Most earthquakes, as we know, are convulsions of the Earth's crust, the most severe of which occur near the edges of the major "plates" which make up the surface of the planet. 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.
The Mediterranean basin is a high-risk area for earthquakes, the danger zone continuing eastwards from there into Asia and on to the East Indies. So too, consistent with the weekend devastation in El Salvador, is the entire west coast of North, Central and South America.
Probably the best-remembered earthquake, almost in living memory, was that of April 18th, 1906, in San Francisco; over 435 people were killed and some 3,500 injured out of a total population of some 400,000, and more than half the city was destroyed by a tremor that, it is estimated, would have registered about 8.3 on the Richter scale.
But there have been 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 over a million people died. Nearly as many died in an earthquake that struck the Shanxi and Henan provinces of China on February 2nd, 1556, while in modern times the 1976 earthquake in Tangshan, also in China, killed 240,000.
The most powerful tremor ever known, on the other hand, is believed to have been the Lisbon earthquake of November 1st, 1755, with an estimated magnitude of 8.9. On the morning of All Saints' Day the sky was bright and cloudless over the city, and the churches were thronged with worshippers to mark the Holy Day. Suddenly and without warning, two violent earthtremors shook the city to its foundations.
Rows of houses fell like dominos, chasms opened in the streets, and churches collapsed upon their congregations. An onlooker some distance out to sea described the entire city as "waving to and fro like a wind-blown field of corn". Of the 275,000 inhabitants of Lisbon at that time, some 15,000 died immediately in the devastation of the tremors and the tidal wave which followed; perhaps twice that number succumbed to fire, disease and other hardships in the aftermath.