What in the world is a tsunami and how does it work?
Essentially, a tsunami is a giant wave moving through the ocean, but it works differently than the waves that crash on our beaches on a regular basis. Let’s explore the difference between the two and how the tsunami can be so devastating.
Before we discuss how a tsunami forms, let’s review some wave terminology. The crest is the highest point on the wave, while the trough is the lowest point on the wave. The wave can be measured by the wavelength, which is the length from crest to crest or from trough to trough, and by the wave height, which is the vertical distance between the crest and the trough. Be careful not to confuse the wave height with amplitude. Amplitude is only half of the wave height. Take a look at the diagram to help clear things up, or check out this resource.
The waves that we are used to, the small breakers that roll onto Sanibel and Captiva beaches everyday, are caused by wind blowing across the surface of the ocean. These waves don’t extend far below the surface, but as they reach shallower water the wave height increases, and they become unstable, breaking onto shore.
On the contrary, tsunamis have a completely different origin. Tsunamis are incorrectly called ‘tidal waves’ because they actually have nothing to do with the tides, but result from an underwater disturbance like an earthquake.
When an earthquake occurs on the ocean floor, the entire water column is displaced and the energy from the earthquake is transferred to the water. Unlike with wind-driven waves, when a tsunami is created, all the water from the surface to the ocean floor is set in motion.
A tsunami is not a single crest, but a series of several crests that radiate out from the disturbance. Out in the open ocean, this wave can travel up to 500 miles per hour, but can go by unnoticed. With wavelengths up to 125 miles and swells not rising more than a few feet above normal sea level, it is easy for a tsunami to sneak past a ship out in the deep, open ocean with out being recognized.
As this wave moves through the ocean, remember that it has an enormous amount of energy, the energy that was transferred from the earthquake (or other disturbance). Two things can play into the energy that the wave holds- the amplitude and the speed at which the wave is traveling. As the wave approaches shallower water, the wave slows down. To compensate, the wave’s amplitude increases, creating a really tall wave as it reaches shore.
The trough of the wave reaches the coast first and the water recedes much farther than normal. Then the crest plunges ashore, capable of moving water several miles inland. The enormous amount of energy and the sheer volume of water carried by the wave can be devastating to coastal communities, as has been seen by the recent destruction in Japan. See a NASA image of the flooding effects in Japan here. The fact that tsunamis can travel so fast makes them hard to prepare for, but to learn more about how NOAA alerts areas of possible tsunamis, check out this video.
Just another reminder of how powerful the ocean is!