The bulk modulus relates hydrostatic compressive pressure to fractional volume compression. In isotropic three-dimensional linear elasticity, the spherical part of the stress tensor is , so uniform dilation has strain energy density . Positive bulk modulus and positive shear modulus make the isotropic elastic stiffness tensor positive on nonzero symmetric strains.
For an isotropic linearly elastic solid, equal compressive principal stresses produce three equal normal strains . Adding them gives the displayed volume strain, or bulk modulus . It is a small-strain relation using the pressure change, not an arbitrary absolute pressure reference.
The pressure response to compression at fixed entropy, . Seismic compressions are commonly approximately adiabatic; a background density gradient follows the corresponding derivative only when composition and entropy assumptions also apply.

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  1. Linear elasticity
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The bulk modulus, often denoted by the symbol \( K \), is a measure of a material's resistance to uniform compression. It quantifies how incompressible a substance is; the higher the bulk modulus, the less compressible the material. Mathematically, the bulk modulus is defined as the ratio of the change in pressure to the relative change in volume of the material.