Seismic waves are vibrations that travel through the Earth, typically following an earthquake or large explosion. Their analysis is fundamental to the field of seismology. Studying how these waves move allows scientists to map the interior of our planet, revealing the boundaries and physical states of layers like the core and mantle. Their varying speeds and behaviors also provide the data necessary to locate the epicenter of an earthquake and assess its potential impact. The waves are broadly categorized into two main groups: body waves, which travel through the Earth’s interior, and surface waves, which are confined to the upper layers.
Primary Waves
Primary waves, commonly called P-waves, are the fastest seismic waves. P-waves are compressional, or longitudinal, waves, meaning they move material by pushing and pulling it in the same direction the wave is traveling. This push-pull mechanism allows P-waves to travel through any type of medium, including solids, liquids, and gases. The ability of P-waves to propagate through all states of matter makes them useful for probing the Earth’s interior structure. For example, P-waves slow down significantly when they encounter the liquid outer core, providing a clear indication of that layer’s fluid nature. While they typically cause minor shaking, their rapid arrival is often utilized in early earthquake warning systems.
Secondary Waves
Secondary waves, or S-waves, are the second type of body wave. S-wave velocity is typically about 60% slower than P-waves, traveling between 3 and 4.8 kilometers per second near the Earth’s surface. Unlike the compressional motion of P-waves, S-waves are shear, or transverse, waves. The particle motion is perpendicular to the direction the wave is moving, causing the ground to shake from side to side.
This shear motion requires a medium that can resist a change in shape (rigidity). Consequently, S-waves can only travel through solids, as liquids and gases lack the necessary rigidity. The observation that S-waves disappear when they reach a certain depth provided evidence that the Earth’s outer core is a molten, liquid layer. S-waves generally cause more intense, higher-amplitude shaking than P-waves.
Surface Waves
Surface waves are generated when the P and S body waves reach the Earth’s surface. These waves are confined to the outer layers of the planet, traveling along the surface, and are the slowest seismic waves. Despite their slower speed, surface waves possess the largest amplitude and are responsible for the most intense and destructive ground shaking experienced during an earthquake. Their motion is complex and divided into two subtypes.
One subtype is the Love wave, named after the British mathematician A. E. H. Love. Love waves move the ground with a purely horizontal, side-to-side shearing motion. This horizontal twisting is particularly damaging to the foundations of structures and buildings. Love waves are the fastest of the surface waves, but still slower than both P and S body waves.
The second subtype is the Rayleigh wave, named for Lord Rayleigh. Rayleigh waves create a rolling motion that causes the ground to move both up and down and side to side in an elliptical path, similar to ocean waves. Rayleigh waves arrive last but typically carry the greatest destructive power due to their large vertical and horizontal displacement.