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Tsunamis are unlike wind-generated waves, observed on a local lake or at a coastal beach. The wind-generated one can be caused by a storm in the Pacific. It rhythmically rolls in, one wave after another and might have a period of about 10 seconds and a wave length of 150 m. A tsunami, however, can have a wavelength in excess of 100 km and period on the order of one hour. Due to this, tsunamis behave as shallow-water waves. A wave becomes a shallow-water wave when the ratio between the water depth and its wavelength gets very small. Because the rate at which a wave loses its energy is inversely related to its wavelength, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances with limited energy losses. Back (When a Tsunami strikes) | Next (Preparing for the arrival of a tsunami) |