Earthquakes release energy that travels through the Earth as seismic waves, which cause the ground shaking people experience. These waves propagate from the earthquake’s focus (hypocenter) and are categorized based on their path. Seismic waves vary in speed and motion, meaning they arrive at a location at different times and cause different amounts of movement.
The Two Types of Body Waves
The first seismic waves to arrive are body waves, which travel through the Earth’s interior from the earthquake’s focus. These waves are generally high frequency and travel much faster than the later-arriving surface waves.
The fastest seismic wave is the Primary wave (P-wave). It travels by compressing and expanding material, similar to a sound wave, vibrating the ground parallel to the direction of movement. P-waves can travel through solids, liquids, and gases at speeds between 4 and 8 kilometers per second in the Earth’s crust.
The Secondary wave (S-wave) arrives after the P-wave, moving more slowly, typically between 2.5 and 4 kilometers per second. S-waves are shear waves that shake the ground perpendicularly to the direction of propagation, causing side-to-side or up-and-down motion. A key characteristic is their inability to travel through liquids, which helped scientists understand the Earth’s liquid outer core. Body waves generally cause less overall damage than surface waves.
Characteristics of Surface Waves
Following the body waves are the surface waves, which are confined to the shallow layers of the Earth’s crust. These waves are slower than both P-waves and S-waves but are responsible for the vast majority of destructive energy released during an earthquake. Their energy is concentrated near the surface, where human-built structures are located.
The two main types of surface waves are Love waves and Rayleigh waves, generated when body waves reach the surface. Love waves are the faster surface wave, characterized by purely horizontal motion that moves the ground side-to-side in a shearing motion. This transverse movement is damaging to building foundations.
Rayleigh waves create a complex, rolling motion similar to an ocean wave, moving the ground both vertically and horizontally in an elliptical path. They are the slowest seismic waves and are recorded last on a seismograph. Rayleigh waves often have the largest amplitude (height of motion), making them highly destructive.
Why Surface Waves Cause the Most Ground Movement
Surface waves, specifically Love and Rayleigh waves, produce the most severe ground movement due to their unique motion, high amplitude, and sustained duration. These waves are the primary driver of the intense ground shaking that causes widespread structural damage.
The severity of movement results from their high amplitude compared to body waves, displacing the ground over a greater distance. Rayleigh waves are notable for their large amplitudes, which are visually apparent as a rolling, undulating effect on the ground surface.
The complex, often low-frequency motion of surface waves is particularly damaging to structures. The purely horizontal shearing of Love waves can twist and destabilize foundations that are not engineered to resist lateral stress. The rolling, elliptical motion of Rayleigh waves applies both vertical and horizontal stress to buildings, challenging structural design.
Although surface waves arrive last, they linger longer than body waves, extending the duration of severe shaking. This sustained, powerful motion, combined with their large amplitude, delivers more energy to surface structures.
Energy Concentration
The concentration of energy is a final factor. Surface waves travel only along the Earth’s surface and lose energy more slowly with distance than body waves, which travel in all directions through the Earth’s volume. This energy concentration in the shallow crust confirms that Love and Rayleigh waves cause the most severe ground movement and destruction.