Seismic waves

Seismic waves are vibrations that propagate through Earth or along its surface. Earthquakes generate them as released energy travels away from a rupture; explosions and other impulses can also produce seismic waves. Seismographs record ground motion, and the arrival patterns of the waves provide evidence about both the source and the materials they pass through.

Body waves travel through Earth's interior. Primary, or P, waves are compressional: particles move back and forth along the direction the wave travels. They are the fastest common seismic waves and arrive first at a station. Secondary, or S, waves are shear waves, with particle motion perpendicular to the direction of travel; they arrive after P waves and do not propagate through liquids. The absence of direct S waves through Earth's liquid outer core is among the evidence used to infer its state.

Love and Rayleigh waves travel along or near Earth's surface. Love waves move the ground horizontally, perpendicular to the direction the wave travels; Rayleigh waves produce a rolling, elliptical particle motion. Surface waves generally arrive after body waves at distant stations and can produce large ground motion. Their behavior depends on the structure of the near-surface layers.

Differences in wave speed and arrival time allow seismic stations to estimate how far away an earthquake occurred; measurements from several stations can help locate its epicenter. Waves change speed and direction, reflect, refract, or convert to other wave types when they encounter boundaries between materials. Patterns such as seismic shadow zones reveal information about Earth's internal structure. Seismic-wave measurements are also used in geological and engineering surveys.