Tsunami: Facts versus Movie Myths

A tsunami or tidal wave is one of those disasters that are rare, difficult to prep for, but interesting to discuss. If you live inland, you might still be interested in this article, in case you are on vacation near the ocean, or simply because it is a fascinating topic. This particular prepping and survival post discusses the difference between a tidal wave in movies and recent real tidal waves. There are several significant differences between the movie version and a real tsunami, and knowing the differences might affect your response.

The height of tidal waves is one thing that movies get wrong or exaggerate. The typical movie tsunami is hundreds of meters high, taller than many skyscrapers. Strictly-speaking, it’s not impossible for a tidal wave to be that high. A comet or asteroid impact in the deep ocean can generate a tsunami hundreds of feet high, 100 km or more from the site of impact. See the Impact: Earth! effects calculator at Purdue University. But the typical tsunami, caused by an undersea earthquake, is several meters in height, or less. See the NOAA Tsunami site. The largest tsunami in recorded history is said to be the 2004 Indonesia tsunami, with a height of 33 meters (per Wikipedia).

But here’s an important point on the height of tidal waves: it doesn’t take much height to do a great deal of damage. The 33 meter 2004 Indonesia tidal wave killed over 200,000 persons. The 2011 northern Japan tidal wave, which struck the Fukushima nuclear power plant, was only 10 meters high. A movie-sized tidal wave would be extremely damaging, but no such tidal wave has occurred in recorded history.

Here’s a clip from a movie with a fairly realistic tidal wave striking a beach resort. The movie is about the 2004 Indonesian tidal wave. Their depiction of the height of the wave and its effects is about right.

The speed of tidal waves is faster in the open ocean and slower as it approaches land. But in the open ocean, the wave might be only a few meters high and very broad. A ship might not even notice the wave. However, in the movie version, the wave is huge and breaking in the open ocean — not very realistic (even for the worst tsunami in recorded history). Here’s a video of a large ship riding out a tsunami. The wave doesn’t break over the ship, but it does toss this huge cruise ship like a toy boat.

A tidal wave travels “at speeds averaging 450 (and up to 600) miles per hour in the open ocean.” That’s about the speed of a 747 Jet. But as “the waves approach the coast, their speed decreases and their amplitude increases.” (NOAA Tsunami site) Still, it seems to me that the movie version of a tidal wave, for dramatic effect I’m sure, is rather slow as it nears shore.

This video explains how an undersea earthquake caused the 2011 Japan tidal wave. I’ve taken a look at some video of the 2011 Japan tsunami. The effect is not one large wave breaking on shore. It looks more like a river flowing across the landscape. Here’s one from a CCTV camera, and here’s a second one (the footage was sped up): Route 45 in Iwate Prefecture.

One thing that surprises me in viewing these videos of real tidal waves is the amount and size of the flotsam that is carried along by the water. Since the tidal wave tends to flow, rather than break, onto the shore, it has tremendous power to pull objects along with it. In the videos above the Fukushima tsunami, you can see many cars carried along by the flowing water. And here’s another video where the tsunami is knocking down houses and pushing them along. The power of the wave is impressive, especially given that the Fukushima wave was only about 10 meters high.

This PDF — Intergovernmental Oceanographic Commission. 2012.
Tsunami, The Great Waves, Second Revised Edition.
— discusses your response and survival, if there is a tsunami. Some useful quotes from that PDF follow:

“Tsunamis can move faster than a person can run.”

“Sometimes a tsunami initially causes the water near the shore to recede, exposing the ocean floor.”

“Tsunamis generated in distant locations will generally give people enough time to move to higher ground. For locally generated tsunamis, where you might feel the ground shake, you may only have a few minutes to move to higher ground.”

What can you do? If you feel an earthquake, and you are near the coastline, don’t wait for a tsunami warning to be announced. Move inland or to higher ground. If you see the water by the ocean receding suddenly, don’t go closer to the water to get a better look. Move to higher ground.

Also, stay away from rivers and streams that lead to the ocean, as the tsunami can travel along that path. The same is true for low lying roads along the ocean. The flow of water can travel along that path and sweep away all the cars. I would rather be a few floors up in a sturdy building (like a hotel) on the beach, than in a car trying to drive away from the wave. However, most single-family homes near the water are not designed to withstand a tidal wave.

If you are on a boat when there is a tsunami warning, your options are move to deep ocean water (if your boat is large enough). Or get to shore and higher ground before the tsunami strikes. A tsunami can take a long time to reach shore, if the earthquake that caused it was hundreds of miles away. If the earthquake was close enough for you to feel it, the tidal wave is probably not far behind.

The movie version of a tidal wave is one super-large wave. But a real tsunami is often a series of waves, and the largest wave might not be the first. “A tsunami consists of a series of waves with crests arriving every 5 to 60 minutes. Often the first wave may not be the largest. The danger from a tsunami can last for several hours after the arrival of the first wave.” So don’t assume that the first wave is the only one.

I used to live in Florida, a couple of miles inland, but the land there is very flat. I don’t think we were more than a few meters above sea level. Now I live in Massachusetts, closer to the ocean, but definitely on higher ground. A movie-sized tidal wave would wipe us out. Fortunately, a normal-sized tsunami would be only a few meters high, and we’d probably be OK. The west coast of the U.S. is much more likely to see a tsunami warning, since the Pacific rim has more earthquakes.

– Thoreau

One Response to Tsunami: Facts versus Movie Myths

  1. “It doesn’t take much height to do a great deal of damage.” So true, and very frightening. Thanks for the article.