The immense energy released during an earthquake travels away from its source in the form of vibrations known as seismic waves. These waves radiate outward through the Earth and are the direct cause of the ground shaking that people experience. Seismic waves are categorized by their travel path, speed, and the specific motion they impart to the rock and soil. These differences mean that some waves arrive first and cause a slight tremor, while others arrive later and are responsible for the most significant ground movement and destruction.
Body Waves: Compression and Shear Motion
The first seismic waves to reach a location travel through the Earth’s interior and are known as body waves. These waves are divided into two main categories based on their particle motion. The fastest are Primary waves (P-waves), which move through the ground with a compressional, push-pull motion, similar to a sound wave. P-waves alternately compress and expand the material in the direction of travel, and they can pass through solids, liquids, and gases.
Following the P-waves are the Secondary waves (S-waves), which travel more slowly. S-waves move particles in a shear motion, shaking the ground perpendicular to the direction the wave is moving. Because liquids cannot support the necessary shear stress, S-waves can only travel through solid materials. Body waves are generally less damaging than later arrivals because their energy is distributed across a large, three-dimensional area.
Surface Waves: Horizontal and Rolling Movement
The body waves generate surface waves when they reach the planet’s surface. These waves travel along the Earth’s uppermost layers, similar to ripples on a pond, and are significantly slower than body waves. Surface waves are characterized by large amplitude and lower frequency, making them effective at causing widespread ground movement.
One type of surface wave is the Love wave, which causes a purely horizontal, side-to-side shearing motion. Particles move parallel to the surface but perpendicular to the direction of wave travel, creating strong lateral displacement. The other type is the Rayleigh wave, which is the slowest seismic wave and imparts a complex rolling motion. Rayleigh waves move particles in an elliptical path, combining both vertical and horizontal movement.
Why Surface Waves Cause the Most Intense Shaking
Surface waves generate the most intense ground shaking because their energy is restricted to a shallower travel path. Unlike body waves, which spread energy throughout the Earth’s volume in three dimensions, surface waves are confined to the near-surface layers. This two-dimensional spreading means the energy dissipates at a much slower rate as the wave travels away from the epicenter.
Because their energy is concentrated near the surface, surface waves maintain a significantly larger wave amplitude over a greater distance. Wave amplitude is the direct measure of ground displacement, and this larger amplitude translates directly to more pronounced ground movement. The slowest-traveling Rayleigh waves often exhibit the largest amplitude and lowest frequency, making them the primary source of the rolling and heaving motion associated with the most destructive shaking.
How Different Wave Types Affect Buildings and Infrastructure
Each distinct wave motion places different types of stress on human-built structures. The initial P-waves cause a rapid but usually brief compressional jolt, which rarely results in structural failure. The subsequent S-waves introduce strong vertical and horizontal shearing forces. These shear waves are much more damaging than P-waves and can cause columns and foundations to twist and crack.
The two types of surface waves are responsible for the most catastrophic structural outcomes. Love waves, with their intense, purely horizontal side-to-side motion, apply extreme lateral stress to foundations and support structures. This horizontal displacement is highly effective at tearing apart rigid structures, often leading to foundation failure and the collapse of bridges.
Rayleigh waves, with their rolling, elliptical motion, cause structures to heave and sink as the ground moves both up-and-down and back-and-forth. This complex, undulating ground movement is particularly destructive, often resulting in the total structural failure of buildings as they are lifted and dropped by the passing wave.