Sunday morning’s wave of death across the east coast was caused by a tsunami — the first time that such a wave has hit Indian shores. Here is a primer on tsunami: A tsunami is a series of waves, generated in a body of water by an impulsive disturbance that vertically displaces the water column. Earthquakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies, such as meteorites, can generate tsunamis. Tsunamis attack coastlines, causing devastating damage and loss of life. In the open ocean, where the water may be 10,000 ft deep, a tsunami may rise only 3 ft above the surface, with its crests as far as 950 km apart. To ships, it is barely noticeable. But the tsunami can race along at 800 km/hr and when it reaches shallow coastal water, is truly devastating. What happens to a tsunami as it approaches land? The wave slows down as it approaches a beach or coastal shelf — but its rear is still traveling at enormous speed. As a result, the whole mass of water piles upon itself, creating a much higher wave, which breaks with overwhelming force on the shore. The wave crests bend as the tsunami travels (called refraction). Wave refraction is caused by segments moving at different speeds as the water depth along the crest varies. In the Pacific Ocean, where the typical water depth is about 4,000 mts, a tsunami travels at about 700 km Because the rate at which a wave loses its energy is inversely related to its wave length, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances with little energy loss. The first sign of an approaching tsunami is the sudden draining away of sea tide. What does ‘‘tsunami’’ mean? Tsunami is a Japanese word with the English translation, ‘‘harbour wave.’’ ‘‘Tsu’’ means harbour, while ‘‘nami’’ means wave. In the past, tsunamis were sometimes referred to as ‘‘tidal waves’’ by the general public, and as ‘‘seismic sea waves’’ by the scientific community. The term ‘‘tidal wave’’ is a misnomer; although a tsunami’s impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. How do tsunamis differ from other water waves?Tsunamis are unlike wind-generated waves, which many of us may have observed on a local lake or at a coastal beach, in that they are characterised as shallow-water waves, with long periods and small wavelengths. A tsunami, on the other hand, can have a wavelength in excess of 100 km and period on the order of one hour. In a tsunami, the sea is usually calm, but in waves, the sea can be rough. How do quakes generate tsunamis? Tsunamis (as in Sunday’s incident) can be generated when the sea floor abruptly deforms.
