Lateral variations in lowermost mantle shear wave anisotropy beneath the north Pacific and Alaska

Garnero, Edward J.; Lay, Thorne

doi:10.1029/96jb03830

Cited by 113 publications

(80 citation statements)

References 66 publications

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“…Positive shear wave splitting ( V SH > V SV ) has been detected in several high D

^{′′}

shear velocity regions underlying present or past subduction zones, such as beneath the Caribbean, Alaska, and North Siberia (e.g., Garnero & Lay, 1997; Kendall & Silver, 1996; Thomas & Kendall, 2002). Thus, many studies attribute it to positive D

^{′′}

radial anisotropy due to slab deformation and/or the collision of slabs with the CMB.…”

Section: Discussionmentioning

confidence: 99%

“…Most observations of anisotropy suggest that V SH is higher than V SV in regions of D

^{′′}

where the shear velocity is relatively high (see Nowacki et al, 2011, for a recent review). These include the D

^{′′}

region beneath Alaska (e.g., Garnero & Lay, 1997; Wysession et al, 1999), the Caribbean (Kendall & Silver, 1996), the Indian Ocean (Ritsema, 2000), and Siberia (Thomas & Kendall, 2002). The pattern of anisotropy is more complex within the LLSVPs and the transition zones between LLSVPs and the high‐velocity regions of D

^{′′}

.…”

Section: Introductionmentioning

confidence: 99%

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Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

2018

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Observations of shear wave anisotropy are key for understanding the mineralogical structure and flow in the mantle. Several researchers have reported the presence of seismic anisotropy in the lowermost 150–250 km of the mantle (i.e., D ′′ layer), based on differences in the arrival times of vertically (SV) and horizontally (SH) polarized shear waves. By computing waveforms at a period > 6 s for a wide range of 1‐D and 3‐D Earth structures, we illustrate that a time shift (i.e., apparent splitting) between SV and SH may appear in purely isotropic simulations. This may be misinterpreted as shear wave anisotropy. For near‐surface earthquakes, apparent shear wave splitting can result from the interference of S with the surface reflection sS. For deep earthquakes, apparent splitting can be due to the S wave triplication in D ′′, reflections off discontinuities in the upper mantle, and 3‐D heterogeneity. The wave effects due to anomalous isotropic structure may not be easily distinguished from purely anisotropic effects if the analysis does not involve full waveform simulations.

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“…Positive shear wave splitting ( V SH > V SV ) has been detected in several high D

^{′′}

^{′′}

radial anisotropy due to slab deformation and/or the collision of slabs with the CMB.…”

Section: Discussionmentioning

confidence: 99%

“…Most observations of anisotropy suggest that V SH is higher than V SV in regions of D

^{′′}

where the shear velocity is relatively high (see Nowacki et al, 2011, for a recent review). These include the D

^{′′}

^{′′}

.…”

Section: Introductionmentioning

confidence: 99%

Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

2018

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“…[13] The transverse isotropy D 00 layer with high velocity anomaly is possibly caused by the lattice preferred orientation in the slab material descending into the lower mantle [Kendall and Silver, 1996;Garnero and Lay, 1997]. In the Antarctic Ocean region, Richards and Engebretson [1992] propose that the subduction has started $180 Ma, indicating the anisotropy of shear wave velocity can be attributed to the paleo-slab material in the D 00 layer beneath the Antarctic Ocean.…”

Section: Discussionmentioning

confidence: 99%

“…The D 00 layer beneath Alaska and the Caribbean Sea regions show shear wave anisotropy [Kendall and Silver, 1996;Garnero and Lay, 1997]. S waves passing through the D 00 layer beneath these regions show arrivals of longitudinal (SV) components several seconds later than those of the transverse (SH) components, indicating that there is a 2 -3% velocity difference between SV and SH waves.…”

Section: Introductionmentioning

confidence: 99%

Thick and anisotropic D″ layer beneath Antarctic Ocean

Usui

Hiramatsu

Furumoto

et al. 2005

Geophysical Research Letters

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Waveform modeling and travel times analyses of S, ScS and SKS phases recorded at the broad‐band permanent station SYO in the Antarctic are used to determine the shear wave velocity structure and transverse isotropy in the D″ layer beneath the Antarctic Ocean. The SH wave structure has a discontinuity with the velocity increase of 2.0% at 2550 km. The SV structure is similar to PREM model. The magnitude of the anisotropy is highest at the top of D″ layer and lowest at the core‐mantle boundary. The D″ layer beneath the Antarctic Ocean is significantly thicker than those beneath Alaska and the Caribbean Sea. We attribute this anisotropic D″ layer to paleo‐slab materials. The subduction in and around the Antarctic Ocean has started ∼180 Ma and is the one of the oldest in the world. It has provided a large amount of the slab materials in the lowermost mantle.

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“…These are associated either with topography of the D 00 discontinuity or lateral variations in seismic velocities in the D 00 layer [Weber, 1993;Lay et al, 1997;Liu et al, 1998]. Anisotropy has also been established in many regions of the D 00 layer [Lay et al, 1998b;Kendall and Silver, 1996;Matzel et al, 1996;Garnero and Lay, 1997]. This is in a drastic contrast with the mantle above it, which is isotropic [Meade et al, 1995] and is characterized by heterogeneities of much larger scale and smaller amplitude, most likely associated with the subducting plates [Grand, 1994;Grand et al, 1997;van der Hilst et al, 1997].…”

Section: Introductionmentioning

confidence: 96%

Small‐scale convection in the D″ layer

Solomatov

Moresi

2002

J. Geophys. Res.

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[1] Small-scale convection has been suggested as a possible explanation for seismic heterogeneities in the D 00 layer of the Earth's mantle. Recently developed scaling laws for convection with realistic viscosities allow quantitative assessment of this hypothesis. Large temperature and viscosity contrasts across the thermal boundary layer at the core-mantle boundary suggest that small-scale convection starts at the bottom of the thermal boundary layer and propagates upward before the thermal boundary layer as a whole becomes unstable. The convection boundary is likely to become a chemical boundary as a result of mixing within the convective layer. This implies that the D 00 discontinuity can represent both convective and chemical boundary and that the presence or absence of small-scale convection can be responsible for the observed intermittent nature of the D 00 discontinuity. Most lateral heterogeneities in the D 00 layer are concentrated near the convection boundary, consistent with seismic data. The length scale of lateral temperature variations, the topography of the core-mantle boundary, and the viscosity of the D 00 layer are in agreement with observational constraints. The thickness of the secondary thermal boundary layer formed at the bottom of the convective layer is similar to the thickness of the ultralow-velocity zone. The magnitude of lateral variations in temperature can only marginally be responsible for lateral variations in seismic velocities. Variations in the topography of the convection boundary and chemical heterogeneities are likely to be more important factors. A large seismic velocity drop in the ultralow-velocity zone cannot be explained by thermal effects alone and must be caused by other factors such as partial melting.

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Lateral variations in lowermost mantle shear wave anisotropy beneath the north Pacific and Alaska

Cited by 113 publications

References 66 publications

Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

Thick and anisotropic D″ layer beneath Antarctic Ocean

Small‐scale convection in the D″ layer

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