We are accustomed to thinking of an earthquake in terms of tremors on dry land. But, as we have seen with the events in Papua New Guinea last weekend, a seismic incident may well occur beneath the ocean floor a thousand miles away, and yet have a potential for destruction greater than any inland continental tremor.
The vibrations of an underwater earthquake, particularly when it occurs in shallow waters, often result in the sudden inundation of adjacent shores by what are popularly known as "tidal waves". The term is something of a misnomer, since the phenomenon has no direct connection with the tides. The word by which it is more properly described is Japanese: tsunami means a "harbour wave".
In the open sea tsunamis are deceptively unobtrusive, with an amplitude of only a few feet. Moreover, there may be as much as 600 miles from crest to crest, so even though they may glide along at several hundred miles per hour, the crests arrive infrequently: a ship rising and falling these few feet over such a long period does not produce a sense of motion, so a tsunami passes by unnoticed. As it approaches a shelving beach, however, and the depth of the water underneath decreases, the wave experiences a consequent decrease in speed. The only way in which it can conserve its energy is to build up its height - and this it does with quite dramatic consequences.
The arrival ashore of a tsu-nami is a very frightening experience. Generally the first wave is only a sharp swell, and relatively unimpressive. But this is followed by a vigorous sucking of the water away from the shore, as the first great trough of the tsunami wave-train nears the shore. Then comes the first of the huge waves, sweeping all before it as it crashes inland from the shore. The rather long interval between the waves often lulls potential victims into a false belief that the tsunami has passed: many additional deaths have arisen from people returning too soon to their inundated homes, before the last of the killer waves has come.
Sometimes, but not always, it is possible to predict the arrival of tsunamis. Not surprisingly, the oceanic disturbance is often directly related to the earthquake magnitude, and since shock waves from an underwater earthquake travel through the solid earth some 15 times faster than a tsunami does through water, a network of seismographs in vulnerable areas can give sufficient warning for the populace to head for higher ground. Indeed, often in such cases, people may even feel the tremor for themselves and take appropriate action.