Global 3D Model of Mantle Attenuation: A Study Based on Seismic Normal Modes

Academic Background

The structure and dynamic processes of Earth’s interior have always been at the core of geoscientific research. Mantle convection is the primary mechanism driving plate tectonics, volcanic eruptions, and earthquakes. However, traditional seismic tomography models primarily rely on variations in seismic wave velocities, making it difficult to distinguish between thermal and compositional origins of mantle structures. Temperature and compositional changes typically affect compressional and shear wave velocities in the same proportion, which limits the ability of velocity-based models to interpret mantle structures.

To further understand the evolution of mantle convection, researchers need to incorporate seismic attenuation data. Attenuation refers to the intrinsic loss of energy as seismic waves propagate through Earth’s interior. It is sensitive to physical properties such as temperature, partial melting, and grain size but less responsive to compositional changes. Therefore, attenuation data can provide new constraints on whether mantle structures are thermally or compositionally driven. However, current global three-dimensional (3D) attenuation models are mainly focused on the upper mantle, while research on lower mantle attenuation remains limited.

Source of the Paper

This paper was co-authored by Sujania Talavera-Soza, Laura Cobden, Ulrich H. Faul, and Arwen Deuss, affiliated with Utrecht University, Massachusetts Institute of Technology, and Vassar College, respectively. The paper was published in Nature in 2024, titled Global 3D Model of Mantle Attenuation Using Seismic Normal Modes.

Research Process and Results

1. Research Objectives and Methods

The goal of this study was to construct a global 3D attenuation model of the mantle using seismic normal modes to reveal the thermal and compositional origins of mantle structures. Normal modes are the vibrational modes of the Earth as a whole and can capture large-scale structural variations within the Earth. Unlike body waves and surface waves, normal modes can simultaneously measure elastic and anelastic structural changes, thereby avoiding errors caused by energy redistribution (e.g., focusing and scattering) in traditional methods.

The study employed a two-step inversion approach: 1. Step One: Splitting functions were measured by inverting normal-mode spectra. Splitting functions describe the frequency splitting phenomenon caused by lateral heterogeneity within the Earth. The researchers analyzed spectral data from 104 major earthquakes and obtained 16 anelastic splitting functions. 2. Step Two: These splitting functions were used to construct a 3D attenuation model. The researchers parameterized the model using spherical harmonics and B-splines, ultimately producing a 3D attenuation model encompassing both the upper and lower mantle.

2. Key Findings

Attenuation Characteristics of the Upper Mantle

In the upper mantle, the study found that regions of high attenuation are strongly correlated with regions of low wave velocity, indicating that thermal sources dominate these areas. For example, the mantle beneath mid-ocean ridges exhibits high attenuation and low wave velocity, consistent with previous research. These results suggest that attenuation in the upper mantle is primarily driven by temperature variations.

Attenuation Characteristics of the Lower Mantle

In the lower mantle, the study observed the opposite phenomenon: regions of high attenuation are found in the circum-Pacific region, where seismic wave velocities are higher. Conversely, Large Low Shear Velocity Provinces (LLSVPs) exhibit low attenuation. By comparing these observations with laboratory data, the researchers hypothesized that the circum-Pacific region may be relatively cold with smaller grain sizes, while LLSVPs are hotter with larger grain sizes.

Mineral Physics Modeling

To further explain these observations, the researchers performed calculations using a viscoelastic model based on laboratory data. The results showed that increasing temperature or decreasing grain size reduces shear wave velocity and increases attenuation. The high attenuation and high wave velocity in the circum-Pacific region can be explained by lower temperatures and smaller grain sizes, while the low attenuation and low wave velocity in LLSVPs can be attributed to higher temperatures and larger grain sizes.

3. Conclusions and Significance

This study is the first to construct a global 3D attenuation model of the mantle, revealing significant differences in attenuation characteristics between the upper and lower mantle. The results indicate that attenuation in the upper mantle is primarily driven by temperature variations, while attenuation in the lower mantle is influenced by both temperature and grain size. These findings provide new insights into the evolution of mantle convection, particularly the role of LLSVPs as long-term stable “mantle anchors.”

4. Research Highlights

• Global 3D Attenuation Model: The first global 3D attenuation model encompassing both the upper and lower mantle, filling a gap in existing research.
• Application of Normal Modes: The use of seismic normal modes to measure attenuation avoids errors caused by energy redistribution in traditional methods.
• Integration of Mineral Physics Models: Laboratory data and viscoelastic models reveal the influence of temperature and grain size on mantle attenuation.

5. Other Valuable Information

The study also notes that the high viscosity and coarse grain size of LLSVPs make them long-term stable mantle structures, possibly related to the crystallization process of Earth’s early magma ocean. Additionally, the high attenuation in the circum-Pacific region may be associated with the accumulation of subducting slabs, which undergo phase transformations as they enter the lower mantle, leading to grain size readjustment.

Summary

By combining seismic normal modes with mineral physics models, this study reveals the 3D structure of global mantle attenuation and its underlying physical mechanisms. These findings not only deepen our understanding of mantle convection evolution but also provide new directions for future geodynamic research.