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Imagine a wave as tall as a 10-story building that can approach land at 100 miles per hour.

It uproots and destroys everything it encounters from large trucks to entire buildings. And then the wave recedes, pulling the destruction back with it. More waves come and then go back, between five minutes to an hour apart. Tsunamis are difficult to predict because they can be caused by disasters across the ocean. Entire towns can be demolished. At times, thousands of people drown or are killed when stricken by floating debris. When the tsunami finally stops, missing loved ones, power outages, lack of clean water and more cause mass confusion.

The devastation has only just begun.

What is a Tsunami?

A tsunami—harbor wave, in Japanese—is a wave that can reach a height of 100 feet. Sometimes tsunamis aren’t large, breaking waves, but appear as a rapidly rising water level, which is why they are sometimes wrongly called tidal waves. Small tsunamis happen almost daily due to earthquakes and other disturbances, but are too far away from land to cause any damage. Wave trains, or the series of waves, can travel as fast as 500 miles per hour deep in the ocean but will only rise a few feet. In shallow water, near the coast, the waves crash against the ocean floor, lowering the wave’s speed and raising its height. Waves recede and, if a receding wave collides with an oncoming wave, the kinetic energy released forces the forward-moving wave to mount higher.

Tsunamis can be caused by underwater volcanic eruptions, underwater landslides, ice falls and as a result of the impact from a meteorite falling into the ocean. But the most common cause of a tsunami is an undersea earthquake. When two tectonic plates push together, it causes an earthquake that pushes energy up into the ocean. This displaces water, and lots of it, in different directions, one into the sea and one toward the coastline. By the time these killer waves reach land, they are tall and powerful enough to sweep away entire villages.

About 80 percent of tsunamis occur within the Pacific Ocean’s Ring of Fire, a zone full of volcanic and seismic activity. This is not to say tsunamis that occur elsewhere aren’t dangerous. History’s deadliest tsunami occurred in the Indian Ocean in 2004. Locations hit by tsunamis include Japan, Hawaii, Chile, Indonesia and Portugal, to name a few.

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Hundreds of thousands of people are killed or injured in tsunamis. Victims— people and animals—drown, are electrocuted or are killed by explosions or floating debris. Tsunamis destroy cliffs and beaches, along with trees and vegetation. The rush of water ruins infrastructure of homes, businesses and bridges costing as much as billions of dollars in damages. Ships and large objects can be carried miles inland, adding to the destruction.

Coastal areas flood, causing problems with sewage and contaminating drinking water. Diseases such as malaria spread. Sea creatures can die from pollution in the sea. If a tsunami hits a nuclear plant, as one did in March 2011 in Japan, radiation can become a danger, which can result in birth defects, cancer or death years after the tsunami occurred. This is all not to mention the emotional and mental toll on those fortunate enough to survive. Victims often suffer from Post Traumatic Stress Disorder years after the tsunami, or even their entire lives.

How to Survive a Tsunami

The first way to prepare for a tsunami is to know where tsunamis are likely or even possible to occur. Coastal areas near fault lines or near volcanoes are obvious places to consider, particularly if they are near the Pacific, but tsunamis have begun and then raged through entire oceans. Earthquakes as far as Chile have caused tsunamis in Hawaii.

They can happen minutes, hours or even a day after the initial earthquake (depending on how far away it was). And just because a tsunami hasn’t happened in an area or isn’t predicted to happen in an area, doesn’t mean it won’t. A tsunami can hit any ocean shoreline.

It might start as a mere rumble, but a loud, roaring sound coming from the ocean could also indicate a tsunami. Survivors have compared the sound to that of a freight train. Another telling sign that a tsunami may be coming is a rapid fall or rise of ocean waves near the coast. A receding ocean may precede a tsunami by five minutes or fewer, according to experts. So, if you see a rapidly receding ocean, get out of there. Fast. (If you’re already on a boat in the ocean, going deeper to sea would be safer than trying to head toward the shore, where the waves increase in height.) Animals also tend to know when a tsunami is coming so if you see some leaving, coupled with other signs, you should probably follow suit.

Evacuate immediately if you hear a tsunami warning. Do not try to get a closer look. Ever. You’re too close if you can see it. Know that there will be chaos all around you if a tsunami strikes. The ocean will roar. People will be running, screaming and even dying around you. Staying calm can save lives. Head for the hills or a mountain, anywhere on higher ground (preferably 100 feet above sea level) or travel two miles inland. If you can’t do that fast enough, find a sturdy, concrete building with at least 10 stories, as that’s as high as waves can reach (but, if possible stay away from buildings that are near water because water can crack or harm walls). Go to the highest floor or a rooftop. If you can’t do that, climb a tall, sturdy tree and hold on. Stay away from downed power lines or anything could electrocute or fall on you.

