Subsections

The Origin of Seismic Anisotropy in the D"

Sanne Cottaar, Paul Cupillard, Allen McNamara, Barbara Romanowicz, and Rudy Wenk

Introduction

The D" zone at the base of the mantle is a boundary layer, both chemically and thermally. Complex dynamic processes are the cause of lateral heterogeneities, with sharp boundaries at the edge of superplumes (e.g. Toh et al., 2005). Another characteristic of D" is the presence of strong and laterally varying anisotropy. A global long-wavelength model for S-wave velocity and radial anisotropy shows that, in general, SH phases are faster than SV phases (Panning and Romanowicz, 2006). This observation leads to the idea that flow causes anisotropy by alignment of anisotropic minerals.

The goal of this research is to investigate seismic anisotropy and sharp velocity boundaries in the D" region by combining geodynamics, mineral physics, and seismic modeling. Different seismic velocity models will be created to test hypotheses of possible microscopic and macroscopic processes. Comparing data computed for the models with real data will confirm or rule out these processes as a possibility for the lowermost mantle.

This contribution is an overview of ongoing research.

Geodynamics

The two-dimensional geodynamical model (provided by Allen McNamara) is refined to emphasize deformation in D" (McNamara and Zhong 2004). Lagrangian tracers travel through the lowermost part of the model, providing strain information. Horizontal shear deformation increases near the CMB, while vertical deformation is strong in the upwelling region. An example of development along a tracer is shown in Figure 2.46. A great number of data points of several tracers are combined to create a static (snapshot) model.

Figure 2.46: Top figure shows the depth of the Langragian tracer in the geodynamical model with time. The bottom figure shows the development of strain. Horizontal strain is higher deeper in the mantle and vertical strain increases in the plume.
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Mineral Physics

Calculating the seismic velocities from the strain requires knowing elastic constants and deformation mechanisms at the temperature and pressure conditions of D". These are measured for the most abundant minerals: perovskite ($MgSiO_3$) and periclase ($MgO$).

We predict the seismic response of a suite of different mineral phases and assemblages. Post-perovskite is proposed as one of the possible anisotropic minerals. In fast regions, the top of the D" is characterized by a velocity discontinuity, possibly caused by the pPv transition of $MgSiO_3$ (e.g. Wookey et al., 2005). For pPv we will try different elastic constants and deformation systems. Disparate results for elastic constants are found computationally by Stackhouse et al. (2005) and Wentzocovitch et al. (2006). Various slip-systems are found by theoretical work and in deformation experiments (e.g. Merkel et al., 2007). It has also been suggested that ferropericlase, $(Mg,Fe)O$, might be the dominant cause of seismic shear anisotropy (e.g. Marquardt et al., 2009).

Although other origins of anisotropy, like SPO and melt inclusions, have been proposed, they are not considered in this project.

Synthetic Data

Slices of two-dimensional velocity models extend the model in the third dimension and will be implemented in sandwiched C-SEM (Coupled Spectral Element Method, Capdeville et al.,2003). Sandwiched-CSEM couples the SEM part in the lower mantle to a computationally faster 1D normal mode solution in the rest of the Earth. Eventually, the data will be calculated down to periods of $\sim$10 seconds. The predicted waveforms will be compared to observations.

Real Data

Several phases are used to study the D". The S-wave, the core-reflected S (ScS) and the diffracted phase (Sdiff) have shown that D" is anisotropic.

Data from deep earthquakes are needed to avoid source-side anisotropy in the upper mantle. Unfortunately, the azimuthal coverage of this data is not very good. Vertical transverse isotropy (VTI) can, however, be measured along one path by the splitting of S-waves into SH and SV phases. Coverage has been greatly improved by the USArray, providing a large number of stations at the correct distance from deep earthquakes in the region of Indonesia and Fiji-Tonga. These paths cover the boundaries of the Pacific superplume.

Before measuring anisotropy in D", the phases need to be corrected for the receiver side anisotropy. The SKS has a shorter path in D", but an almost similar path through the upper mantle. Therefore, we correct the Sdiff phase with values measured for SKS at similar azimuth using (a modified version of) SplitLab (Wüstefeld et al., 2007). Figure 2.47 shows examples of how this decreases and increases anisotropy in D".

Figure 2.47: Observations for 7.1 $M_{w}$ event at 530 km depth in Mindanao, Phillipenes. Top four figures show data before and after correction for station HLID at a distance of 108$^{\circ}$. The lower four are for station A04A at 101$^{\circ}$.
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Future Work

Besides varying a number of assumptions with the models, several major improvements are needed to make the models more realistic. One of the improvements will be to use a three-dimensional model providing more realistic strains.

Possible mineral phase transitions will be implemented. This could test the occurrence of sharp horizontal and vertical velocity boundaries or transitional zones.

Acknowledgements

This project is funded by NSF’s CSEDI program under grant number NSF EAR-0757608.

References

Capdeville, Y., B. Romanowicz, and A. To, Coupling spectral elements and modes in a spherical earth: an extension to the ``sandwich'' case,Geophys. J. Int., 154, 44-57, 2003.

McNamara, A.K. and S. Zhong, The influence of thermochemical convection on the fixity of mantle plumes, EPSL 222, 485, 2004.

Merkel S, A.K. McNamara, A. Kubo, S. Speziale, L. Miyagi, Y. Meng, T. S. Duffy, H.-R. Wenk, Deformation of (Mg,Fe)SiO3 post-perovskite and D'' anisotropy, Science, 316, 1729-1732, 2007.

Marquadt, H., S. Speziale, H. J. Reichmann, D. J. Frost, F. R. Schiling and E. J. Garnero, Elastic Shear Anisotropy of Ferropericlase in Earth's Lower Mantle, Science, 324, 224, 2009.

Panning, M. and B. Romanowicz, A three dimensional radially anisotropic model of shear velocity in the whole mantle,Geophys. J. Int., 167, 361-379, 2006.

Stackhouse, S., J.P. Brodholt, J. Wookey, J.-M. Kendall and G.D. Price, The effect of temperature on the seismic anisotropy of the perovskite and post-perovskite polymorphs of MgSiO3, EPSL 230, (1-2), 1-10, 2005.

Toh, A., B. Romanowicz, Y. Capdeville and N. Takeuchi, 3D effects of sharp boundaries at the borders of the African and Pacific Superplumes: observation and modeling, Earth and Planet. Sci. Lett.,233, 137-153, 2005.

Wentzcovitch, R.M., B.B. Karki, M. Cococcioni, and S. de Gironcoli, Thermoelastic properties of MgSiO3-perovskite: insights on the nature of the Earth's lower mantle, PRL 92, doi:10.1103/PhysRevLett92.018501, 2004.

Wookey, J., S. Stackhouse, J. Kendall, J. Brodholt, and G. D. Price, Efficacy of the post-perovskite phase as an explanation for lowermost-mantle seismic properties, Nature, 438, 1004, 2005

Wüsterfeld, A., G. Bokelmann, C. Zaroli and G. Barruol, SplitLab: A shear-wave splitting environment in Matlab, Computers and Geosciences 34(5), 515-528, 2007.

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