Japan Today

Tsunami survival by the numbers


It shouldn’t be hard to remember the sheer force that a tsunami can unleash on land. And with the recent quakes in Chile, many parts of the world wonder if another is not too far away. But there’s an important thing about a tsunami that is not often discussed, and that’s how big it has to be to jeopardize your life.

Recently a Twitter user posted a photo of this safety poster which has caused those who saw it to wish it was seen all over the country. You might be able to understand this graphic which shows the water level versus the probability of death without understanding the Japanese, but let’s look at it a little more closely.

30 cm – 0.01% chance of death “A healthy adult can manage to stand. It’s difficult to walk.”

This might seem odd to imagine a one foot wave as a tsunami, but tsunamis and the waves you find at windy day on the beach are different in nature. The poster is not referring to wave heights but to the rise in water level.

Although it’s simply a water level, the force of the flowing water beneath is intense. Luckily, at only 30 cm able-bodied adults should be alright and able to move about.

50cm – 4.8% chance of death “Cars and containers float. You can remain standing if you hang onto something.”

Even at 50 cm, you still have a good shot at survival, but now we start to see some more extensive damage to property. At this point you might be thinking tsunamis aren’t all that dangerous, but things are about to escalate quickly.

70 cm – 71.1% chance of death “The force of water above the knees gets stronger. Even a healthy adult would get swept away.”

It would seem that the height of your knees plays a crucial role in surviving a tsunami. Once the water level rises above them somewhere around 70 cm, your chances of dying increase 13 fold.

100 cm – 100% chance of death “You cannot stand. You’ll be hit by floating debris. The probability of death is high.”

With the water just a little over your waist, your chance of survival drops to zero. It appears that even if you were to somehow manage to withstand the massive currents of the water you’d likely be hit by something else flowing within it.

As the person who originally tweeted this image said, “this is for all you guys who underestimate a 1-meter tsunami.” Many netizens agreed saying, “I did underestimate the tsunami. It’s nothing to mess with” and “I thought I was going to die when one flooded my area. It’s dangerous to think lightly of the water.”

The main point of the poster is not just to take tsunamis more seriously but to also understand how serious the warnings are if they come to your area. If someone’s telling you to leave immediately, err on the side of caution and do so.

Source: Twitter via Hamusoku

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This is kind of interesting because until recently - even after the quake and tsunami of 2011, I had a highly mistaken Hollywoodized view of a tsunami as a giant wall of water breaking over everything.

But - correct me if I am wrong - that is not accurate, right? It is more like a sudden rise in sea level, rather than an actual wave, as the footage of the real event seems to show. Nothing shows a "wave" as such, more the sea level rising. And rising. And rising. And it was the fast flowing nature of it that seemed to be so deadly. The footage of Sendai airport was particularly shocking. From nothing, to what only seemed like a foot or so of water. No big deal. Until it actually swept away a helicopter.

3 ( +3 / -0 )

It is more like a sudden rise in sea level, rather than an actual wave

One way it was explained to me that was quite graphical and easy to understand was this:

Next time you're having a bath, fill your washing bowl to the brim with water. That's the sea. Dabble your fingers in the water, see the ripples spread to the edges of the bowl, a few of them slopping over the edge. That's ordinary waves on a beach. Splash your hand about a bit more, see how more water splashes over the edge. That's storm waves. Kick the bowl over, watch all the water surge out at once and carry away your towel, soap, shampoo bottle. That's a tsunami.

Where the washing-bowl explanation differs from a real tsunami of course is that the water from the bowl flows harmlessly down the drain, while the water in a tsunami, after it's surged up over the land, will return to the sea causing nearly as much damage on the way back as it did on the way in.

4 ( +4 / -0 )

Finally a useful article in this category! I guess without ever having experienced a tsunami one cannot really fathom the power and danger that is behind it, even at a harmless sounding one meter height.

3 ( +3 / -0 )

Isn't it not only height but also the speed of the water that counts? For example water that is only 30 cm's high but has a higher speed than water of 70 cm high of a lower speed?

0 ( +1 / -1 )

Very useful article. Please, more like this, JT!

2 ( +2 / -0 )

Good article. This, combined with tsunami projections for your local area can be very useful. For example, here in Osaka, they forecast about 1 hour and 50 minutes between a Nankai Trough earthquake and a tsunami landing in downtown Osaka. Here are the Osaka depth projections http://www.pref.osaka.lg.jp/kikikanri/tsunamishinsuisoutei/.

0 ( +0 / -0 )

I saw a documentary about tsunamis that said it is worse if it happens to cities with a lot of buildnings because the water between the buildnings will gain even more speed due to these features. Along with debris and people around, it's death unless you can get higher up.

0 ( +0 / -0 )

I watched the original posting of this video on YouTube and it summed up perfectly for me what a tsunami can do. I can't locate the original on YouTube, so here's a news re-broadcast:


Note how the video starts with a little trickling up the street, and note how quickly it changes to a raging torrent that moves buildings off their foundations. The power of water on the move cannot be underestimated!.

0 ( +0 / -0 )

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