The earthquake struck at 5.26 a.m. local time, a sudden shift in the complex geology of the Asian highlands, where three huge blocks of the planet's crust push and grind against each other.
The Arabian plate is constantly pushing north-eastwards against the Asian plate in the Zagros region, producing a flurry of earthquakes along the northern side of the Gulf. The Asian plate, in turn, is pushing against a landmass known as the central Iranian block. This last huge lump of crust is roughly triangular and relatively rigid, but moving north-east, deforming slowly along its north and eastern flanks.
On average, such tectonic plates move only a few centimetres each year. In practice, masses of rock that should ideally slide imperceptibly past each other become wedged fast at depth, locked for years. Then suddenly they give way, to shift at speeds of up to 8,000 k.p.h. over very short distances of a metre or thereabouts. This happened yesterday morning along the eastern flank of the central Iranian block near the stone and brick town of Bam, in the mountainous Kerman province near the Baluchistan border of Pakistan.
These mountains are fissured by seismic faults running north to south. Along one of these, before dawn, there was what geologists call a strike-slip event - measured provisionally at 6.7 magnitude on the Richter scale - that sends shock waves radiating away at almost the speed of sound.
"You have one block sliding past another horizontally," said Roger Musson, of the British Geological Survey.
"With a vertically moving fault you may get a scarp which shows as a step in the landscape, whereas here what you will get are roads torn apart, so you will see what was previously a straight road going along suddenly with a kink in it."
Any building straddling the fault would be torn apart. But the real damage is from the shock waves generated by the sudden movement. The fastest waves would send the terrain shivering up and down at a measurable fraction of the force of gravity. The first waves would be followed swiftly by a second set of waves that would shake the bedrock from side to side. Buildings of breeze block, brick and sun-dried mud would be badly shaken, or dismantled altogether.
"In that part of the world there is a lot of brick building, and probably a lot of it not of very good construction," Dr Musson said. "Reports I have heard say all the brick buildings in Bam are more or less badly damaged. Good construction at a low technology level usually revolves around using plenty of wood, and wood is in short supply in those regions. Often these traditional Iranian houses are so weak that really quite modest shaking will bring them down."
Earthquakes are inevitable but engineers and scientists say huge death tolls could be avoided, provided that governments and city authorities enforce building codes.
In the last four years, despite a decade of international effort to reduce the risks from natural disaster, earthquakes have claimed an average of 10,500 lives a year.
The people most at risk are the poor, crowded into slums on the least stable land in the cities - and urban populations have increased dramatically in the last 50 years. - (Guardian service)