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Seawater Displacement
A subduction earthquake occurs when one of the tectonic plates that covers the Earth shifts below an adjoining plate. When the second plate moves upward, it displaces hundreds of thousands of tons of seawater. The result is a column of water that reaches from the ocean floor to the surface. In the deepest parts of the ocean, the column could reach over 6,000 meters (20,000 feet) below the surface. Subduction earthquakes are the trigger for most tsunamis but other violent events, such as landslides, submarine slides, calving glaciers and meteorite strikes can also displace enough seawater to cause a tsunami.
Speed of Travel
That column of seawater with all its stored energy travels across the ocean at the speed of a jetliner. Some waves can reach between 805 and 965 kilometers (500 to 600 miles) per hour. At these speeds, the tsunami can cover the distance from the coast of Asia to the United States Pacific coast in about 24 hours.
Wave Height
A powerful tsunami traveling rapidly across the ocean may appear as a ripple on the surface in the open sea, often little more than .5 meter (1.6 feet) in height. Passengers and crew aboard an ocean liner may not even feel anything when a tsunami packing destructive forces passes beneath them. The ocean depth diminishes as the wave approaches shore. Friction caused by the water moving against the shallow bottom slows the wave down, but the energy still does not dissipate. That forces the wave to move upward, the only direction it has left to go. Eventually, the back of the wave reaches the slowing, leading edge, which causes the tsunami wave to climb to heights greater than 100 feet.
Wave Piling
Typically, the wave trough reaches the shore first. Water recedes from the beach and bays empty, but that only lasts for a few minutes before the tsunami wave slams the shoreline with the full fury of its stored energy. Often, this first wave is only the beginning. Tsunamis typically contain more than one large wave. If the distance between each wave is short, they end up piling into one another, creating a massive tsunami. If the distance between waves is longer, a series of tsunami waves batter the coastline, with each one as dangerous and damaging as the one before it.