Seismic waves are vibrations of energy that travel through the Earth, typically generated by earthquakes, volcanic eruptions, or large explosions. Seismic energy is released in distinct forms that exhibit both longitudinal and transverse motion. The two main categories are body waves, which travel through the Earth’s interior, and surface waves, which travel along the planet’s outer layer. Analyzing the motion of these waves helps scientists understand the physical forces of an earthquake and map the Earth’s deep interior.
Defining the Two Primary Body Waves
The body waves, Primary (P) waves and Secondary (S) waves, are distinguished by how they move particles. P-waves are longitudinal waves, meaning particles vibrate in the same direction the wave is traveling. This motion involves a sequence of compressions and rarefactions. P-waves are the fastest seismic waves, which is why they are the first to be recorded by seismographs.
S-waves are transverse waves, where particle movement is perpendicular to the wave’s direction. This causes the material to shear or shake side-to-side or up-and-down. Because S-waves are slower than P-waves, they arrive second at a seismic station. This difference in motion dictates their behavior as they move through the Earth.
How Different Waves Travel Through Materials
The motion of each body wave determines its ability to travel through the Earth’s different states of matter. P-waves, being longitudinal compression waves, involve volume changes and can propagate through solids, liquids, and gases. Their ability to travel through all materials makes them a reliable signal for detecting seismic events globally.
S-waves rely on a shearing motion that requires the material to possess rigidity, or the ability to resist a change in shape. Since liquids and gases lack this rigidity, S-waves cannot travel through them and are completely blocked. The absence of S-waves in certain areas is primary evidence that the Earth has a liquid outer core. Tracking the pathways and speeds of both P-waves and S-waves helps scientists map the boundaries and physical properties of the Earth’s internal layers.
The Destructive Power of Surface Waves
A separate category known as surface waves causes the most significant ground shaking and damage. These waves travel along the Earth’s surface, generated when P and S waves reach the near-surface layers. Surface waves are slower than body waves, but they carry a larger amplitude and concentrate energy near the surface where structures are located.
There are two main types of surface waves: Love waves and Rayleigh waves.
Love Waves
Love waves exhibit a purely transverse motion in the horizontal plane, causing the ground to shift rapidly from side to side. This lateral motion is damaging to building foundations and infrastructure not designed to withstand strong horizontal forces.
Rayleigh Waves
Rayleigh waves produce a complex, rolling motion that is both vertical and horizontal, moving particles in a retrograde elliptical path. This combined movement creates the characteristic ground roll that can cause structures to sway and topple.