Earthquakes, mass movements above or below water, volcanic eruptions and other underwater explosions, landslides, large meteorite impacts comet impacts and testing with nuclear weapons at sea all have the potential to generate a tsunami. However, 86 % of all Tsunamis result from "seaquakes".

The term “tsunami” comes from the Japanese words “Tsu” meaning “harbor” and “Nami” meaning wave. The term was created by Japanese fishermen who returned to port to find the area surrounding their harbor devastated, although they had not been aware of any waves in the open water.

Tsunamis have been historically referred to “tidal waves” by the general public because as they approach land, they take on the characteristics of a violent on-rushing tide rather than the sort of cresting waves that are formed by wind action upon the ocean(with which people are more familiar). However, since they are not actually related to tides, which result from the imbalanced, extraterrestrial, gravitational influences of the moon, sun, and planets, the term is considered misleading and its usage is discouraged by oceanographers.

Their size and power can produce devastating effects; travelling across oceans and reshaping entire coastlines.

Megatsunamis are usually caused by a very large impact or landslide into a body of water explosive volcanic events, or meteor impacts when the water cannot disperse in all directions. Underwater earthquakes do not normally generate such large tsunamis.

But huge landslides and the mega-tsunami that they cause are extremely rare. The geological record suggests that megatsunamis generated by the collapse of flank of a volcanic island, their most common cause, may occur every few thousand years. The last one happened 4,000 years ago on the island of Réunion.

Many people have the mistaken belief that tsunamis are single waves. They are not. Instead tsunamis are "wave trains" consisting of multiple waves. In many tsunami events the shoreline is pounded by repeated large waves. A tsunami can cause massive damage thousands of miles from its origin, so there may be several hours between its creation and its impact on the coast, more time than it takes for seismic waves to arrive.

The difference between tsunamis and normal waves or wind-generated waves caused is the extreme distances between wavelengths. Wave lengths is the distance from one wave crest to the next wave crest, which can be between 100 and 300 km. The wind-generated swell one usually sees at beach, for example, rhythmically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m. A tsunami, on the other hand, can have a wavelength in excess of 100 km and period on the order of one hour.

As a result of their long wave lengths, tsunamis behave as shallow-water waves. A wave becomes a shallow-water wave when the ratio between the water depth and its wave length gets very small. Because the rate at which a wave loses its energy is inversely related to its wave length, tsunamis not only propagate at high speeds, they can also travel great, trans-oceanic distances with limited energy losses.

As a tsunami leaves the deep water of the open ocean and travels into the shallower water near the coast, it transforms.  This is because a tsunami travels at a speed that is related to the water depth - hence, as the water depth decreases, the tsunami slows. Consequently, as the tsunami's approaches the shore and travels into shallower water, its speed diminishes, it starts to compress. Because of this shoaling effect, a tsunami, imperceptible at sea, may grow to be several meters or more in height near the coast. When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore. However, the tsunami's energy flux, which is dependent on both its wave speed and wave height, remains nearly constant.

Just like other water waves, as tsunamis rush onshore, they begin to lose energy - part of the wave energy is reflected offshore, while the shoreward-propagating wave energy is dissipated through bottom friction and turbulence. Despite these losses, tsunamis still reach the coast with tremendous amounts of energy, reaching a maximum vertical height onshore above sea level, often called a run-up height, of 10, 20, and even 30 meters.

But tsunamis are a global phenomenon. They can occur in all larger seas of the world. They are possible wherever large bodies of water are found, including inland lakes, where they can be caused by landslides. Thus, fatal tsunamis occur in geologically less active oceans such as the Atlantic, the Indian Ocean or the Mediterranean as well.

There is often no advance warning of an approaching tsunami. However, since earthquakes are often a cause of tsunami, an earthquake felt near a body of water may be considered an indication that a tsunami will shortly follow.

One of the early warnings comes from nearby animals. Some scientists speculate that animals may have an ability to sense subsonic waves from an earthquake minutes or hours before a tsunami strikes shore. Hence, many animals sense the danger and flee inland and to higher ground before the water arrives.  Humans, on the other hand, head down to the shore to investigate.

This is because the first part of a tsunami to reach land is a trough rather than a crest of the wave. Therefore, the water along the shoreline may recede dramatically, exposing areas that are normally always submerged. If the slope is shallow, this recession can exceed many hundreds of meters. People unaware of the danger, may remain at the shore due to curiosity, or for collecting fish from the exposed seabed.

If one is near a beach when a strong earthquake is felt, or sees the sea water starting to recede or if one hears a loud roar, the following safety procedures can help save lives:

• Moving IMMEDIATELY a few kilometers to the interior, away from the coast. Not waiting for a formal warning.
• Listening to the radio or televison to get the latest emergency information and evacuating, if asked to do so.
• Staying away from the beach.
• Climbing to higher grounds, if possible.
• Avoiding river valleys.
• Escaping into higher buildings may offer protection, however this is not guaranteed. The building could be swept away by the tsunami or could even collapse.
• NOT lingering even for one minute to rescue belongings or luggage. Only taking the most important items: money, mobile phone, warm clothes, and getting out.
• Not immediately returning to the danger zone, after the tsunami has hit,. A tsunami is a series of waves. Often a second, even larger wave may arrive a few minutes after the first wave. Further waves can reach land even hours after the first wave.
• It is best to WAIT outside the danger zone until officials give the 'all-clear' signal.