P, S, Love, and Rayleigh waves — Earth's interior revealed by seismology
P-waves (compressional, vP ≈ 6–8 km/s) travel through solids and liquids; S-waves (shear, vS ≈ 3.5–4.5 km/s) only through solids — their absence in Earth's shadow zone revealed the liquid outer core (Oldham 1906, Gutenberg 1914). Surface waves Love and Rayleigh carry most earthquake energy. The ratio vP/vS = √(κ/μ+4/3) determines Poisson's ratio of the medium.