Waves, Tides and Tsunamis

You have probably seen waves on a body of water. But have you ever thought about what waves actually are? Waves are a special type of energy transfer. 

• Electromagnetic waves transfer energy through vibrations in electric and magnetic fields.
• Sound waves transfer energy through the vibration of particles of air, water and other solids.
• Water waves transfer energy through the vibration of water molecules.

In this backgrounder, we are going to be taking a closer look at waves in water.

Water waves can take many forms. There are internal waves that happen deep underwater. We don’t tend to see these waves at the water’s surface. There are also surface waves that occur on the surface of water. These are the waves that you are probably familiar with. Some of these surface waves may be tidal waves, which are caused by tides. There are also giant surface waves called tsunamis. In this background we will focus on the science of surface waves.

Wind-generated Waves

Waves can be measured and described according to three characteristics. These include:

1. Wavelength, which refers to the distance between wave crests;
2. Amplitude, which is the height of the wave from the resting point called the equilibrium;
3. and period, which is the time in between crests.

Most surface waves happen because kinetic energy from moving air is transferred to water. These waves can range from small ripples to waves big enough to surf on and everything in between.

The size of wind-generated waves depends on five factors. These include:

1. The speed at which the wind blows;
2. The distance over which the wind blows. This is called the fetch;
3. The width of the area affected by the fetch;
4. The amount of time that the wind blows over the given area; and
5. The depth of the water.

 Some waves can get very large! Let’s look at two types of these waves and what makes them happen.

Did you know?

The Beaufort scale is a way sailors describe wind and sea conditions. You can learn more about it in our article about wind. 

Rogue Waves

Rogue waves are large waves that occur far out at sea. Some happen as a result of storms but others do not. This makes them very hard to predict. For a long time, people were not able to prove the existence of rogue waves even though they believed them to be the cause of many shipwrecks. It wasn’t until fairly recently that scientific research was able to finally prove that rogue waves exist using satellite imagery. Using radar images from the European Space Agency, satellites found ten rogue waves, each 25 metres (82 ft.) or higher.

Did you know?

Rogue waves are also known as freak waves, monster waves, killer waves and extreme waves.

Let’s start by looking at what happens when two waves head towards each other.

As you can see, when two waves meet, they can either add together to make a crest twice as high or cancel each other out. Scientists knew about wave interferencewave interference, but what they could not understand was how rogue waves could become more than twice the amplitude of waves in the area. What was even stranger was that they sometimes occurred where the sea was calm.

Try this!

Make your own waves using this introductory wave simulator and then explore what happens when you have two waves using the interference simulator.

What scientists did to try to explain rogue waves was to find a way to reproduce them in the lab. They were able to do this, and found some interesting results. They discovered that it takes a rare and unique combination of forces from winds and ocean currents to make a rogue wave. Even though they are rare, scientists have found that rogue waves are still much more common than predicted.

In a similar way, rogue holes can form in the ocean. They are the opposite rogue waves. In a rogue hole, the water level is lower by twice the normal average height. Just like rogue waves, rogue holes are very dangerous for ships. 

Tsunamis

Tsunami is a Japanese word meaning ‘harbour wave’. This type of surface wave is not caused by wind. Tsunamis are a series of massive waves generated when a large amount of water is displaceddisplaced vertically. Any seismicseismicevent, such as an earthquake or underwater volcanic eruptions can lead to a tsunami.

Earth’s crust is made of massive slabs of rock many kilometres thick. These slabs, known as tectonic plates, may shift and move over time, causing earthquakes. When the plates move upward, they in turn push the water above them upward. At the same time, gravity is pushing downward on the water. These opposing forces create waves. Volcanic eruptions, landslides and meteorite impacts can also cause tsunamis in a similar way.

Tsunami waves are different from the wind-generated waves you can typically observe at the beach. Far out at sea, tsunami waves are hardly noticeable. Their amplitude is small and they have much longer periods and wavelengths. For example, in the open ocean, tsunami wavelengths may be a hundred kilometres long with periods of an hour. Normal waves have wavelengths around 100 metres with periods of a few seconds.

Did you know?

Japanese people call Tsunamis ‘harbour waves’ because they only see these waves when they get closer to land.

In the deep, open ocean, the energy of a tsunami is spread out over a large area and volume of water. As the waves go from deep to shallow waters, their amplitude increases. This is called shoaling. Shoaling happens because the energy of waves is transferred to a smaller volume of water. Once the waves reach the ocean floor, they slow down. But their energy is not lost. The energy is instead used to make their amplitude even greater. By the time the waves reach land, a powerful tsunami may be up to 10 to 20 metres in height!

Although tsunamis are fairly unpredictable, there are certain patterns that can let people know that a tsunami is coming.

