By Paul Rincon
BBC News Online science staff
The devastating earthquake at Bam, Iran, in 2003 was caused by the rupture of a rare, hidden fault that is invisible at the surface, experts say.
The earthquake destroyed buildings both new and old
This fault runs directly under the city of Bam and, combined with the density of settlement, may have been responsible for the high death toll.
Data shows the main shock on this fault was followed by a smaller one 10 seconds later at a fault 5km away.
Details have been published in the journal Geophysical Research Letters.
The magnitude 6.5 earthquake destroyed the town in the southeast of the country on 26 December last year, killing more than 26,000 people.
A fault is a fracture in the Earth along which blocks of crust on either side have moved relative to one another.
When fractures do not rupture all the way to the surface they are known as blind faults. These features are usually associated with thrust faults, which are formed by compressive stresses.
Blind thrust faults are buried under the uppermost layers of crust, but they cause the surface layers to fold over them as they deform, forming a tell-tale hill at the surface that identify them to scientists.
After the Bam earthquake, seismologists focussed their attention on several known faults in the area with clearly visible surface traces that reveal their location.
The researchers used a technique called Interferometric Synthetic Aperture Radar interferometry (Insar) to map the deformation of the Earth's surface in images from the European Space Agency's Envisat satellite.
They discovered that the main rupture in the earthquake occurred on a fault to the west of those known from their obvious exterior features.
This previously unknown fault was not associated with any surface features. What is more, this blind fracture turned out to be a "strike-slip" fault, in which the blocks of crust move sideways relative to one another.
The quake flattened the fortress of Bam, said to be the world's largest mud brick structure
"This is - as far as I know - the first well-documented case of a blind strike-slip fault," Dr Eric Fielding, of the US space agency's Jet Propulsion Laboratory in Pasadena, California, told BBC News Online.
"The fault was right in the middle of the city. It was the combination of the large population so close to the fault that really caused the devastation at Bam.
"Even very recent buildings - less than 10 years old - were brought down by this earthquake because the shaking was so strong."