Tsunamis in scientific terms are also called as seismic sea waves.
The reason of their occurrence is the sudden change in the surface of sea which is usually earthquakes and large landslides. Tsunamis are not "tidal waves" but people disregard them as that. In reality they are not caused due to tidal actions. Every earthquake cannot create Tsunami. In order to create it, it must be under or near the ocean, must be very huge and able to create vertical movements. Almost every oceanic regions present in the world are prone to Tsunamis but Pacific Ocean is much more vulnerable to such earthquakes and thereby, Tsunamis. They are not very powerful deep inside the ocean since they carry small amplitudes and speed of around 800 kms per hour but it dissipates its energy after reaching the outer ocean. In fact the ocean becomes a ‘pond' to Tsunamis. Moreover, there are numerous waves to accumulate into it.
Tsunamis are very energetic, powerful and enormous amount of energy can spread onto a large volume of water deep within sea. When they reach coastline or shallow water their energy concentrates into smaller volume. Its speed tends to decrease but the amplitude heightens up to a dangerous level which can be 50 feet or even more than that covering the entire island.
Tsunamis are formed after occurrence of earthquakes, landslides or volcanic eruptions under the sea floor. When such movements are happened under the deep sea, huge amount of energy releases as a consequence of quick upward and bottom movements. For example after occurrence of volcanic eruptions the oceanic floor very quickly moves upward several hundred feet. When such thing happens, huge amount of ocean water gets pushed upwards resulting into formation of a wave. Similarly a huge earthquake can lift several thousands of square kilometers of sea surface which further results into formation of huge waves. In fact The Pacific Ocean is the oceanic region which is very much prone to such Tsunamis. The reason behind this is also the numerous amounts of geological activities undertaken by it.
In the open ocean Tsunamis may appear to be very small and that's why they get unnoticed until they reach shallow waters or the coastline. Waves occurring after such movements carry large wavelengths. As a result these wavelengths can take gigantic and dangerous forms thereby resulting into Tsunamis. There is no wonder that such Tsunamis can cause massive destruction around the coastal life and huge amount of loss of life.