Physical fitness can only help here. Being able to run fast or having the strength, speed, and agility needed to climb a tree, not to mention be able to fit on a branch, could save your life. If you aren’t able to evacuate quickly enough, you can still survive if you’re a strong swimmer. (Not a strong swimmer? Take lessons.) You’ll want to know how to swim, with debris such as parts of houses, large trucks or ships coming at you.

Move with the current and keep your legs up, so they don’t get caught on something that can pull you underwater. Shed loose clothing or anything on you that could get caught on other items. Make use of anything you can use to float.

Continue to be on the alert after the waves begin. Just because you survive a first wave, second or even a third, doesn’t mean you’re in the clear. Waves can keep coming for as long as an hour after the first. Wait until official word is given that it’s safe to return. Get away from the beach and stay away from rivers and streams because tsunamis can travel up those bodies of waters as well. Although those tips may help someone survive during a tsunami, the best way to ensure survival is to prepare before the first wave strikes.

Tsunami Warnings

The National Oceanic and Atmospheric Administration has its own network of radio stations that broadcast continuous weather information directly from the nearest National Weather Service office. It’s called NOAA Weather Radio All Hazards and it airs official Weather Service warnings, watches and forecasts around the clock. So listen in, if you know of a recent earthquake or disaster. Sign up to receive early alert text messages from NOAA’s tsunami warning center online.

The National Tsunami Hazard Mitigation Program recommends knowing if your work, school, your children’s school, or any other place you and loved ones frequent are in a tsunami hazard or evacuation zone. If they are, find out the location’s height above sea level and distance from the coast (you can use Google Earth). This might affect evacuation plans. Find out what they are. Knowing where your children are supposed to be picked up in the event of a tsunami is far safer than racing to their school only to realize they’re gone and you’ve put yourself in harm’s way needlessly.

Next, come up with your own evacuation plan. Map out routes to take you to safety, preferably at least two miles inland or 100 feet above sea level. Think of alternative routes. Practice so you know them so well that no matter what is going on, day or night, you can get there. Make sure your family knows the evacuation plan. If you’re a tourist, learn about the places you are staying and visiting and plan accordingly. Learn the local tsunami protocol.

Have an emergency kit ready (more on specific gear later). Basic supply kits should include one gallon of water per person for three days; a three-day supply of non-perishable food; a battery-powered radio with batteries; a flashlight and batteries; a whistle; a wrench or pliers; a dust mask; a map; a cell phone with inverter or solar chargers, and a manual can opener.

Tsunami Gear Guide

It’s always a good idea to have a basic survival kit at hand in case a disaster strikes, but if you live in or are visiting a coastal area, arm yourself with these tools to make your chances of survival that much greater.

Personal Flotation Device

PFDs, generally referred to as life vest or jackets, provide buoyancy—the force in pounds needed to keep a person’s head and chin above water. (Generally, adults need an extra seven to 12 pounds of buoyancy to stay afloat.) PFDs should fit snuggly. Many PFDs include extras such as a knife holder. PFDs are also available for dogs.

 

 

 

 

 

 

Dry Suit

Watersport dry suits, unlike wetsuits, prevent water from entering, keeping you dry and warm while immersed in water. If you have the chance to get one of these on before a tsunami strikes, do so. The suit can also protect the wearer from polluted water and sharp edges of debris.

 

 

 

 

 

 

 

 

Water filter

For survival, there isn’t much we need more than water. After a tsunami strikes, there’s often chaos and it can be days or even weeks before help arrives. Depending on a few factors such as temperature and activity, a person can live three days without water. Water filters can come in the form of micro-filters, survival straws or even UV-purifying water bottles.

 

 

 

 

 

 

Signal Flare

Signal flares can shoot high enough in the sky to alert others to your location. You might escape a tsunami only to get stranded somewhere, unable to leave safely or you may be too injured to move. A flare can signal others you need help and possibly save your life.

 

 

 

 

 

 

 

Dry Bag

You might be wondering where you’re going to put all this stuff while trying to escape a tsunami. A dry bag, that’s where. Typically made of nylon and waterproof, one of these is a must-have. Go ahead and put gloves, a whistle and other survival gear in there, while you’re at it.

 

 

 

 

 

 

 

Submersible two-way radio

Having a radio to hear the news and warnings before, during and after a tsunami is essential. If you get separated from a loved one and you each have one of these, you’ll be able to communicate in all the panic. And, of course, ensure the radio remains functioning by purchasing a weatherproof device.

 

 

 

 

 

 

Editor’s note: A version of this article first appeared in the Doomsday 2016 issue of American Survival Guide.