I offer up the following as a definition for what we call a tsunami. Tsunami is of Japanese origin. “Tsu” meaning harbor and “Nami” waves. You may also hear these waves referred to as tidal waves or even seismic waves. Both of those can be somewhat misleading in that tsunami waves can come about by other means. For instance, tsunami waves are not at all related to astronomical tides that come about from solar, lunar, or even other planetary influences that are affected by gravity.
Now that we know from whence the term tsunami comes, we will begin to look into what causes them. Most videos or pictures we see of tsunamis show huge walls of dangerous waves crashing onto the shore creating severe damage and death in the area of impact. While they may begin as nearly normal-sized waves the first wave is generally not the largest. Typically, the killer wave is the second thru fourth or even after those. One wave will flood the shoreline and then recede so far back that large portions of the ocean floor are exposed. The next waves washes ashore bringing the debris from the first. These waves continue to build and come every five to sixty minutes sometimes reaching heights of over one hundred feet at up to five hundred miles per hour causing widespread destruction and serious loss of life. Where do they come from?
There are several ways that a tsunami can be created. Most lines of logic state that they come from violent shifting of the ocean floor. Usually, this shift is caused by an earthquake, landslide, or lava seeping into the sea. However, there can be other reasons for tsunamis. One such other reason is a meteorite impact. According to http://tsunami.org/what-causes-a-tsunami/ earthquakes are the number one cause of a tsunami (72%). With landslides and unknown phenomena nearly tying at 10-11%.
The surface of the earth is comprised of a series of tectonic plates. You can picture them as a puzzle. Some form continents and others form oceans and seas. These plates move several inches each year and nothing abnormal occurs. Sometimes they bump into or move away from one another and that is called a plate boundary. These boundaries when moving toward one another are termed subduction and when they move away from one another it is spreading. The plates can move under one another, on top of one another, or past one another on the ocean floor. This movement is what causes a tsunami because it is displacing a large amount of water as it moves, changing its equilibrium.
How tsunamis form
Once the heavy plate slips beneath the lighter one, it creates a bulge in the sea floor. When the seafloor cracks open from the tension of this slippage the water is shoved forcefully toward the surface causing the earth to crack and shake. These are termed fault lines. As the water rises and then flattens it creates giant ripples. Sometimes this movement occurs underneath the sea floor and other times it can be generated from land or air-based incidents.
The most dangerous of tsunamis come about due to under the sea tectonic plate movements. In the deepest portion of the ocean, these waves may only appear a foot or more in height. As they come ashore the water gets shallower and that slows the waves down. Slowing the wave down increases their height. The upper portion of the wave moves more rapidly than the bottom and that also increases their height.
The lowest portion of the wave hits shoreline first, that is called the trough. A vacuum effect is created from this motion. What occurs is the coast is sucked into the ocean or sea. When this happens it should serve as a signal that something worse is yet to come. The waves in a tsunami come successively and are called wave trains. Sometimes you may only see a rapidly surging tide coming along the shore as a signal that more may be heading your way. At best, you may have five minutes to plan your escape.
Above the water a landslide, earthquake, space-borne objects falling onto the earth, or volcanic eruption can create tsunamis. These incidents are also known to disrupt the water beneath the surface and spawn tsunamis. If a tsunami is created from one of these means they are typically short-lived.
The Pacific Ocean is host to most tsunamis because of the earthquake activity from tectonic shifting in those subduction zones. This is sometimes referred to as the “Ring of Fire” and nearly ninety percent of all earthquakes take place along this area. There are other locales that also experience some of this phenomenon. Those are; Alaska, Japan, Chile, and Indonesia.
Generally, earthquakes of magnitude seven or above that occur near, within sixty miles, the ocean or sea spawn tsunamis.
Sources:
http://tsunami.org/what-causes-a-tsunami/
https://www.nationalgeographic.com/environment/article/tsunamis