For example, the ocean may recede, or pull back suddenly from the shore prior to the arrival of the wave. This can signal that a tsunami is not far behind. During the December 26, 2004 tsunami in the Indian Ocean, this occurred on many shorelines, giving enough warning to many people to go to higher ground.

Image - Text Version

Shown is a colour photograph of a Tsunami Hazard Zone public sign on a wooden post. Palm trees can be seen in the distance behind the sign.

There is an illustration on the sign that has a bright blue background and a white silhouette of a large wave on the left, and a person climbing upwards on the right side. Below the illustration is the text "In case of earthquake go to high ground or inland".

 

Also, an earthquake tends to happen just before a tsunami. Earthquake tremors travel more quickly through the earth than through the water. This means that when people living in tsunami-prone areas feel an earthquake, they can move to higher ground. Unfortunately, there isn’t always time to do this.

Luckily, large tsunamis happen very rarely. In Canada, the biggest tsunami ever recorded was in 2007, when a 38 metre high tsunami happened after a landslide in Chilliwack, British Columbia. Thankfully it happened in the winter and no one was hurt. This was not the case in 1929 when an Earthquake caused a 13 metre high tsunami in Grands Banks Newfoundland. 25 people were killed and many buildings and houses were destroyed.

Not just big…MEGA!

Some tsunamis have wave heights that are much larger than regular tsunamis. These are sometimes Megatsunamis. The tsunamis discussed above were caused by earthquakes and undersea volcanic eruptions. Known megatsunamis have been caused by large scale landslides and impact events, such as the collision of a meteorite or asteroid with the Earth. This is the case of the asteroid which created the Chicxulub crater and probably caused the dinosaur extinction in Yucatán about 65 million years ago. Scientists think it generated megatsunamis as high as 3 000 metres!

The highest wave ever recorded was caused by a giant landslide at the head of Lituya Bay in Alaska on July 9, 1958. This wave was 524 metres high.

Misconception Alert

People sometimes call tsunamis tidal waves. Tsunamis are not created by tides and they are much less predictable than waves associated with tides.

Tides

If you have been to a beach near an ocean, you may have noticed that sometimes the water comes further up the beach at some times of day. This rise and fall of sea levels is known as the tidetide. When the water is at its highest level, it is called high tide. When the water is at its lowest level, it is called low tide. In between, the water is either slowly rising or slowly falling. 

Tides are basically very, very long ocean waves with a wavelength that may be as long as half the planet. This is why they are also known as Tidal waves. 

This incredible pushing and pulling of ocean water is caused by gravitational forcesgravitational forces exerted by the Sun and the Moon. The Sun’s gravity is stronger than the Moon’s, but the Sun is 400 times farther away from the Moon. This is why lunar tides are twice as strong as solar tides.

So how do lunar tides work?

Lunar tides happen because the pull of the Moon’s gravitational force changes the shape of Earth’s gravitational fieldgravitational field. This makes the pull of gravity weaker in line with the Moon and stronger perpendicular to the Moon.

Image - Text Version

Shown is an illustration indicating the locations of stronger and weaker gravity on Earth as a result of the Moon.

In the centre of the image is an illustration of a top down view of the Earth. To the right of the Earth, in a line with the centre of the Earth, is an illustration of the Moon. A thin pink line cuts across both the Earth and the Moon.

Surrounding the Earth are drawings of small, black arrows. The arrows indicate the direction of the gravitational field. Near the top and bottom of the Earth, the arrows are pointing towards the Earth. These arrows indicate that the gravitational pull towards the Earth is strongest at these locations.

Near the left side and right side of the Earth, the arrows are pointing away from the Earth. In between, the direction of the arrow gradually changes directions. These arrows indicate that the gravitational pull towards the Earth is weakest at these locations.

 

Where the pull of gravity is stronger, water in the oceans is squeezed. This pushes water into parts of the ocean where the water is squeezed less. This movement of water causes a ‘bulge’ in the ocean in the direction of the Moon and on the side of the Earth opposite from the Moon.

Tides that face the Moon are called Sublunar Tides. Tides that face away from the moon are called Antipodal Tides.

Image - Text Version

Shown is an illustration indicating the locations of tidal bulges around the Earth.

This illustration is the same as the previous one with some small changes. A large pale, blue coloured oval is superimposed on the Earth so that it encompasses the field arrows. This indicates where water is squeezed and pushed out. On the side of the Earth near the Moon, the bulge is labelled as a Sublunar Tide. On the side of the Earth opposite the Moon, the bulge is labelled as an Antipodal Tide.

 

As the Moon orbits around the Earth, tide levels change along with it. When the solar and lunar tides are in the same direction, they are known as Spring Tides. When they are perpendicular to each other, they are known as Neap Tides.

Did you know that the highest variation from low to high tide is in Canada? The Bay of Fundy between New Brunswick and Nova Scotia experiences high tides up to 16 meters high twice a day! Watch just how fast this can happen in the video below. No time for a nap on the beach!