The energy released during an earthquake propagates away from the source as vibrations known as seismic waves. Seismologists classify these vibrations based on the path they take through the planet and the type of motion they impart to the material. These waves are separated into two primary classifications based on their travel path.
The Fundamental Division: Body Waves and Surface Waves
The two main classifications of seismic waves are body waves and surface waves, defined by the regions of the Earth they traverse. Body waves travel through the planet’s interior, moving through the crust, mantle, and core from the earthquake’s focus. They are the fastest seismic waves, arriving first at monitoring stations. Surface waves are generated when body waves reach the surface and propagate along the outermost layers of the Earth. The distinction in their paths is the primary factor determining their speed, amplitude, and potential for causing damage at the surface.
Characteristics of Body Waves
Body waves are divided into Primary waves (P-waves) and Secondary waves (S-waves), differentiated by their motion and speed. P-waves are compressional waves that move material back and forth in the same direction the wave is traveling. They are the fastest seismic waves and can travel through solids, liquids, and gases, allowing them to pass through the Earth’s liquid outer core. S-waves, or shear waves, travel significantly slower than P-waves. Their motion is a shearing movement, shaking the material perpendicular to the direction of wave travel, which prevents them from traveling through fluids.
Characteristics of Surface Waves
Surface waves are the slowest seismic waves, arriving last, but they are often the most destructive because their energy is concentrated along the Earth’s shallow surface. These waves possess a much higher amplitude than body waves, which contributes to increased ground shaking. The two subtypes are Love waves and Rayleigh waves. Love waves are the faster surface wave type and produce a horizontal shearing motion, moving the ground side to side and creating a twisting effect highly damaging to structures. Rayleigh waves are slightly slower and produce a rolling, elliptical motion involving both vertical and horizontal movement, which leads to intense shaking and widespread structural damage.