You may think of a surface wave as just a bunch of moving water or–in the case of an earthquake–moving dirt. Find out more about surface waves, and take a quiz to test your understanding.
Waves of Warning
In late August of 2011, a Twitter user in New England had a surprising experience. A few minutes before 2 p.m.
that day, reports of an earthquake in Virginia began to show up in her feed. A little over a minute later, a mild earthquake rocked her own city. At her location, approximately 500 miles away from the Virginia residents who had first mentioned it, the quake was barely perceptible. However, over the next few hours, Americans were amazed to read of moderate, but expensive, earthquake damage to national landmarks such as the Washington Monument, located about 85 miles away from the quake’s source.What was going on? Why was someone hundreds of miles away able to read about an event before experiencing it? The answer isn’t time travel, but the differential speeds of primary and surface waves.
What Is a Surface Wave?
In physics, a surface wave can occur along any boundary of two different substances.
The seismic type of surface wave happens at the boundary between air and rock—the surface of the earth. In other words, this is not a wave that travels in a relatively straight line through the earth from the earthquake’s focus, or breakage point of subsurface rock, to the observer’s location.Paths through the deep layers of the earth are the quickest routes from the focus to the human or seismometer observing the quake.
Therefore, the seismic waves that take that path are the first to arrive, and scientists call these the body waves. They include the P- and S-waves. Our New England Twitter user was hit by the P- and S-wave just seconds after they were generated in the capital, but she didn’t feel them. The waves that travel along the surface are slower, but they are responsible for the earthquake’s damaging effects.
Surface waves are classified by the type of motion they transmit.
Two of the most important types are Rayleigh waves and Love waves.Rayleigh waves have an up-and-down rolling motion that many people describe as feeling like riding in a ship on the ocean. They are also called ‘surface roll’ waves.Love waves have a back-and-forth motion, like a sidewinder rattlesnake traveling over the sand.
The combination of the Rayleigh waves’ vertical rolling action and the horizontal stresses from the Love waves is the source of most direct damage from earthquakes (i.e., damage not caused by secondary effects like a fire or tsunami).
Both of these wave types are named for the scientists who predicted them.
More Examples of Surface Waves
So far, we have been thinking about the way surface waves work during an earthquake. But, you don’t have to wait for a catastrophic seismic event to study surface waves.
Waves on the ocean, as well as the ripples on the surface of a cup of coffee after you add sugar or milk, are surface waves.If you could follow an individual particle of water under the influence of a surface wave, you would see that it’s not being carried forward by the wave. Instead, each ‘piece’ of water rocks forward, up, back, and down, making one complete circle as a wave passes by.
Understanding this motion has helped scientists and engineers design safer buildings for earthquake-prone areas; these buildings’ foundations are ready to ‘roll with the punches’ of an earthquake instead of rigidly resisting the waves’ energy.
Wherever two different substances meet and form a surface boundary, you can observe the way energy travels through each substance at a unique velocity. That’s why surface waves exist. They can be as familiar as your favorite beverage or as strange and alarming as a massive earthquake. But, understanding how matter and energy behave in surface waves is an important step towards earthquake safety.
- Surface wave: (in physics) occurs along any boundary of two different substances
- Focus: breakage point of subsurface rock
- Rayleigh waves: have an up-and-down rolling motion like an ocean liner
- Love waves: have a back-and-forth motion like a sidewinder snake
Once you’ve viewed the lesson and absorbed facts about surface waves, prove your ability to:
- Illustrate surface waves
- Compare and contrast Rayleigh and Love waves
- Note examples of surface waves