Seismic waves are vibrations that travel through Earth, resulting from events like earthquakes or large explosions. Seismologists classify this energy into two main categories based on their travel path: Body waves and Surface waves. Understanding these wave types is fundamental to interpreting the mechanics of an earthquake, from the initial rupture deep underground to the destructive shaking felt at the surface. This classification provides a framework for analyzing the speed, motion, and impact of seismic energy as it propagates through Earth’s structure.
Body Waves: Movement and Subtypes
Body waves travel directly through the interior of the Earth. These waves are the first to be recorded by seismographs because they travel the fastest through the planet’s dense, deep layers. Their speed and capacity to penetrate the Earth’s interior make them invaluable for mapping the planet’s internal structure.
The Primary wave, or P-wave, is the fastest type and moves with a compressional, push-pull motion. The particles of the medium vibrate parallel to the direction the wave is traveling, similar to sound waves. This compressional motion allows P-waves to travel through solids, liquids, and gases, including Earth’s molten outer core. They are felt as a subtle, initial jolt and are less destructive than the waves that follow.
The Secondary wave, or S-wave, arrives after the P-wave because it travels more slowly. S-waves move with a shear motion, causing particles to vibrate perpendicular to the direction of wave travel, shaking the ground up-and-down or side-to-side. S-waves can only propagate through solid materials. Their inability to travel through liquids provided the initial scientific evidence that Earth possesses a liquid outer core.
Surface Waves: Movement and Subtypes
Surface waves are generated when body waves reach the uppermost layers of the Earth and are confined to travel along the surface. Because their energy is concentrated in this shallow zone, their amplitude is significantly greater than that of body waves. These waves are slower than body waves, arriving last at a seismic station, but are responsible for the most intense ground shaking.
The Love wave, named after A.E.H. Love, moves the ground with a horizontal shearing motion. This causes the surface to shift from side to side perpendicular to the wave’s path. This transverse motion is particularly effective at damaging building foundations and twisting structures, as most buildings are not designed to withstand strong lateral forces. Love waves travel slightly faster than Rayleigh waves.
The Rayleigh wave, named for Lord Rayleigh, produces a rolling, retrograde elliptical motion, similar to the movement of an ocean wave. This motion combines both horizontal and vertical ground displacement, making them highly destructive. Because Rayleigh waves cause the ground to move in multiple directions at once, they contribute significantly to the prolonged, violent shaking experienced during an earthquake.
Comparing Velocity, Amplitude, and Destructiveness
The speed hierarchy is consistent: P-waves are the fastest, followed by S-waves, and finally, Surface waves (Love and Rayleigh) are the slowest. This velocity difference is the basis for earthquake early warning systems, as the detection of the fast, less damaging P-waves provides a short window of time before the arrival of the slower, more destructive waves.
Body waves possess a lower amplitude because their energy disperses throughout the Earth’s interior. In contrast, surface waves maintain a higher amplitude because their energy is trapped and concentrated along the surface layer. This confinement means that ground displacement from surface waves can be measured in centimeters or even meters during a powerful earthquake.
Despite their slower speed, surface waves are the main cause of damage to human structures. Their large amplitude and the nature of their motion—horizontal shearing from Love waves and rolling from Rayleigh waves—exert stress on buildings. Because the slower surface waves trail the body waves, they often prolong the shaking felt at a location, increasing the time during which structural fatigue can lead to collapse.