A Bound on Mantle Shear Modulus from Wave Speeds and the Density Ceiling

This study computes the mantle shear modulus from stated inputs and places the result against both the published observational value and the density ceiling of Einstein-Cartan torsion gravity, in which spin-sourced torsion caps the effective density of neutron matter near 9.72e57 kilograms per cubic metre. The computation gives: largest layer shear modulus 293 GPa; stress at a coseismic strain of one part in ten thousand 29 MPa; absolute intact-rock stress bound 2.9 GPa; typical stress drop as a fraction of the modulus 1.71e-05. The standard geophysical account reproduces the measured behaviour of the silicate mantle within the uncertainty of its inputs, and the largest correction the ceiling could contribute at terrestrial densities is far below the precision of the observations, so the measurement is consistent with the frame while also naming the threshold at which it would cease to be. The comparison separates two questions that are often merged: how stiff the mantle is, which seismology answers well, and how much shear stress it can store before failing, which is set by friction and damage rather than by elasticity and is far below any ceiling derived from density alone. The study is a computation from stated assumptions, not a new measurement.