Our planet is encrusted with a fragmented layer of solid rock whose segments, rather like a cracked eggshell, float as a series of plates on the molten "white" beneath. The edges of these plates - which we know as "faults" - define the major earthquake zones.
In some regions the plates slide past each other smoothly without consequence; in others they do so in a kind of "stick-slip" motion. Sometimes they may "stick" for several decades, then slip suddenly by several yards to produce, unexpectedly and often tragically, an earthquake.
Tuesday's tremor in Turkey is reported to have measured 7.2 on the Richter Scale. Before the introduction of this scale, devised by Charles Francis Richter in 1935, estimates of the intensity of an earthquake were based on observations of the local damage.
Most widely used was that devised in 1902 by Giuseppe Mercalli; and it was not dissimilar in concept from the Beaufort Scale of wind force used by meteorologists. The disadvantage of the Mercalli Scale, however, was that it did not provide a unique figure for a given seismic disturbance; the "strength" of a tremor varied depending on the distance of the epicentre from the observation point.
The Richter Scale, on the other hand, is a measure of the total energy released by an earthquake, a unique objective figure deduced from seismographic readings. Richter had hoped to construct a method for categorising earthquakes into a few broad bands, but in the event his scale was so good that the strength of a tremor can now be specified accurately to the nearest tenth of a magnitude.
For practical purposes the scale runs from 0 to 9, but it is not a scale of equal increments; on the contrary, an increase of one unit signifies an earthquake about 30 times as great as one corresponding to the lower number. The most powerful earthquake ever known, in Lisbon on November 1st, 1755, is believed to have had a magnitude of about 8.9. Probably the best-remembered earthquake was that of April 18th, 1906, in San Francisco which killed over 400 people and injured 3,500 (out of a total population of 400,000), and which would have registered about 8.3.
But there have been much worse disasters. The greatest loss of life is believed to have occurred in July 1201, when the entire eastern half of the Mediterranean basin was rocked by a massive tremor and more than one million people perished, and much more recently the 1976 earthquake in Tangshan, China, killed 240,000.