A model links deep mantle reflections to aligned minerals
Some seismic reflections beneath subduction zones could arise from mineral alignment rather than a sharp change in composition. A single peer-reviewed simulation study links depth-dependent deformation of bridgmanite to structures near a thousand kilometres below the surface. Modelled wave signals resemble some existing observations beneath western Java, but assumptions about mineral contributions and flow geometry limit how broadly the explanation can be applied.
Science··Morning
Crystal orientation offers a mechanism for deep reflections
Seismic reflections beneath descending tectonic plates may sometimes emerge from aligned crystals without a sharp boundary in chemical composition. A single peer-reviewed simulation study published on October 5 explores that possibility for bridgmanite, a major mineral of the lower mantle. The calculations combine plate movement, mineral deformation and wave propagation to investigate existing observations of structures at middle-mantle depths.[1]
Depth-dependent deformation changes wave speeds
Crystals sharing a preferred orientation can transmit waves at different speeds in different directions, a property called anisotropy. Pressure changes the way bridgmanite deforms around a descending slab in the model. The resulting texture produces reflectors near a thousand kilometres depth and deeper in some strongly deformed regions. Calculated fast and slow shear-wave speeds differ by about 1.5 per cent. The proposed physical mechanism complements explanations involving phase changes, chemistry or temperature.[1]
Modelled waves resemble some western Java observations
Synthetic wave records calculated from the crystal textures test whether the reflections could be detected. Comparison with existing seismic measurements beneath western Java, in Indonesia, gives qualitative agreement for some directional features. The model reproduces measured delay times unevenly. A reflector’s visibility also varies with wave frequency, numerical resolution and how sharply deformation changes.[1]
A two-dimensional flow calculation supplies the underlying model, with a third dimension added for calculating waves. Bridgmanite supplies the dominant directional contribution, while the directional contributions of other mineral phases receive a different treatment. These assumptions constrain the interpretation of where alignment offers a possible boundary mechanism. The work calculates a possible origin for reflections rather than recovering a deep-mantle sample or predicting an earthquake.[1]
