A surface wave is a mechanical wave that travels along the interface between two different media, transferring energy along that boundary. This phenomenon occurs in various natural settings, from the oceans where water meets air, to the solid Earth where the crust meets the atmosphere. The wave is a propagation of disturbance, moving energy without transporting the medium itself. The cause of a surface wave dictates its initial properties, but its subsequent behavior is shaped by the properties of the media it travels through.
Generation of Water Surface Waves
The most common surface waves on Earth’s oceans and lakes are generated by the transfer of kinetic energy from wind to the water surface. This process begins with a gentle breeze creating microscopic disturbances known as capillary waves, where surface tension is the primary force attempting to flatten the water. As wind speed increases, the friction and pressure differences exerted by the wind transfer more energy, causing the wave’s size and wavelength to grow.
Once the wavelength exceeds approximately 1.73 centimeters, gravity takes over as the dominant restoring force, and the waves transition into larger gravity waves. The ultimate size and power of wind-generated waves depend on three factors: the speed of the wind, the duration of time the wind blows, and the fetch, which is the uninterrupted distance over the water that the wind acts upon. A long fetch allows for sustained energy transfer, enabling the waves to build into significant swells that can travel thousands of kilometers from their origin.
Surface waves can also be generated by the gravitational pull of the Moon and the Sun, creating the very long-period waves known as tides. This gravitational interaction causes massive bulges in the ocean water, resulting in a surface wave with wavelengths spanning hundreds to thousands of kilometers. Tsunamis are generated by massive, rapid displacement of water, such as from an underwater earthquake, volcanic eruption, or large landslide. This sudden vertical movement of the seafloor pushes the entire water column, creating waves whose energy extends from the surface to the ocean floor.
Generation of Seismic Surface Waves
Seismic surface waves, which travel along the Earth’s outermost layer, are caused by the sudden release of stored energy during an earthquake. This process begins deep within the Earth with the abrupt slip along a fault line, which initially generates body waves: P-waves (compressional) and S-waves (shear). These body waves travel through the Earth’s interior, but they are not the surface waves themselves.
The destructive surface waves are created when the P and S body waves reach the interface between the solid Earth and the atmosphere. At this boundary, the energy from the body waves converts and becomes trapped near the surface, forming two distinct types of seismic surface waves. Love waves cause particles to move horizontally, perpendicular to the direction the wave is traveling, resulting from the shearing motion of S-waves interacting at the surface.
Rayleigh waves involve a rolling motion, causing particles to move in a retrograde elliptical path that combines vertical and horizontal displacement. Rayleigh waves are slower than Love waves, but their rolling action often produces the strongest ground shaking and the most extensive damage. Both Love and Rayleigh waves travel much slower than their P and S wave precursors, but their amplitude decreases exponentially with depth, meaning their energy is concentrated at the ground level where it can affect human structures.
Environmental Factors Shaping Surface Waves
After a surface wave is generated, the medium it travels through dictates its behavior, modifying the initial input of energy. For ocean waves, a major transformation occurs through wave shoaling, where the wave moves from deep water into progressively shallower water near the coast. When the water depth becomes less than half of the wave’s wavelength, the wave begins to “feel” the bottom, causing its speed to decrease significantly.
To conserve the energy flux, this reduction in speed is compensated by an increase in wave height, causing the wave to grow taller and its wavelength to shorten. The intrinsic properties of the wave, such as its period (the time between successive wave crests), are set at the point of generation and remain constant, but the changing water depth alters the wave’s form. This shoaling effect applies to all water waves, causing both wind-driven swells and long-wavelength tsunamis to increase in height as they approach land.
For seismic surface waves, the speed and amplitude are controlled by the composition of the Earth’s crust and the near-surface layers. Wave velocity is governed by the material’s density, elasticity, and rigidity, with waves traveling faster through denser, more rigid rock. Lateral variations in the crustal structure, such as changes in rock type or the presence of sedimentary basins, can cause seismic surface waves to be refracted, reflected, or amplified, significantly influencing the shaking felt at the surface.