* Ahmadinejad wants 5 mln people to leave Tehran
* Big quake overdue, could destroy capital - seismologists
* Population 10-15 mln, growth shows no sign of slowing
By Robin Pomeroy
TEHRAN, April 22 (Reuters) - President Mahmoud Ahmadinejad has warned Iranians of a possible nuclear strike by the United States, but it is an even more deadly threat that has prompted him to ask 5 million of them to evacuate the capital.
Like the people of San Francisco, Tehranis know their sprawling metropolis is due for a massive earthquake. In Iran, where building standards have not advanced as quickly as the population, some estimate millions could be killed or maimed.
In an Islamic society where disasters are often seen as acts of God, Ahmadinejad told housing officials they could no longer rely on the power of prayer to save Tehran from annihilation.
"Tehran has 13 million inhabitants. If an incident happens, how can we manage it? Therefore, Tehran should be evacuated," said Ahmadinejad, a former mayor of the city, announcing financial aid for people who move to towns with a population of less than 25,000.
"At least 5 million people should leave Tehran," he said.
When the last major earthquake hit, in 1831, Tehran was tiny compared to the metropolis where today the work-day population can reach 15 million.
As a huge quake is reckoned to hit the area around every 150 years, seismologists say one is now well overdue.
"If such a thing does happen in Tehran it will be the biggest disaster in humanity," said Farid Mehdian, who headed a seismic study 10 years ago which gave a conservative estimate that half a million people would die in the next 'big one'.
By comparison, the 2003 earthquake that devastated the small city of Bam in southeast Iran, and renewed talk of moving the capital, killed some 30,000 people. The human and economic impact of a big quake in Tehran would be incalculably greater.
It is not only politicians who are talking about the threat.
Leading Friday prayers, the focal point of Iran's religious week, Ayatollah Kazem Sadighi said better observance of Islamic rules on modesty would help ward off an earthquake.
"Those women who dress inappropriately will tempt youngsters and it will finally lead major sins being committed and in that case the wrath of God will be sent upon us," he said.
For veteran seismologist Bahram Akasheh, Ahmadinejad's radical plan does not go far enough.
Akasheh has been arguing for years that the entire capital should be moved far away from the fault lines at the foot of the Middle East's highest mountains, and that its various functions be relocated around the country.
"Maybe we should have four capitals, one for culture, one for politics one for industry and one for economic affairs," said Akasheh, a professor at Tehran's Islamic Azad University.
Akasheh estimates that Tehran faces a 90 percent risk of a quake of Richter scale 6 -- enough to devastate the city -- although he cannot say exactly when.
"Maybe in 50 years. Maybe tomorrow night. Or maybe while I'm speaking to you," he told Reuters in a telephone interview.
SPRAWL AND SMOG
Looking down from the foothills of the snow-capped Alborz mountains, Tehran sprawls as far as the eye can see. Outnumbering the minarets, cranes are busy building new high-rise blocks to house a bulging population.
A semi-permanent haze -- from the exhausts on Tehran's gridlocked streets -- shows another motive for moving some of the population to other parts of a country which is three times the size of France with a similar population.
Mehdian, an architect, said it would take 100 years and billion of dollars to make Tehran's buildings earthquake proof, but he does not think the alternative policy -- moving the masses out of town -- has been properly thought through.
"Of course, if the population of Tehran was 3 million it would be easier to manage the risk, but it's impossible to move the population of Tehran somewhere else."
One problem is where to move them as most of the inhabited areas of Iran are also in earthquake zones. "By moving them (there) you are just moving their graves," Mehdian said.
The main obstacle Ahmadinejad will face is persuading Tehranis to leave Iran's economic, political and cultural heart, independent daily Ettela'at said in an editorial which asked why millions of Iranians had moved to Tehran in the first place.
"For its pollution? Its traffic jams? The impatient and aggressive people? ... Wealth and job opportunities are its attractions."
"The need to reduce Tehran's population is undeniable but no one will leave his home and his job for 200 square metres of land in a small city and a 10 million-toman (around $10,000) low interest loan," the newspaper said.
At a trendy cafe in affluent northern Tehran, 24-year-old industrial design student Reza agreed.
"You can not offer people land and then simply ask them to leave the city ... it does not work, it's not enough." said Reza, who declined to give his surname. "(The policy) should offer them more incentives -- like a decent job."
Indicating the government was also aware of this, Ahmadinejad's first deputy, Mohammad Reza Rahimi, on Saturday announced a plan to relocate some ministries, companies and other organisations to outside Tehran.
A Tehran cafe owner who is no fan of Ahmadinejad, whose re-election last June brought thousands of Tehranis onto the streets in protest, said he feared a heavy-handed approach.
"It is a good idea but the way that they are going to implement it is very important because there is a possibility that the vulnerable sectors of society get trampled under the feet of the big shots." (Additional reporting by Ramin Mostafavi; Editing by Jon Hemming)