P and S speeds in an isotropic elastic solid
For a homogeneous isotropic linearly elastic solid of densityρ, P waves have vP²=(λ+2μ)/ρ=(K+4μ/3)/ρ and S waves have vS²=μ/ρ.
Key knowledge
For a homogeneous isotropic linearly elastic solid of densityρ, P waves have vP²=(λ+2μ)/ρ=(K+4μ/3)/ρ and S waves have vS²=μ/ρ. The tensor-strain convention is ε_ij=(∂_iu_j+∂_ju_i)/2 with σ_ij=λtr(ε)δ_ij+2με_ij and K=λ+2μ/3. Wave propagation is distinct from particle displacement.
Values and equations
vP=√[(K+4μ/3)/ρ]; vS=√(μ/ρ); σ_ij=λtr(ε)δ_ij+2με_ij
- vP,vS
- compressional and shear phase/group speeds in the nondispersive model m s⁻¹
- ρ
- mass density kg m⁻³
- λ,μ,K,σ_ij
- Lamé coefficient, shear/bulk moduli and stress components Pa
- ε_ij,δ_ij
- small tensor strain and Kronecker delta 1
Conditions and scope
- Small strain, positive density, stable bulk modulus K>0 and shear modulusμ>0, local homogeneous isotropic coefficients, no attenuation or frequency dispersion in this model.
- A fluid with zero elastic shear modulus lacks this bulk elastic S branch; it can still support compressional sound and other types of disturbance. Anisotropy, viscoelasticity, layering, pores and fluids can change measured seismic response; speed alone does not uniquely identify composition.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Read actual printed14–16 P-wave force balance/substitution and P/S speed formulas. Used the displayed bulk-modulus identity, with the standard tensorial factor2 verified from ε definition; explicitly excluded the earlier printed13 shear-stress factor typo and the blanket claim that these isotropic speeds apply to every seismic mode/material.
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Entry details
Permanent link to this entry ↗- Record identity
- uni-earth-seismic-elastic-speeds
- Revision
- core.1
- Structured reference record
- University core expansion