropy, the splitting for (sub-)vertical phases (i.e. azimuthal anisotropy), and the splitting for subhorizontal phases. The patterns in radial anisotropy confirm our earlier results in 2-D. Observations of radial anisotropy and splitting in subhorizontal phases are mostly consistent with our models of post-perovskite with (010)-slip and (001)-slip. Our model of (001)-slip predicts stronger splitting than for (010)-slip for horizontally propagating phases in all directions. The strongest seismic anisotropy in this model occurs where the slab impinges on the core-mantle boundary. The azimuthal anisotropy pattern for (001)-slip shows fast axis directions at the edges of the slab (sub-)parallel to flow directions, suggesting horizontal flows may be mapped out in the lowermost mantle using seismic observations.",
keywords = "Body waves, Composition of the mantle, Plasticity, diffusion, and creep, Rheology: mantle, Seismic anisotropy",
author = "Sanne Cottaar and Mingming Li and McNamara, {Allen K.} and Barbara Romanowicz and Wenk, {Hans Rudolf}",
year = "2014",
doi = "10.1093/gji/ggu244",
language = "English (US)",
volume = "199",
pages = "164--177",
journal = "Geophysical Journal International",
issn = "0956-540X",
publisher = "Wiley-Blackwell",
number = "1",

}

TY - JOUR

T1 - Synthetic seismic anisotropy models within a slab impinging on the core-mantle boundary

AU - Cottaar, Sanne

AU - Li, Mingming

AU - McNamara, Allen K.

AU - Romanowicz, Barbara

AU - Wenk, Hans Rudolf

PY - 2014

Y1 - 2014

N2 - The lowermost few hundreds of kilometres of the Earth's mantle are elastically anisotropic; seismic velocities vary with direction of propagation and polarization. Observations of strong seismic anisotropy correlate with regions where subducted slab material is expected. In this study, we evaluate the hypothesis that crystal preferred orientation (CPO) in a slab, as it impinges on the core-mantle boundary, is the cause of the observed anisotropy. Next, we determine if fast polarization directions seen by shear waves can be mapped to directions of geodynamic flow. This approach is similar to our previous study performed for a 2-D geodynamic model. In this study, we employ a 3-D geodynamic model with temperaturedependent viscosity and kinematic velocity boundary conditions defined at the surface of the Earth to create a broad downwelling slab. Tracers track the deformation that we assume to be accommodated by dislocation creep. We evaluate the models for the presence of perovskite or post-perovskite and for different main slip systems along which dislocation creep may occur in post-perovskite [(100),(010) and (001)]-resulting in four different mineralogical models of CPO. Combining the crystal pole orientations with single crystal elastic constants results in seismically distinguishable models of seismic anisotropy. The models are evaluated against published seismic observations by analysing different anisotropic components: the radial anisotropy, the splitting for (sub-)vertical phases (i.e. azimuthal anisotropy), and the splitting for subhorizontal phases. The patterns in radial anisotropy confirm our earlier results in 2-D. Observations of radial anisotropy and splitting in subhorizontal phases are mostly consistent with our models of post-perovskite with (010)-slip and (001)-slip. Our model of (001)-slip predicts stronger splitting than for (010)-slip for horizontally propagating phases in all directions. The strongest seismic anisotropy in this model occurs where the slab impinges on the core-mantle boundary. The azimuthal anisotropy pattern for (001)-slip shows fast axis directions at the edges of the slab (sub-)parallel to flow directions, suggesting horizontal flows may be mapped out in the lowermost mantle using seismic observations.

AB - The lowermost few hundreds of kilometres of the Earth's mantle are elastically anisotropic; seismic velocities vary with direction of propagation and polarization. Observations of strong seismic anisotropy correlate with regions where subducted slab material is expected. In this study, we evaluate the hypothesis that crystal preferred orientation (CPO) in a slab, as it impinges on the core-mantle boundary, is the cause of the observed anisotropy. Next, we determine if fast polarization directions seen by shear waves can be mapped to directions of geodynamic flow. This approach is similar to our previous study performed for a 2-D geodynamic model. In this study, we employ a 3-D geodynamic model with temperaturedependent viscosity and kinematic velocity boundary conditions defined at the surface of the Earth to create a broad downwelling slab. Tracers track the deformation that we assume to be accommodated by dislocation creep. We evaluate the models for the presence of perovskite or post-perovskite and for different main slip systems along which dislocation creep may occur in post-perovskite [(100),(010) and (001)]-resulting in four different mineralogical models of CPO. Combining the crystal pole orientations with single crystal elastic constants results in seismically distinguishable models of seismic anisotropy. The models are evaluated against published seismic observations by analysing different anisotropic components: the radial anisotropy, the splitting for (sub-)vertical phases (i.e. azimuthal anisotropy), and the splitting for subhorizontal phases. The patterns in radial anisotropy confirm our earlier results in 2-D. Observations of radial anisotropy and splitting in subhorizontal phases are mostly consistent with our models of post-perovskite with (010)-slip and (001)-slip. Our model of (001)-slip predicts stronger splitting than for (010)-slip for horizontally propagating phases in all directions. The strongest seismic anisotropy in this model occurs where the slab impinges on the core-mantle boundary. The azimuthal anisotropy pattern for (001)-slip shows fast axis directions at the edges of the slab (sub-)parallel to flow directions, suggesting horizontal flows may be mapped out in the lowermost mantle using seismic observations.

KW - Body waves

KW - Composition of the mantle

KW - Plasticity, diffusion, and creep

KW - Rheology: mantle

KW - Seismic anisotropy

UR - http://www.scopus.com/inward/record.url?scp=84906806366&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=84906806366&partnerID=8YFLogxK

U2 - 10.1093/gji/ggu244

DO - 10.1093/gji/ggu244

M3 - Article

AN - SCOPUS:84906806366

VL - 199

SP - 164

EP - 177

JO - Geophysical Journal International

JF - Geophysical Journal International

SN - 0956-540X

IS - 1

M1 - ggu244

ER -