Seismic Waves

P-waves, S-waves, and surface waves through Earth's interior

Earthquakes generate three main types of seismic waves. Primary (P) waves are compressional — particles move parallel to propagation, like sound waves. They're fastest (~6-8 km/s in crust) and travel through solids and liquids. Secondary (S) waves are shear — particles move perpendicular to propagation. Slower (~3.5-4.5 km/s in crust), they cannot pass through Earth's liquid outer core, creating the S-wave shadow zone. This shadow zone was key evidence for a liquid outer core. Surface waves (Love and Rayleigh) travel along Earth's surface and cause the most ground shaking. Love waves move horizontally; Rayleigh waves create elliptical rolling motion. Seismographs at different distances record arrival time differences (S-P interval), which determine earthquake distance. Three stations triangulate the epicenter.