When a fault ruptures, energy travels away from the source as seismic waves. Seismometers identify different arrivals because each wave type moves through the Earth in a different way. Those arrival times help locate earthquakes, while their amplitudes and frequencies help characterize the source and expected ground motion.
P waves: the fastest arrivals
Primary, or P, waves compress and expand material in the direction they travel. They move through solids and fluids and are usually the first earthquake waves recorded at a station. The time difference between P and later arrivals is one of the classic clues used to estimate distance from an earthquake.
S waves: slower shear motion
Secondary, or S, waves move material perpendicular to their direction of travel and do not propagate through liquids. They arrive after P waves and often produce stronger side-to-side or vertical motion. The absence of S waves through the Earth’s outer core was historically important evidence that the outer core is liquid.
Surface waves and local ground response
Surface waves travel along the Earth’s exterior and can have large amplitudes and long durations, especially for major earthquakes. Near the surface, soft sediments may amplify or prolong certain frequencies. Buildings respond differently depending on their height, stiffness and design, so the same incoming wave field can produce different consequences across a city.
Frequently asked questions
Which seismic wave arrives first?
P waves generally arrive first because they travel faster than S and surface waves.
Why can earthquake early warning work?
A system can detect fast P waves and estimate an earthquake before slower, stronger shaking reaches locations farther from the source.
Do all seismic waves cause the same motion?
No. Different wave types have different particle motions, speeds, frequencies and paths.