Editors’ Vox is a blog from AGU’s Publications Department.
Earthquake waves travel through Earth’s interior and can be recorded by seismometers at distances of thousands or even tens of thousands of kilometers. These seismic waves traverse different parts of the mantle at different speeds, which depends on the material they are traveling through. Sometimes, the speed of shear waves – in the same material – depends on their vibrational direction, a phenomenon called seismic anisotropy.
If seismic anisotropy is present, analogous to optical birefringence, the shear wave can split up into a fast and a slow traveling component. This primarily happens when waves travel through the lowermost mantle and upper mantle or crust. The effects of seismic anisotropy in the lowermost mantle on an SKS seismic wave are displayed in Figure 1.
A new article in Reviews of Geophysics focuses on how seismic anisotropy in the lowermost mantle can be inferred, and what it tells us about Earth. We asked the lead author to give an overview of seismic anisotropy, describe recent scientific advances, and outline what challenges remain.
What kinds of insights can observations of seismic anisotropy tell us about Earth’s deepest mantle?
Seismic anisotropy can be caused by mantle deformation due to convective flow, therefore, measurements of seismic anisotropy are often used to answer questions about the flow of material in Earth’s mantle. The lower boundary layer of mantle convection, the base of the mantle, experiences much deformation, which is often connected to the processes that are happening in the mantle above. Therefore, measurements of seismic anisotropy can potentially tell us whether cold material sinking down from above – perhaps remnants of subducted slabs – is causing deformation, or whether deformation is caused by heat induced upwellings rooted at the core-mantle boundary which may eventually become plumes.
This is important because we would like to better understand how mantle convection is happening as a whole. For example, we still do not understand the dynamic relationships between different lower mantle structures. Two large structures whose nature still remains largely unclear are two continent-sized features with below-average seismic velocities, the so-called large low-velocity provinces (LLVPs) (e.g., Garnero et al., 2016).
All previously identified locations of lowermost mantle seismic anisotropy are shown in Figure 2. Our current understanding is that these locations do not strongly correlate with any deep Earth structures, indicating that different mechanisms and deformation scenarios can cause it.
What are the main causes of seismic anisotropy?
Seismic anisotropy in Earth’s mantle can be caused by two potential mechanisms. In the deep Earth, materials may have elastic properties significantly different from the surrounding material. If their shapes are directionally oriented, so-called shape-preferred orientation can be induced, and seismic waves can travel faster in certain directions than in others (e.g., Kendall & Silver, 1998).
Additionally, deformation can lead to the preferential alignment of individual crystals in an aggregate, so-called crystallographic-preferred orientation, which can also be cause of seismic anisotropy (e.g., Kocks et al., 2000). We believe that either of these mechanisms can potentially cause seismic anisotropy in the lowermost mantle, but that crystallographic-preferred orientation may be the dominant mechanism.
Why is it hard to infer seismic anisotropy in the lowermost mantle? And what has led to recent progress?
The main challenge when studying lowermost mantle anisotropy is that the seismic waves that are commonly used for this also potentially sample upper mantle anisotropy. Therefore, we must infer where exactly the seismic anisotropy is sampled, but the measurements that we are usually making integrate the effects of seismic anisotropy over the whole raypath. This is why techniques have been developed which use seismic phases that have very similar upper mantle raypaths and different lowermost mantle raypaths. In such cases, it is often argued that if the phases show a different anisotropic signature, seismic anisotropy must be present in the lowermost mantle. In other cases, seismic waves are used that have particularly long raypaths through the lowermost mantle.
In general, all techniques to infer lowermost mantle anisotropy make a variety of assumptions, and it is important to understand which assumptions are sufficiently accurate. Such detailed tests have been made possible by recent progress in the efficient numerical computation of seismograms (e.g., Leng et al., 2016; Fernando et al., 2024), which has been applied to seismograms for Earth models that include seismic anisotropy (e.g., Nowacki and Wookey, 2016; Wolf et al., 2022). Such numerical simulations have shown which assumptions are reasonable to make and have led to the development of new techniques to measure lowermost mantle anisotropy.
What additional research, data, or modeling efforts are needed to overcome these challenges?
The presence of seismic anisotropy has been suggested in 25% of lateral lowermost mantle locations (Figure 2). However, these measurements have been made using different techniques that have various strengths and shortcomings, therefore, we still do not understand the global distribution of lowermost mantle anisotropy.
Going forward, it will be important to use uniform methods to map lowermost mantle anisotropy globally – both techniques and data for such an effort already exist. To interpret such results, it will be important to better understand the properties of lowermost mantle materials at the relevant pressures and temperatures, and to realistically implement such constraints in global models of mantle flow. This requires further progress in multiple geophysical areas. It is important that experts from these areas, which include geodynamics and mineral physics, combine their expertise in multidisciplinary approaches to illuminate the nature of the lowermost mantle.
Figure 3 (below) is a schematic diagram of the types of regions and flow scenarios that can potentially be elucidated via a combination of seismic anisotropy observations, mineral physics constraints, and geodynamic simulations. For example, using measurements of seismic anisotropy, deformation caused by subducting slabs or upwelling flow in the deepest mantle can be imaged. With future improvements of anisotropy measurements, their interpretations, and analyses of bigger seismic datasets, measurements of seismic anisotropy, combined with constraints from other fields, will shed a more detailed light on dynamic deep mantle processes and structures.
— Jonathan Wolf (jonathan.wolf@yale.edu; 
0000-0002-5440-3791), Yale University, USA
Editor’s Note: It is the policy of AGU Publications to invite the authors of articles published in Reviews of Geophysics to write a summary for Eos Editors’ Vox.
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