Tsunami - the giant, killer waves

Early in 2004, a devastating earthquake hit Iran killing over 40,000 people. The deadliest tornado in history tore through Bangladesh in 1989. 1,300 people died and 50,000 were left homeless. In 1970, a cyclone hit Bangladesh, killing 500,000 people. This is the worst natural disaster of the 20th Century. The worst ever flood in history, hit China in 1887 as the Yellow River broke its banks and claimed a million lives. A super-volcano exploded in Sumatra in 70,000 BC. It blasted out enough debris to bury greater London to a depth four times the height of Canary Wharf.


Such natural phenomena are all the more catastrophic as they could in turn trigger what are called tsunami - the most destructive waves in the ocean. Pronounced tsoo-nah-mee, and translated as harbor wave from Japanese - it is a series of waves generated in a body of water by an impulsive disturbance that vertically displaces the water column.


What causes a tsunami?

Contrary to popular belief, these waves are not caused by the tide or the wind. 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. Tides actually are a result of the imbalanced, extraterrestrial, gravitational influences of the moon, sun, and planets. Earlier these waves were termed as "seismic sea waves" by scientists. This is misleading. "Seismic" implies an earthquake-related generation mechanism, but a tsunami can also be caused by a non-seismic events. In fact tsunamis are generated by landslides, volcanic eruptions, explosions, and by the impact of cosmic bodies, such as meteorites besides underwater earthquakes. These disturbances cause the seabed to move swiftly and shift a large amount of water and disrupt the sea surface. A train of waves is set in motion traveling away from the source of disturbance.


About four out of five tsunamis happen within the Pacific Ring of Fire, a zone of frequent earthquakes and volcanic eruptions roughly matching the borders of the Pacific Ocean. Along the ring's edges, giant slabs of the earth's crust, called tectonic plates, grind together. Sometimes the plates get stuck, and pressure builds, causing the plates to suddenly come apart and slam into a new position. This jolt causes an earthquake and waves are formed as the displaced water mass, acting under the influence of gravity, attempts to regain its equilibrium. When large areas of the sea floor elevate or subside, a tsunami can be created.


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 down and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can create an impulsive force that uplifts the water column and generates a tsunami. Super marine landslides and cosmic-body collisions disturb the water from above - momentum from falling debris is transferred to the water into which the debris falls. Fortunately such tsunamis, unlike the earthquake-generated ones, dissipate quickly and rarely affect coastlines far away from the source area.


Why are tsunamis different?

Wind-generated waves that occur in lakes or at coastal beaches are shallow-water waves, with long periods and wavelengths. The typical wind-generated swell might have a period of about 10 seconds and a wavelength of 150 meters. A tsunami, on the other hand, can exceed wavelengths of 100 km and period of the order of one hour.


Such long wavelengths are deceptive - they make the tsunamis seem like shallow-water waves. For a shallow-water wave the ratio between the water depth and its wavelength is very small. In the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at over 700 km/hr. 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.


A tsunami can move across the ocean at 500 miles an hour, matching the speed of a jet plane. In deep water its waves are only a few feet high, but when the waves approach shore, they increase in energy and height. As the water depth decreases, the tsunami slows down. The energy flux, which is dependent on both its wave speed and wave height, remains nearly constant. Even as the tsunami's speed diminishes when it enters shallower water, its height grows. This shoaling effect helps a tsunami that is imperceptible at sea, to grow to be several meters in height on approaching the coast.


On reaching land 

Even before it hits the land, there is a massive vacuum effect - A tsunami can suck all the water out of a harbor. The sea bottom is filled with flopping fish and stranded boats. This is because waves are made out of crests (high points) and troughs (dips) between crests. When a trough hits land first, the water level drops drastically. Usually another wave reaches the shore about 15 minutes later, then another and another, for atleast two hours. When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide, or a series of breaking waves. Tsunamis can strip whole beaches of sand that may have taken years to accumulate and can uproot trees and other coastal vegetation. The rapidly moving water can inundate or flood hundreds of meters inland. Tsunamis may reach a maximum vertical height onshore above sea level, often called a run-up height of 10, 20 and at times 30 meters. A tsunami hits the coast with tremendous amounts of energy. It can grow more than 100 feet tall and flatten whole villages, devastating property and life.


The largest recorded tsunami reached Siberia's Kamchatka Peninsula in 1737 - it measured 210 feet above sea level. Tsunamis have killed more than 50,000 people in the past century. To save lives, scientists established the Pacific Tsunami Warning System, based in Hawaii, in the U.S.A. Its network of earthquake detectors and tide gauges detects quakes that may cause a tsunami and therefore help anticipate and plan how to escape from these killer waves.