Seismic waves are mechanical waves of energy that travel through Earth or other planetary bodies. They originate from sudden movements, such as earthquakes, volcanic eruptions, magma movement, large landslides, or human-made explosions. These vibrations transmit energy through the Earth’s interior and along its surface, providing insights into our planet’s hidden structure and dynamic processes.
Primary Waves (P-waves)
Primary waves, or P-waves, are the fastest seismic waves and the first to arrive at a seismic station. These compressional waves cause material to alternately compress and expand in the direction of wave travel, similar to how sound waves move.
P-waves can travel through solids, liquids, and gases. Their speed varies depending on the density of the material, making them invaluable for probing all layers of the Earth, including its liquid outer core.
Secondary Waves (S-waves)
Secondary waves, or S-waves, are shear waves that arrive at seismic stations after the faster P-waves. Unlike P-waves, S-waves displace the ground perpendicular to the direction of wave propagation, causing a shaking or crosswise motion, similar to shaking a rope.
A key characteristic of S-waves is their inability to travel through liquids or gases; they can only propagate through solid materials. This limitation is due to the fact that fluids do not support shear stress. The absence of S-waves in certain regions of Earth’s interior provides crucial evidence about the composition and state of those layers, particularly the liquid outer core.
Surface Waves
Surface waves are a distinct category of seismic waves that travel along the Earth’s surface, similar to ripples on water. They generally move slower than P-waves and S-waves, which are known as body waves because they travel through the Earth’s interior. Despite their slower speed, surface waves often cause the most significant damage during an earthquake due to their larger amplitude and longer duration.
There are two primary types: Love waves and Rayleigh waves. Love waves, named after British mathematician A. E. H. Love, exhibit a horizontal shearing motion, moving the ground from side to side perpendicular to the wave’s direction of travel. They are typically faster than Rayleigh waves and are confined to the crust.
Rayleigh waves, named after Lord Rayleigh, create a rolling or elliptical motion, moving the ground both up and down and forward and backward. This motion resembles ocean waves and often accounts for the majority of the shaking felt during an earthquake. Surface waves diminish in amplitude with increasing depth, meaning their effects are strongest at the surface.
Unlocking Earth’s Secrets with Seismic Waves
Seismologists utilize the varying speeds and travel paths of P, S, and surface waves to construct a detailed picture of Earth’s internal structure. By analyzing seismograms—the recordings from seismometers—scientists determine how long it takes for different waves to arrive at various stations around the globe. This information allows them to infer the composition, density, and physical state of the Earth’s layers, including the crust, mantle, and core.
The observation that S-waves cannot pass through the outer core, while P-waves can, provided the definitive evidence that Earth’s outer core is liquid. Differences in wave velocity and refraction patterns also reveal boundaries between layers and variations within them. Beyond revealing Earth’s deep interior, seismic waves are instrumental in locating earthquake epicenters and assessing seismic hazards, contributing to early warning systems and resource exploration.