Theory of the magnetic susceptibility of stressed rock

Abstract

The stress-dependence of magnetic susceptibility of igneous rock is calculated from the multidomain theory of magnetic grains, assuming that the magnetic mineral is pure magnetite whose saturation magnetization, magnetocrystalline anisotropy and saturation magnetostriction are known. The susceptibility χ measured in the direction of a linear compression σ is given by d(ln χ)/dσ = −1.0y × 10⁻⁴ cm²/kg, in excellent agreement with measurements on a number of rocks by S. P. Kapitsa who obtained values between −0.8 × 10⁻⁴ and −1.3 × 10⁻⁴ cm²/kg. The stress-dependence of susceptibility is sufficient to cause readily observable local geomagnetic disturbances during earthquakes.