Everything About Tsunamis

Podcast Transcript

They are some of the most devastating natural disasters on Earth. They can strike without warning, or sometimes you might have several hours’ notice.

Their effects can be limited to small areas, or they can devastate communities on opposite sides of the world.

They have killed hundreds of thousands of people and have been responsible for billions of dollars in damage.

Learn more about tsunamis, what causes them and how devastating they can be, on this episode of Everything Everywhere Daily.

Once again, let’s start out with definitions to kick off this discussion.

A tsunami is a series of large waves which are created by an underwater displacement of a large volume of water.

Tsunamis are almost always caused by earthquakes, which is why 80% of all tsunamis occur in the Pacific Ocean. The ring of fire surrounding the Pacific is where most earthquakes occur, especially those near a large body of water.

The word tsunami comes from the Japanese word for “harbor wave”. “Tsu” means harbor, and “nami” means wave.

The term tidal wave is often used synonymously with the word tsunami, however, this term has fallen out of favor. This is because a tsunami really has nothing to do with tides.

There is an event called a tidal bore, which is a wave made by tides that can travel up rivers and estuaries and can even be surfed, but they are regular and are not tsunamis.

That being said, what exactly happens with a tsunami?

If you’ve been next to a body of water, you are familiar with waves. Waves come in, and waves go out. Even if there are very strong winds and waves are very big, they still go in and out.

There are two major components to a wave. The amplitude, which is the height of the wave, and the wavelength, which is the distance from one wave to another.

Many people think that a tsunami is a wave with a very large amplitude, and this is not necessarily the case. They will usually have a larger than usual amplitude, but the height of the wave isn’t a tsunami’s defining characteristic.

Some of the largest waves in the world appear on the North Shore of the island of Oahu in the winter. Likewise, there are other places like Mavericks Point just south of San Francisco and other places around the world that can generate very high waves.

These monster waves are not tsunamis, even though they are very large. These waves still go in and out like a normal wave, except that they are just larger than normal.

What defines a tsunami is the wavelength of the wave. A normal wavelength will be measured in meters, and a big swell might be measured in tens of meters.

A tsunami will have a wavelength that is measured in kilometers. Instead of a wave that goes in and out, a tsunami will come in…..and keep coming in. That is why a tsunami is so dangerous.

Tsunami waves can grow in height as they approach land and the water is forced upward as there is no sea below it. A wave just a few meters in height can wind up being tens of meters in height once it hits land.

How how does such a wave get created?

They are almost always by earthquakes, but they sometimes might form from other underwater events such as a volcano or a landslide.

Let’s look at what happened during the horrific 2004 tsunami that struck the Indian Ocean.

A 9.1 magnitude earthquake struck off the north shore of the island of Sumatra in Indonesia. There was a massive length of a region called the Sunda Megathrust which ruptured. This was a 1,300 kilometer or 810-mile length of the seafloor which was very suddenly thrust up.

This movement of the seafloor displaced the water which had been there, pushing it up and causing the wave.

If you are in the middle of the ocean on a boat during a tsunami, you probably wouldn’t notice anything. Because the wavelengths are so long, the entire sea around you would rise and fall, and you probably wouldn’t even notice that anything is happening.

A wave has a peak, the high part, and a trough, the low part. The first part to hit land is usually the trough.

One thing that tsunami survivors often report is that before the water hits them, the sea will often disappear. If you remember back to my episode on the 1755 Lisbon Earthquake, after the earthquake struck, observers reported the harbor emptied out and they could see all of the debris on the seafloor.

Similar reports from southern India before a tsunami reported ruins of ancient structures could be spotted when the sea retreated.

This is perhaps the biggest warning signal to anyone in a coastal area that a tsunami is about to hit. When the sea retreats, you have a few minutes, maybe five minutes, before the peak of the wave is going to start coming in.

When the water comes in there is no stopping it. It will probably tear down all but the most sturdy buildings. Cars will be thrown around like toys. Telephone poles will snap and will be carried along with all the rest of the debris which is picked up by the wave.

