Seismic Wave Speeds

Geophysics · 1828

Equation: v_p = √((K + 4μ/3)/ρ) · v_s = √(μ/ρ)
Field: Geophysics
Named After: Siméon Denis Poisson
Year: 1828

Symbols

v_p compressional (P) wave speed · v_s shear (S) wave speed · K bulk modulus (incompressibility) · μ shear modulus (rigidity) · ρ density · liquids have μ = 0, so no S-waves

Significance

P-waves arrive first and pass through anything; S-waves arrive second and cannot cross liquid — and their shadow on the far side of earthquakes is how we know Earth's outer core is molten. Timing the two arrivals locates quakes and X-rays the planet's interior, layer by layer.