Elasticity of Hydrous SiO₂ Across the Post‐Stishovite Transition in the Lower Mantle

Elastic properties of Si 0.95 H 0.21 O 2 with hydrogarnet substitution (4H + = Si 4+ ) across the post‐stishovite transition (28–42 GPa) are determined up to 70 GPa using ab initio calculations and a pseudo‐proper type Landau model. At 28 GPa, elastic coefficients C 11 and C 12 converge, and the average shear and compressional velocity ( V S and V P ) decrease by a maximum of 25.5% and 5.2%, respectively. Hydrogarnet substitution reduces ambient elastic moduli and sound velocities, and shifts shear softening to lower pressure. 2%–13% Si 0.95 H 0.21 O 2 may cause a V S anomaly of −0.5% to −2.6% at 700–820 km depth, explaining low V S layers beneath North America and the European Alps. Additionally, 20 vol % SiO 2 in subducted basalt, with decreasing water content from 3.2 wt% to zero, could cause a V S anomaly of up to −7(4) % from 700 to 1,900 km depth, aligning with seismic scatterers identified in some subduction regions.