There will usually be more than one wave. Just like with a regular wave that goes in and out, a tsunami will go in and out as well, but it just takes much longer due to its long wavelength.

If you haven’t already seen it, take a few minutes to watch the many videos taken of the 2011 tsunami which hit Japan. There were dozens of cameras that managed to capture the event. It can give you a more visceral feel for just how powerful a tsunami can be.

Because of the nature of the wave, a tsunami can travel incredibly fast. It is not uncommon for a tsunami to travel 500 miles per hour across the ocean. The total energy of the wave is conserved quite well, however, the energy of the wave is distributed across a larger area the further away from the epicenter it gets.

Think of concentric rings emanating out from a point. Each ring gets larger and the energy of the wave is spread out amongst the perimeter of the ring.

That is why the most dangerous tsunamis are the ones that occur very close to land. If it happens right off the coast, there is very little warning for the people who live closest to the epicenter.

I actually got to experience this first hand. I was in Maui during the tsunami that hit Japan in 2011. Tsunami warnings went out and we had hours to prepare for the tsunami to hit. I obsessively followed reports from other islands between Japan and Hawaii, in particular Guam.

When people in Guam were reporting slightly high water up to people’s knees, I knew that nothing would probably happen in Hawaii….and it didn’t.

Many locals, when they got the warning, went to the harbor if they had boats and went out to sea.

The damage in California was limited to some boats in a harbor that was knocked together that weren’t taken out.

There are now several tsunami warning systems in place around the world. The United States created the National Tsunami Warning System after the devastating 1964 Alaskan earthquake, the tsunami from which killed over 100 people in Alaska and Hawaii.

In the 1990s the Deep-ocean Assessment and Reporting of Tsunamis or DART system was created. This is a network of deep ocean buoys which have GPS sensors and satellite communications which can read water levels.

So long as you aren’t too close to the epicenter of the earthquake which causes a tsunami, most people should have sufficient time to get to safety. The dramatic increase in the number of people with cell phones should allow far more people to be notified faster to prevent another disaster like what occurred in 2004.

There is another type of tsunami that you don’t normally hear about, but it is still a threat. It is called a megatsunami.

A megatsunami differs from a regular tsunami in how they are formed. Tsunamis are formed by an underwater displacement of water. This is a thrust of the seafloor or an underwater landslide.

A megatsunami is created by a sudden displacement into water. They are usually created by a landslide on land which falls into the water, or by a water impact of a meteorite.

If you have ever thrown a large rock into water, you’ve created a small model of a megatsunami.

The “mega” in a megatsunami comes from the amplitude of the wave. Where tsunamis are defined by their long wavelengths, a megatsunami is defined by its large wave heights.

Megatsunamies are rather rare. Regular tsunamis of some sort will happen several times a year around the world.

The most notable natural megatsunami we know of occurred in 1958 at Lituya Bay in the Alaskan Panhandle.

An 8.0 earthquake caused a massive landslide of 40 million cubic yards to get dumped suddenly into the waters of Lituya Bay.

The wave it created was enormous. It killed two people who were in boats and the water knocked down trees 1,720 feet or 524 meters above sea level.

In October 1963 in Italy, a landslide in the lake above the Vaiont Dam resulted in a megatsunami with a wave that was 250 meters or 820 feet high.

The most recent megatsunami occurred in 2015 in Taan Fiord, Alaska. A landslide caused the wave which when it hit the opposite side of the fjord was 193 meters or 633 feet high.

So, what should you do if you are in an area where there is a tsunami warning?

Basically, head inland to high ground depending on how much time you have. If you don’t have time, seek high ground in a very sturdy building. Buildings can and do collapse from the power of the water, so don’t assume just being above the water will protect you. Climb a tree as a last resort.

Tsunamis are some of the most powerful forces in nature. The 2004 Indian Ocean tsunami killed almost a quarter of a million people in 14 countries. Thankfully with advances in detection and early warning, a tsunami disaster the like of which we saw in 2004 will never happen again.

The associate producers are Thor Thomsen and Peter Bennett.

If you would like to support the show, you can do so over at Patreon.com.

Remember, if you leave a review or send in a question, you too can have it read on the show.