We are accustomed to thinking of earthquakes in terms of tremors on dry land. But, as we have seen with the tragic events in the Indian Ocean over the 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, writes Brendan McWilliams
"Seaquakes", as they are sometimes called, as a general rule do little direct damage of themselves, beyond breaking the odd submarine cable or disturbing bottom-feeding fish.
But as a secondary effect, the underwater vibrations often give rise to long gentle swells on the surface of the ocean, which, on reaching a shoreline, pile up into massive towers of water, sometimes 50 or 100 feet in height.
The popular name for these phenomena, "tidal waves", is a misnomer, since they have little or nothing to do with tides; the proper name, "tsunami" - meaning "harbour wave" - evolved because the Japanese coastline is one particularly prone to these lethal monsters.
Under the influence of very high winds, ordinary sea waves are an awe-inspiring sight, and may measure a few hundred feet from crest to crest, travelling at speeds of up to 60 miles per hour. But tsunamis are, by contrast, deceptively unobtrusive, and virtually undetectable away from the coast.
They may be as much as 600 miles from crest to crest, and their height is only a few feet. It is the tsunami's "speed" which is remarkable - and the greater the depth of water the greater its speed; in deep ocean it can glide along at 500 m.p.h., or even faster.
But the crests pass only at the rate of about one an hour, and a ship rising and falling these few feet over an hour does not produce a noticeable sense of motion.
It is for this reason that in some places prone to tsunami the local fishermen, at the first signs of an attack, adopt the apparently lemming-like approach of rushing to their boats, their idea being that they may be able to ride out the surge at sea.
As it approaches a shelving beach, however, and the depth of the water underneath decreases, a tsunami, forcibly slowed down, conserves its energy by building up its height - which it does to quite remarkable effect.
The arrival ashore of a tsunami 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. The rather long interval - 15 minutes to an hour - between successive waves often lulls potential victims into a false belief that the danger is over; many additional deaths associated with such events have arisen from people returning too soon to their inundated homes.
Many tsunami casualties could be avoided if these waves could be predicted accurately. Sometimes, indeed, they can: not surprisingly, tsunami generation is often directly related to the earthquake magnitude, and since shock waves from an underwater 'quake 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. Moreover, in such cases, people may even feel the tremor for themselves and take appropriate action.
Japan has a sophisticated network of over 150 seismographs, located at strategic points around the islands and operated by the Japanese Meteorological Agency.
When a signal from any one of these seismographs exceeds a certain threshold, warning messages are transmitted.
But advance warnings of this kind are not available to the less sophisticated populations of the Indian Ocean and many parts of the Pacific. And then again, some of the largest tsunamis are known to occur when the measured seismic shock is deceptively low. This happens when the wave has been caused, not by the tremor itself, but by a related underwater landslide, or by tremor-induced "slumping" of the ocean bed. The detection of these "unannounced" tsunamis by careful examination of seismic records is one of the more challenging problems faced by geophysicists.