Body Waves in Earth's Interior
Earthquakes generate two types of body waves that travel through Earth's interior. P-waves (compressional) travel fastest and arrive first, while S-waves (shear) follow. Their velocities depend on the elastic moduli and density of the material they traverse, providing a direct probe of Earth's internal composition and state.
Ray Paths and Snell's Law
Seismic rays bend continuously as they descend into Earth because velocity generally increases with depth due to increasing pressure. Snell's law governs this refraction: the ray parameter p = sin(i)/v remains constant along each ray. At sharp discontinuities (Moho, core-mantle boundary), rays reflect and refract, generating distinct arrivals on seismograms.
The Shadow Zone
Earth's liquid outer core creates one of geophysics' most dramatic observations: a P-wave shadow zone between 103° and 142° angular distance, and a complete S-wave shadow beyond 103°. S-waves cannot traverse liquids (zero shear modulus), while P-waves are strongly refracted at the core-mantle boundary. Weak arrivals in the P shadow revealed the solid inner core in 1936.
Seismic Energy and Magnitude
Earthquake magnitude scales quantify the energy released at the source. The moment magnitude (Mw) is derived from seismic moment M0 = µAD, where µ is rigidity, A is fault area, and D is average displacement. The logarithmic energy-magnitude relation means a magnitude 9 megathrust releases energy comparable to several hundred million tons of TNT, while a magnitude 3 is barely felt.