Andy Nowacki

Deformation of the Lowermost Mantle from Seismic Anisotropy

The lowermost part of the Earth’s mantle—known as D″—shows significant seismic anisotropy, the variation of seismic wave speed with direction. This is probably due to deformation-induced alignment of MgSiO3-post-perovskite (ppv), which is believed to be the main mineral phase present in the region. If this is the case, then previous measurements of D″ anisotropy, which are generally made in one direction only, are insufficient to distinguish candidate mechanisms of slip in ppv because the mineral is orthorhombic. Here we measure anisotropy in D″ beneath North and Central America, where material from subducting oceanic slabs impinges on the core–mantle boundary, using shallow as well as deep earthquakes to increase the azimuthal coverage in D″. We make more than 700 individual measurements of shear wave splitting in D″ in three regions from two different azimuths in each case. We show that the previously assumed case of vertical transverse isotropy (where wave speed shows no azimuthal variation) is not possible, and that more complicated mechanisms must be involved. We test the fit of different MgSiO3-ppv deformation mechanisms to our results and find that shear on (001) is most consistent with observations and the expected shear above the core–mantle boundary beneath subduction zones. With new models of mantle flow, or improved experimental determination of the dominant ppv slip systems, this method will allow us to map deformation at the core–mantle boundary and link processes in D″, such as plume initiation, to the rest of the mantle.

Elastic anisotropy of D″ predicted from global models of mantle flow

In order to test the hypothesis that seismic anisotropy in the lowermost mantle is caused by the development of a post-perovskite lattice preferred orientation, and that anisotropy can thus be used as a probe of the dynamics of the mantles lower boundary layer, an integrated model of texture generation in D″ is developed. This is used to predict the elastic anisotropy of the lowermost mantle as probed by global anisotropic tomographic inversions. The model combines the current 3D mantle flow field with simulations of the deformation of post-perovskite polycrystalline aggregates. Different descriptions of single crystal plasticity can lead to model results which are anti-correlated to each other. In models where post-perovskite deformation is accommodated by dislocations moving on (010) or (100), patterns of anisotropy are approximately correlated with the results of tomographic inversions. On the other hand, in models where dislocations move on (001) patterns of anisotropy are nearly anti-correlated with tomographic inversions. If all the seismic anisotropy in D″ extracted from global anisotropic inversions is due to the presence of a lattice preferred orientation in post-perovskite in the lowermost mantle, and if the results of the tomographic inversions are not strongly biased by the sampling geometries, these results suggest that, in contrast to ideas based on the 1D anisotropic signal, deformation of post-perovskite in the lowermost mantle may be accommodated by dislocations moving on (010) or (100). Alternatively, a significant portion of the anisotropic signal may be caused by mechanisms other than the alignment of post-perovskite crystals.

Mid-mantle anisotropy in subduction zones and deep water transport

The Earths transition zone has until recently been assumed to be seismically isotropic. Increasingly, however, evidence suggests that ordering of material over seismic wavelengths occurs there, but it is unclear what causes this. We use the method of source-side shear wave splitting to examine the anisotropy surrounding earthquakes deeper than 200 km in slabs around the globe. We find significant amounts of splitting (≤2.4 s), confirming that the transition zone is anisotropic here. However, there is no decrease in the amount of splitting with depth, as would be the case for a metastable tongue of olivine which thins with depth, suggesting this is not the cause. The amount of splitting does not appear to be consistent with processes in the ambient mantle, such as lattice-preferred orientation development in wadsleyite, ringwoodite, or MgSiO3-perovskite. We invert for the orientation of several mechanisms—subject to uncertainties in mineralogy and deformation—and the best fit is given by updip flattening in a style of anisotropy common to hydrous phases and layered inclusions. We suggest that highly anisotropic hydrous phases or hydrated layering is a likely cause of anisotropy within the slab, implying significant water transport from the surface down to at least 660 km depth.

Microseismic Full Waveform Modeling in Anisotropic Media with Moment Tensor Implementation

Seismic anisotropy which is common in shale and fractured rocks will cause travel-time and amplitude discrepancy in different propagation directions. For microseismic monitoring which is often implemented in shale or fractured rocks, seismic anisotropy needs to be carefully accounted for in source location and mechanism determination. We have developed an efficient finite-difference full waveform modeling tool with an arbitrary moment tensor source. The modeling tool is suitable for simulating wave propagation in anisotropic media for microseismic monitoring. As both dislocation and non-double-couple source are often observed in microseismic monitoring, an arbitrary moment tensor source is implemented in our forward modeling tool. The increments of shear stress are equally distributed on the staggered grid to implement an accurate and symmetric moment tensor source. Our modeling tool provides an efficient way to obtain the Green’s function in anisotropic media, which is the key of anisotropic moment tensor inversion and source mechanism characterization in microseismic monitoring. In our research, wavefields in anisotropic media have been carefully simulated and analyzed in both surface array and downhole array. The variation characteristics of travel-time and amplitude of direct P- and S-wave in vertical transverse isotropic media and horizontal transverse isotropic media are distinct, thus providing a feasible way to distinguish and identify the anisotropic type of the subsurface. Analyzing the travel-times and amplitudes of the microseismic data is a feasible way to estimate the orientation and density of the induced cracks in hydraulic fracturing. Our anisotropic modeling tool can be used to generate and analyze microseismic full wavefield with full moment tensor source in anisotropic media, which can help promote the anisotropic interpretation and inversion of field data.

Can seismic anisotropy in D″ be used to constrain flow patterns in the lowermost mantle?

Observations of shear wave splitting in ScS phases reveal the signature of strong seismic anisotropy in D′′ and suggest that it may be possible to use seismology to probe the dynamics of the lowermost mantle. A key step in this quest is to establish the mechanism leading to anisotropy in D′′ and show how this is linked to mantle flow. In this study we test the hypothesis that the observed anisotropy under Siberia, the Caribbean and North America is caused by deformation-induced crystallographic preferred orientation by combining 3D models of current mantle flow with simulations of texture development and compare the predicted splitting with new seismic observations.

Seismic Anisotropy and Deformation in the Lowermost Mantle

The D\(^{\prime \prime }\) region, which lies in the lowermost few hundred kilometres of the mantle, is a central cog in the Earth’s heat engine, influencing convection in the underlying core and overlying mantle. In recent years dense seismic networks have revealed a wealth of information about the seismic properties of this region, which are distinct from those of the mantle above. Here we review observations of seismic anisotropy in this region. In the past it has been assumed that the region exhibits a simple form of transverse isotropy with a vertical symmetry axis (VTI anisotropy). We summarise new methodologies for characterising a more general style of anisotropy using observations from a range of azimuths. The observations can be then used to constrain the mineralogy of the region and its style of deformation by a lattice preferred orientation (LPO) of the constituent minerals. Of specific interest is the recent discovery of the stability of the post-perovskite phase in this region, which might explain many enigmatic properties of D\(^{\prime \prime }\). Mantle flow models based on density models derived from global tomographic seismic velocity models can be used to test plausible mineralogies, such as post-perovskite, and their deformation mechanisms. Here we show how linked predictions from mineral physics, geodynamical modelling and seismic observations can be used to better constrain the dynamics, mineralogy and physical properties of the lowermost mantle.

Plate Deformation from Cradle to Grave

Seismic anisotropy and deformation in the lowermost mantle.- Mantle anisotropy beneath the Earths mid-ocean ridges.- Deformation of the lowermost mantle from seismic anisotropy.- Predicting lowermost mantle anisotropy using models of mantle flow.