Researchers have identified hydrous minerals in Earth's mantle as a likely source of seismic anisotropy detected beneath subduction zones, resolving a long-standing mystery about why seismic waves travel at different speeds through the planet's interior.
Seismic anisotropy occurs when earthquake waves propagate faster in some directions than others through rock. Scientists have observed this phenomenon consistently beneath subduction zones, particularly near stagnant slabs that sink into the mantle transition zone and the uppermost lower mantle. Despite decades of study, the physical mechanism producing these signals remained unclear.
The breakthrough centers on water-rich minerals that persist in subducting oceanic plates as they sink deep into the mantle. These hydrous minerals possess crystalline structures with directional properties that align with the slab geometry. When seismic waves pass through these oriented minerals, they encounter different crystal orientations depending on travel direction, causing the velocity variations characteristic of anisotropy.
This discovery addresses a fundamental gap in understanding mantle dynamics and plate tectonics. Stagnant slabs represent oceanic plates that have descended into the transition zone but failed to sink further into the lower mantle, instead remaining in a metastable state. The prevalence of anisotropy signals in these regions suggests that hydrous minerals play a crucial role in the mechanical and chemical properties of subducting slabs.
The findings carry implications for geodynamic models and earthquake monitoring. Seismic anisotropy provides a window into slab composition and orientation, information valuable for understanding where subducting plates stall and how they eventually resume descent. Better characterization of these zones improves interpretation of global seismic networks and enhances our understanding of deep mantle convection processes that drive plate tectonics.
Future research will likely focus on quantifying the relationship between hydrous mineral abundance and anisotropy strength