Important Historical Events
Tsunamis were historically referred to as tidal waves because as they approach land they take on the characteristics of a violent onrushing [tide], rather than the sort of cresting waves that are formed by wind action upon the ocean, and which most people are familiar with. However, as they are not actually related to tides, the term is considered misleading, and its use is discouraged by[oceanographer], who prefer the term "seismic sea wave", as it is more scientifically accurate.
Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the earth's crust deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. Waves are formed as the displaced water mass moves under the influence of gravity to regain its equilibrium. When large areas of the sea floor elevate or subside, a tsunami can be created. Large vertical movements of the earth's crust can occur at plate boundaries. Plates interact along these boundaries called "faults". Around the margins of the Pacific Ocean, for example, denser oceanic plates slip under continental plates in a process known as subduction. Subduction earthquakes are particularly effective in generating tsunamis.
Submarine landslides, which often accompany large earthquakes, as well as collapses of volcanic edifices, can also disturb the overlying water column as sediment and rock slump downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can uplift a water column and generate a tsunami.
Large landslides and cosmic-body impacts can disturb the water from above, as momentum from falling debris is transferred to the water into which the debris falls. Generally speaking, tsunamis generated from these mechanisms, unlike the Pacific-wide tsunamis caused by some earthquakes, dissipate quickly and rarely affect coastlines distant from the source area. However if the landslide or cosmic body is large enough, it will create a megatsunami. A megatsunami is a tsunami, usually caused by a collapsing island, asteroid impact, or huge chunks of ice falling into a large body of water, and is hundreds of meters high.
Tsunamis have extremely long periods, 2 minutes to over one hour, and long wavelengths, in excess of 100 km. (Compare a typical wind-generated swell one sees at a surf beach, which might be spawned by a faraway storm and rhythmically roll in, one wave after another, with a period of about 10 seconds and a wavelength of 150 m.)
Typically undersea earthquakes give rise to between 3 and 5 distinct waves (crests), the second or third of which are usually the largest.
In instances where the leading edge of the tsunami is its trough, the sea will recede from the coast half the wave's period before the wave's arrival. If the slope is shallow, this recession can exceed 800 m. People unaware of the danger may remain at the shore due to curiosity, or for collecting fish from the dry sea bottom.
In instances where the leading edge of the tsunami is its first peak, low-lying coastal areas are flooded before the higher second wave reaches them. Again, being educated about a tsunami is important, to realize that when the water level drops the first time, the danger is not yet over.
A wave becomes a shallow-water wave when the ratio between the water depth and its wavelength gets very small. Since a tsunami has a large wavelength, tsunamis act as a shallow-water wave even in deep oceanic water. Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s2) and the water depth. For example, in the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s (about 712 km/hr or 442 mi/hr) with little energy loss even for far distances, while at a water depth of 40 m, the speed is 20 m/s (about 71 km/hr or 44 mi/hr), much slower, but still difficult to outrun.
In deep water, the energy of a tsunami is constant, a function of its height and speed. Thus, as the wave approaches land, its height increases while its speed decreases. While in deep water a person at the surface of the water would probably not even notice the tsunami, the wave can increase to a height of 30 m and more as it approaches the coastline and compresses. Tsunamis can cause severe destruction on coasts and islands, even at locations remote to the source event, where that event itself is not even noticable without instruments.
Tsunamis propagate outward from their source, so coasts in the "shadow" of affected land masses are usually fairly safe. However, tsunami waves can diffract around land masses (as shown in this Indian Ocean tsunami animation as the waves reach southern Sri Lanka and India). They also need not be symmetrical; tsunami waves may be much stronger in one direction than another, depending on the nature of the source and the surrounding geography.
Megatsunamis and seiches
Related to a tsunami is a seiche, an underwater, irregular fluctuation or rhythmic rocking of the water level of a lake. Often large earthquakes produce both tsunamis and seiches at the same time and there is evidence that some seiches have been caused by tsunamis.
The highest tsunami wave ever recorded was very localized: caused by a landslide in Lituya Bay, Alaska in 1958, a tsunami more than 500 m high stripped trees and soil from the steep walls of a fjord. By the time the wave reached the open sea, however, it dissipated quickly. The height of the waves was determined more by the topography of the inlet than by the energy generated by the landslide.