An earthquake releases stored energy that travels away from the source as seismic waves. These waves move through the Earth, causing the ground to shake, but they vary significantly in the type and degree of movement they cause. The differences in wave speed, travel path, and ground motion determine their potential for damage. Understanding these variations helps explain why some earthquake waves are far more destructive than others.
The Two Primary Categories of Seismic Waves
Seismic waves are divided into two major categories based on their path: body waves and surface waves. Body waves travel through the Earth’s interior, originating at the earthquake’s focus and propagating in all directions. They are the first to be detected by seismographs due to their high speed. However, they carry a comparatively lower amplitude when they reach the surface.
Surface waves are generated when body waves travel along the Earth’s outer layers, primarily the crust. They are significantly slower than body waves and arrive later, but they possess a much larger amplitude and lower frequency. Their energy is concentrated near the surface, making them a major factor in earthquake destruction.
Body Waves: Speed and Vertical Motion
The two types of body waves are the Primary wave (P-wave) and the Secondary wave (S-wave). P-waves are the fastest seismic waves, traveling between 4 and 8 kilometers per second in the crust, and are the first to arrive. Their motion is compressional, pushing and pulling material in the same direction the wave travels. P-waves can travel through solids, liquids, and gases. While they are felt as a sudden jolt, they typically cause very little structural damage and often serve as a brief warning.
S-waves, or shear waves, arrive next because they are slower, traveling at roughly 60% of the speed of P-waves. They shake the ground perpendicular to the direction of wave travel, creating a strong side-to-side or up-and-down motion. S-waves cannot travel through liquids, which confirms the Earth’s outer core is fluid. Due to their higher amplitude and forceful shearing motion, S-waves are more damaging than P-waves.
Surface Waves: The Most Destructive Force
The seismic waves that cause the most significant damage are the surface waves: Love waves and Rayleigh waves. These waves are the slowest to arrive but have the largest amplitudes and longest wavelengths, maximizing the intensity of ground movement. Surface waves are the primary reason for structural collapse because their energy is concentrated along the shallow crust where structures are built.
Love waves produce a purely horizontal, side-to-side motion. This motion is particularly destructive because it shears and twists the ground, subjecting building foundations to intense lateral forces. The sideways movement of Love waves can snap utility lines and twist railroad tracks. This shifting can cause widespread foundation failure beneath structures.
The other highly destructive surface wave is the Rayleigh wave, which exhibits a complex, rolling motion in an elliptical pattern. This motion moves the ground both up and down and back and forth, similar to ocean waves. The lifting and dropping action, combined with large amplitude, subjects structures to stresses in multiple directions, increasing the risk of collapse. The high amplitude, low frequency, and confinement to the surface make Love and Rayleigh waves the most destructive forces released during an earthquake.