Tsunami. The word sounds so beautiful. Foreign, exotic, you might think it was the latest Far East must-have delicacy or the next Big Thing in Japanese cartoons. But call it a tidal wave, and suddenly this is one Big Thing that no-one wants.
You can add other adjectives to the tsunami description list. Try fast, try huge. But also try deceptive. For the wave that might tower 30 meters high when it strike the shores of Japan, Hawaii or Indonesia may have passed under countless boats in the deep ocean as barely a ripple. Like many awesome natural phenomena, it's all about the manner of their creation and the manifestation of their energy.
Tsunami (soo-nah-mee), a term adopted officially by scientists in 1963 to cover what previously had the far less memorable tag of seismic sea waves (calling them tidal waves is incorrect), translates from the Japanese as "harbor wave."
Like the terrifying Indian Ocean tsunami that killed thousands in 2004, they are most commonly caused by earthquakes beneath the ocean, though they can also be caused by underwater volcanic eruptions, landslides or, very rarely, asteroid impact – anything, in fact, that suddenly disturbs the equilibrium of the ocean on a large scale.
EARTHQUAKES, LANDSLIDES AND VOLCANOS
Oceanic earthquakes, for example, generate tsunami by deforming the seafloor, either suddenly raising or lowering it, an action that displaces the overlying water. In sub-marine landslides, the equilibrium is altered by the sudden movement of sediment on the sea floor. Above-water landslides unleash falling debris that disturb the water's equilibrium. When a marine volcano is the culprit, the force of the eruption causes the displacement of the water.
NasaA NASA satellite captured this image of deep ocean tsunami waves about 30 to 40 kilometers from Sri Lanka, just hours after the quake that caused the 2004 tsunami. The image was made possible by sunlight reflected on the sea surface.
You can make tsunami-like waves in your bath, if you like. Rest the heel of your palm on the bottom of the bath and jerk your palm downward. Your movement beneath the surface makes waves on top of it. Imagine the earth doing something similar, but with billions of times more power. The energy of the ensuing wave is determined mainly by the amount of the seafloor deformation, but also by its velocity, the water depth near the earthquake source and the efficiency with which energy is transferred from the Earth to the water column above it. Add in a few big figures – like 4,000 meters as an average depth in the Pacific – and you are creating one hell of a wave.
Strangely, if you were far out at sea, you wouldn't even notice a tsunami hitting you. So long as there are thousands of feet of water to contain all that energy, a tsunami might only be about a meter or less tall as it sweeps under your boat, just another bit of swell. But let the water shoal as the wave nears land, and the monster emerges. Decreasing water depth slows the tsunami but if its speed falls, the laws of overall energy balance mean that Nature compensates by making the wave grow. When it finally reaches the shore, the tsunami has become a thing of terror, which can take various forms.
Reefs, bays, entrances to rivers, undersea features and the slope of the shoreline all play a part in how the tsunami strikes land. The single great towering breaking wave of popular image is actually one of the more rare manifestations. A series of breaking waves, a rapidly-changing tide or a bore (a step-like wave with a steep breaking front) are alternatives. At their worst, tsunamis can pile up a 30-meter high wall of water with which to attack the shore, and can flood hundreds of meters inland. A huge landslide that followed an earthquake in Lituya Bay in Alaska on July 9, 1958, created a freak tsunami from hell, sending a wall of water 525-meters high against the opposite shore of the bay. Miraculously, one boat out on the lake at the time survived.
90 RECORDED TSUNAMIS
From 1990 to 2005, there have been about 90 tsunamis recorded worldwide. Until the wave that devastated Southeast Asia, the top 10 of these had claimed just more than 4,000 lives, a figure now dwarfed forever. The worst of these previous killer waves, which struck Papua New Guinea's northern coast on July 17, 1998, is estimated to have claimed 2,200 lives.
