Seismic waves are vibrations that travel through the Earth following a sudden release of energy, typically from an earthquake. These waves are categorized based on their travel path: either through the planet’s interior or along its surface. Surface waves are seismic energy that propagates along the interface between the crust and the atmosphere, meaning they travel only at the surface. They result from seismic energy reaching the ground, where they become trapped and travel outward from the epicenter.
Surface Waves Compared to Body Waves
The fundamental distinction in seismology is between body waves and surface waves, defined by where they travel. Body waves, which include the faster P-waves (Primary) and S-waves (Secondary), move through the Earth’s deep interior layers, refracting and reflecting off boundaries between different materials. Surface waves, by contrast, are confined to the shallowest layers of the crust and the surface itself, acting much like ripples expanding across the top of a pond.
This confinement means that surface waves spread their energy across a two-dimensional area, specifically along the surface, instead of dispersing it throughout the three-dimensional volume of the Earth’s interior. This difference in geometric spreading causes surface waves to attenuate, or lose amplitude, much more slowly with distance than body waves. While body waves travel much faster and arrive first at a seismograph, surface waves have significantly larger particle motions, which contributes directly to their capacity to cause ground displacement.
The Distinct Motions of Love Waves and Rayleigh Waves
Surface waves are further divided into two primary types, each defined by the distinct way they cause the ground to move as the wave passes. The first type, known as Love waves, involves a purely horizontal shearing motion of the ground. This motion is entirely side-to-side and is perpendicular to the direction in which the wave is propagating.
Love waves, named after Augustus Edward Hough Love, have no vertical component to their movement, causing a transverse shaking parallel to the Earth’s surface. This shearing action can be particularly damaging to structures like building foundations and bridge supports, which are not designed to withstand such intense lateral distortion. Love waves are the faster of the two surface waves, making them the first type recorded by seismic instruments.
The second primary type is the Rayleigh wave, which exhibits a complex, rolling motion similar to the movement of a wave across the ocean. Rayleigh waves combine both horizontal and vertical ground movement, causing the particles to move in a retrograde elliptical path. This means the ground rolls backward relative to the wave’s direction of travel as the wave passes underneath.
This combined up-and-down and back-and-forth motion makes Rayleigh waves responsible for much of the observed ground-shaking during an earthquake. The vertical displacement they produce can cause objects to lift and drop, while the horizontal component adds to overall ground instability. Their large amplitude and rolling motion often contribute the most to ground displacement and shaking intensity.
Characteristics of Surface Wave Propagation
Surface waves are characterized by their large amplitude, which is the measure of maximum ground displacement or shaking. Because surface wave energy is trapped near the surface, it spreads out less rapidly than the energy of body waves, enabling them to cause far greater ground movements over long distances. Despite their power, surface waves are the slowest of all seismic waves, traveling at velocities typically less than the shear-wave velocity of the near-surface materials. This low velocity means they are the final signals to arrive at a seismograph, trailing the faster P and S body waves.
The difference in arrival time can be substantial, with the surface wave train often lasting for many minutes after the initial jolt. Another property is dispersion, where the speed of the wave depends on its frequency. Low-frequency, long-wavelength surface waves penetrate deeper into the Earth’s structure and travel faster than high-frequency, short-wavelength waves confined to the immediate surface. As a result, a single surface wave signal spreads out into a long, drawn-out wave train over distance.
Detection and Impact on Structures
Seismographs record surface waves as the final and largest part of the seismic signal, appearing as a long-duration, high-amplitude oscillation on the seismogram. The distinct arrival times and characteristic motions of Love and Rayleigh waves allow seismologists to identify them and use their propagation characteristics to study the structure of the Earth’s near-surface layers. Surface waves are of great concern in earthquake engineering due to their destructive impact on man-made structures. The resulting ground displacement subjects buildings and infrastructure to significant stress.
The side-to-side shearing of Love waves is highly damaging to foundations and rigid structures, causing them to twist and fail laterally. Meanwhile, the rolling motion of Rayleigh waves can cause foundations to lift and settle unevenly, leading to structural collapse. The relatively low frequency and long duration of surface waves are particularly harmful to large, long-period structures, such as high-rise buildings, long bridges, and liquid storage tanks. The slow, intense shaking can cause these structures to resonate, magnifying the displacement and increasing the probability of severe damage or failure. The analysis of surface waves is important for assessing seismic risk and designing earthquake-resistant construction.