What determines how destructive a tsunami will be near the origin and at a distant shore?
Tsunamis arrive at a coastline as a series of successive crests (high water levels) and troughs (low water levels)—usually occurring 10 to 45 minutes apart. As they enter the shallow waters of coastlines, bays, or harbors, their speed decreases to about 50–60 km/h. For example, in 15 m of water the speed of a tsunami will be only 45 km/h. However 100 or more kilometers away, another tsunami wave travels in deep water towards the same shore at a much greater speed, and still behind it there is another wave, traveling at even greater speed. As the tsunami waves become compressed near the coast, the wavelength is shortened and the wave energy is directed upward—thus increasing their heights considerably. Just as with ordinary surf, the energy of the tsunami waves must be contained in a smaller volume of water, so the waves grow in height. Even though the wavelength shortens near the coast, a tsunami will typically have a wavelength in excess of ten kilometers when it comes ashore. Dependi
