Spanning more than 1,600 square kilometers (640 square miles), Tehran is the capital of a country that the United Nations has branded one of the 10 most prone to natural disasters.

"The risk of a big earthquake is very high in Tehran," the head of the city's crisis management and prevention centre, Mazyar Hosseini, told AFP.

"Tehran is located on a seismic line, and every 150 years we should expect a big one," he said as fears mount that one of Iran's next big quakes could strike its metropolis.

"The last big one happened around 1830, so you see it could happen anytime, the probability is high."

On Friday, Iran's southeastern city of Bam was flattened by a quake measuring 6.3 on the Richter scale.

Some 22,000 people were killed, according to the latest provisional toll, and tens of thousands injured. One of the jewels in Persian cultural heritage, the 2,000-year-old Bam citadel, was also completely destroyed.

Tehran straddles three major fault lines. The two major ones run parallel to each other, stretching east to west, one in the north and the other in the south of the city.

The population density in southern Tehran is far greater than in the affluent north, where buildings are slightly more earthquake resistant.

Although the prospect of a massive quake has been whipped up frequently by a worried Iranian press, the authorities have sought to deflect panic by highlighting plans in place to cope with the issue.

"Unfortunately Tehran's expansion is not balanced, some parts are built without adhering to any tremor standards," said Hosseini, refusing to say how much of Tehran falls under that category.

But according to municipal experts, between 55 to 70 percent of the city needs to be renovated, costing in excess of 40 billion dollars.

Iranian quake experts fear that a tremor measuring six on the Richter scale could kill around one million Tehranis, injure four million and level around 65 percent of the city.

"In order to cope with the probable quake we should renovate the worn out areas by awarding long-term loans with low interest rates, build emergency roads and equip Tehran's emergency rescue teams with upgraded equipment," Hosseini added.

Under normal circumstances, Tehran's plethora of streets are some of the most clogged in the world. Traffic jams can leave people stranded for hours.

Experts fear that if a massive tremblor seizes Tehran, it would be almost impossible for rescue teams to reach the victims.

During the 20th century, around 20 big quakes hit Iran, leaving more than 140,000 people dead. Since 1991 alone, and before the Bam disaster, nearly 1,000 earthquakes have claimed some 17,600 lives and injured 53,000 people, according to official figures.

TERRA.WIRE