Important Historical Events
Tsunamis act very differently from typical surf swells; they propagate at high speeds and can travel great transoceanic distances with little energy loss. A tsunami can cause 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.
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.