Seismic Anisotropy in Subduction Zone Geology

Seismic anisotropy in subduction zones refers to directional variations in seismic wave speed caused by the alignment of minerals and structural fabrics within the crust and mantle. As an oceanic plate bends and descends beneath an overriding plate, deformation and fluid release produce lattice-preferred orientation of anisotropic minerals such as olivine and serpentine. These fabrics record both fossil textures within the slab and active flow patterns in the overlying mantle wedge and subslab mantle. Observations of trench-parallel and trench-normal fast-velocity directions, from shallow forearc to depths exceeding 1,000 km, reveal the geometry of corner flow, slab-mantle decoupling and deep mantle convection. Seismic anisotropy thus offers a unique window into plate dynamics, thermal structure and fluid transport, with implications for earthquake segmentation, volcanic arc development and global mass–heat transfer.

Research from Nature Portfolio

Laboratory measurements of antigorite at high pressures and temperatures demonstrate that its thermal conductivity is highly anisotropic, with conductivity along the crystal c-axis three to four times lower than along the b-axis. Numerical models show that when antigorite’s low-conductivity axis is oriented normal to slab dip, heat concentrates in the upper portion of the slab, promoting dehydration embrittlement and driving the upper plane of double seismic zones. This anisotropic heat transport may also trap frictional heat in shear zones, favouring thermal runaway as a mechanism for intermediate-depth earthquakes.

Tomographic imaging of P-wave azimuthal anisotropy beneath the Philippine Sea Plate reveals pronounced N–S fast-velocity directions at 700–900 km depth. These patterns are interpreted as relic flow fabrics of early Cenozoic Pacific lower mantle circulation and identify isolated high-velocity anomalies consistent with remnants of the Izanagi slab. The results provide evidence for long-lived deep mantle deformation mechanisms that persist away from active subduction interfaces.

Seismic Anisotropy in Subduction Zone Geology publication trend

The graph below shows the total number of articles in seismic anisotropy in subduction zone geology across all publications each year (not limited to Nature Index journals).

Technical terms

Seismic anisotropy: Variation of seismic wave speed with direction due to aligned minerals or structural fabrics.

Shear-wave splitting: The separation of a shear wave into fast and slow polarisation components as it traverses an anisotropic medium.

Azimuthal anisotropy: Directional dependence of seismic velocity within a horizontal plane, revealing flow or fabric orientation.

Lattice-preferred orientation (LPO): Alignment of crystal axes in minerals induced by deformation or flow.

Mantle wedge: The region of asthenospheric mantle above the subducting slab and below the volcanic arc, characterised by corner flow.

Slab-mantle decoupling: Mechanical separation between the subducting slab and overlying mantle wedge, inferred from reduced anisotropy.

References

1. Anisotropic thermal conductivity of antigorite along slab subduction impacts seismicity of intermediate-depth earthquakes. Nature Communications (2024).
2. Remnants of shifting early Cenozoic Pacific lower mantle flow imaged beneath the Philippine Sea Plate. Nature Geoscience (2024).
3. Mantle flow underneath the South China Sea revealed by seismic anisotropy. National Science Review (2023).
4. P and S Wave Anisotropic Tomography of the Banda Subduction Zone. Geophysical Research Letters (2023).
5. Azimuthal Anisotropy Tomography of the Southeast Asia Subduction System. Journal of Geophysical Research: Solid Earth (2022).