Tsunami

A tsunami is a giant wave hitting the coast. It is usually caused by an earthquake, volcanic eruption, or underwater landslide (also often caused by an earthquake). At sea, the wave is hardly noticeable, but when it is slowed down by a shallow area, such as near the coastline, it becomes steeper and can rise to a height of tens of meters, with an enormous destructive force. Imagine having a mid-sized building suddenly appear at the edge of the water and fall down on the beach. Now imagine that the building is made of water and as wide as the eye can see, and beyond that. You don't want to be underneath that.

Realistically, the only way to survive a moderate-to-large tsunami is to not be present in the inundation zone when it arrives. Fortunately, if it's detected by local warning systems or if you know what to look for before you can see it, you will likely have at least several minutes to get away before it hits.

Understand

If the epicenter of an earthquake is under the ocean or a large lake, the earthquake will move huge amounts of water. Even a small earthquake can cause an underwater landslide, and an underwater volcano may transform huge amounts of water into steam, both with similar effects. The tsunami risk depends on in what direction the water is moved. In the worst case, such as when the edge of a tectonic moves up- or downwards along the plate boundary, the water will move more or less in a coordinated direction, causing a wave. The wave can be small or enormous. There may be one, but probably there will be more than one, the first one not always being the biggest. And earthquake aftershocks are common, which means that subsequent tsunamis are also common.

Tsunamis can strike the coast mere minutes to many hours after an earthquake, depending on how far away from the coast the epicenter was; tsunamis can travel very far from their origins. For instance the 2004 Indian Ocean tsunami was caused by an earthquake north of Sumatra but also wreaked havoc on the east coast of Africa several hours later. An earthquake near one side of the Pacific Ocean can cause a tsunami both locally and on the opposite side of the Pacific Ocean. This means that you may not feel the earthquake that triggered the tsunami.

Tsunamis may travel as fast as a commercial jet and are often invisible before it is too late, as they grow in height only as they hit the shallower waters near the coast. You need to run before you see it.

Electronic warning systems

There are tsunami warning systems for the Pacific and Indian Oceans. Although they are not perfect and false alarms do occur, their evacuation warnings should be heeded instantly, as the worst that can happen in case of a false alarm is a ruined day (and a good story to tell later), while getting hit by a large tsunami will kill you and everyone around you.

Electronic warning systems operate on two time scales:

If the earthquake was on the other side of the ocean, you may have as much as 12 hours warning before it hits, and that will give you and everyone else plenty of time to pack up and create a traffic jam on the road to the next town inland, where you can usually hang out for a few hours, get the all-clear signal from authorities, and go back (assuming your destination hasn't been destroyed).
On the other end of the scale, if you feel an earthquake and then hear a tsunami warning, your only job is to scoop up your kids and run for your life. That combination means that the tsunami will likely hit within minutes, not hours. Leave everything else behind and focus on getting inland and uphill. In this quick-moving scenario, people moving on foot or bike often have an advantage over people stuck in a traffic jam.

Natural warning signs

Should you find yourself in the unlikely situation of a tsunami where there is no warning infrastructure, there are usually what are referred to as "natural warning signs": nature itself telling you that a tsunami is coming. If an earthquake occurs right on the coast you may only have a matter of minutes to escape, so don't hesitate to evacuate. Some noticeable natural warning signs are:

Water receding and exposing ocean floor that is usually under water at all times. This is a dead giveaway that a larger tsunami is imminent and you have, at best, a couple of minutes to flee from this point on. The further the water goes out, the more of it will come suddenly rushing back to kill anyone who stopped to take pictures or to explore the just-revealed seashells instead of getting out of the tsunami's path.
Notable shocks of an earthquake off or close to the coast. If you are at the coast and feel an earthquake, don't wait for any official warnings, but head inland and to higher ground immediately.
Loud or unusual noises coming from the sea

During a tsunami

During a tsunami warning, follow any designated escape routes and pay close attention to what the authorities are saying. Especially in countries with a Pacific coast line, they are usually well aware of the nature of a tsunami and have refined their emergency plans with earlier tsunamis. The most important thing during a tsunami is obviously to get to high ground. However, trees and even the roofs of houses are a sub-par alternative to mountains, as the enormous force of the tsunami may simply knock you out of the tree or destroy the house you are seeking shelter in (or on the top of).

Under no circumstances should you count on your swimming ability against such a powerful current! Even the best swimmers drown when they're knocked unconscious, and by the time it hits you, that wall of water will be churning with a surprising number of deadly objects, such as trees and boulders and vehicles that used to be parked near the beach. If you are swept away by the water, try to hold onto a floating piece of debris and await rescue.

Do not ever go to the coast to sight-see or watch for a tsunami! This is a surprisingly popular way for people to accidentally kill themselves. Hundreds of people have been killed by tsunamis over the years – sometimes dozens at the same time – because they heard the early warning systems, thought they knew a safe place near the coast to get some killer pictures, and got swept away or crushed when the tsunami hit.

After a tsunami

Be aware that there may be more than one tsunami wave, and they can occur up to 24 hours after the initial wave, so remain evacuated until you are told by officials that it is safe to return.

When returning to buildings, be aware that floodwaters may have caused serious damage to infrastructure. Floodwater may also be contaminated with sewage.

Avoid sightseeing and only travel if necessary. If you have made travel arrangements with an airline or otherwise, it is best to contact them for advice.