Energy at the Edge: Surface Waves Quiz

Because surface waves carry a significant amount of energy and travel along the Earth's crust, they can circle the globe multiple times after a very large earthquake. Seismologists use the arrival times and characteristics of these waves to calculate the magnitude of the event and to better understand the composition of the Earth's outermost layers.

Surface waves are essentially "boundary waves." They exist because of the interface between two different media, like water and air, or rock and air. In the deep, uniform interior of a planet, energy travels as body waves (P and S). The unique rolling or shearing motion of surface waves only develops where the medium ends.

While often called "tidal waves," tsunamis have nothing to do with tides. They are surface waves with extremely long wavelengths caused by a massive displacement of water. In the open ocean, they may be only inches high, but as they approach the coast and undergo shoaling, they can grow into devastating walls of water.

Ocean waves and seismic waves (Rayleigh and Love) are all mechanical surface waves because they involve the physical displacement of matter. Radio waves are electromagnetic and do not require a medium or a boundary between physical substances to travel. Understanding these categories helps scientists predict how different types of energy will move through the Earth's systems.

In shallow water, the speed of a surface wave is directly related to the depth of the water. As the water gets shallower, the wave slows down. This change in speed is what leads to refraction, where waves bend to become nearly parallel to the shoreline, regardless of the direction they were originally traveling in the open sea.

Surface waves are unique because they travel along the interface between different substances, such as the boundary between water and air or the Earth's crust and the atmosphere. Unlike body waves that travel through the interior of a material, these waves concentrate their energy at the surface, which is why they are so visible and impactful.

In deep water surface waves, the particles move in a circular motion. As the energy passes, a water molecule moves up, forward, down, and back to nearly its original position. This combination of longitudinal and transverse motion creates the rolling effect we see on the ocean surface, representing a complex transfer of mechanical energy.

While P and S waves travel faster and reach a location first, surface waves carry more energy and cause more intense ground shaking. Because they travel along the surface where buildings and infrastructure are located, their long-duration rolling and swaying motions are responsible for the majority of structural damage during significant seismic events.

Rayleigh waves are a type of seismic surface wave that moves the ground in a rolling motion, similar to ocean waves. Love waves, on the other hand, move the ground side-to-side horizontally. Both are slower than body waves but are much larger in amplitude, making them the most destructive part of an earthquake's energy release.

Rayleigh waves displace the ground surface both vertically and horizontally in a vertical plane oriented in the direction of travel. This "rolling" motion can be felt for great distances from the epicenter. Because they are surface waves, they cannot travel through the deep interior or liquid core, unlike certain primary body waves.

The energy of a surface wave is strongest at the interface and dissipates with depth. In the ocean, the circular paths of water molecules become smaller and smaller as you go deeper. At a depth equal to about half the wavelength, the water is essentially still, which is why submarines can avoid rough surface weather by diving.

Love waves are named after A.E.H. Love, the mathematician who modeled them. They move the ground in a horizontal shearing motion perpendicular to the direction of travel. This lateral shifting is particularly damaging to building foundations, as most structures are designed to handle vertical loads better than sudden horizontal shifts.

Shoaling occurs because the bottom of the wave starts to interact with the seafloor, causing the wave to slow down. As the back of the wave catches up to the front, the energy is compressed upward, increasing the wave's height. This process is what causes waves to "break" as they approach a beach or shoreline.

In seismology, there is a specific order of arrival. P-waves are the fastest, followed by S-waves. Surface waves, including Love and Rayleigh waves, are the slowest of the bunch. However, their slow speed is offset by their high amplitude and the fact that they stay near the surface, leading to prolonged and violent shaking.

The growth of ocean surface waves depends on how fast the wind blows, how long it blows for, and the "fetch," which is the distance of open water the wind travels over. While the moon influences tides, it does not directly determine the height of standard wind-driven surface waves. These three factors combine to determine the total energy transfer.