Seismic Waves

Seismic waves are elastic waves that carry earthquake energy through the Earth. They divide into body waves (traveling through Earth's interior) and surface waves (traveling along the surface). Body waves come in two types: P-waves (Primary/Compressional): fastest, ~5-8 km/s. Particle motion is parallel to wave direction, like sound waves. P-waves travel through solids, liquids, and gases. On seismograms, P-waves arrive first. Humans feel a sudden vertical jolt. S-waves (Secondary/Shear): ~3-5 km/s. Particle motion is perpendicular to wave direction. S-waves cannot travel through liquids (key evidence that Earth's outer core is liquid). S-waves cause stronger shaking and typically cause more damage than P-waves. Surface waves travel along Earth's surface, slowest but with largest amplitudes, causing the most destruction: • Love waves: horizontal shearing, side-to-side shaking • Rayleigh waves: elliptical motion like ocean waves, rolling motion Seismologists use P-S arrival time differences to calculate epicentral distance. This is the principle behind P-S distance calculators: each second of time difference corresponds to roughly 8 km of distance.