Tremor Site Takes Experts by Surprise : Geography: No similar quake has been reported in the area for 500 years, geophysicist says.

The earthquake that leveled an unknown number of villages in northern Iran on Thursday, causing devastating loss of life, caught experts by surprise because it struck off the beaten path for major earthquakes in that part of the globe.

“We haven’t had such a strong earthquake in that region for at least the past 500 years,” said Mdansour Niazi, an Iranian geophysicist who is now an earthquake consultant in Berkeley.

The region lies to the north of a complex seismic zone called the Alpine Belt that created the Himalayan Mountains and ranks as one of the most deadly earthquake regions in the entire world. The zone marks the collision of two continents, Asia and India. At least 100,000 people have died in major quakes along the belt in this century alone.

However, the villages destroyed by Thursday’s quake are separated from the main seismic belt by a chain of mountains. Why the region should suddenly produce one of Iran’s largest earthquakes of this century is a mystery.

“Previous (seismic) activity in that area does not cause us to expect such a large earthquake,” Niazi said. “So it is catching us by surprise.”

The U.S. Geological Survey estimated the earthquake at magnitude 7.7 and said it struck along the southwestern coast of the Caspian Sea. The Geophysics Center of Tehran University put the figure at magnitude 7.3. Both measurements are several times more powerful than the 7.1 shocker that devastated parts of the San Francisco Bay Area last October.

An official dispatch from Iran termed the earthquake “unprecedented,” and Niazi agreed, at least as far as recent history is concerned.

“In 1678, there was a strong earthquake which damaged the main mosque in Lahijan,” east of the epicenter of Thursday’s quake, Niazi said, “so the area is active. But the bigger quakes have been farther south.”

He added that buildings throughout that area tend to be better suited to withstanding earthquakes than structures in central Iran, but that is because of architectural preferences rather than safety concerns. The region is heavily influenced by Russian architecture, he said, and thus many buildings have wooden roofs, covered with thin metal, instead of the concrete roofs that collapsed and killed so many residents of the Armenian villages that were devastated in December of 1988.

“It’s a beautiful area,” Niazi added.

The region has been shaped by the collision of tectonic plates, the huge slabs of the Earth’s crust on which the continents and the oceans float. When two plates rub together, such as along the coast of California, they produce faults that can cause major earthquakes.

In some areas, however, an oceanic plate is pushed under a continental plate in a process called subduction, and that produces both earthquakes and volcanoes.

The Alpine Belt, which marks the boundary between Asia and the subcontinent of India, began as a subduction zone many years ago when India was on a slow migration north. About 50 million years ago, however, India collided directly with the Asian plate.

“That’s sort of like a head-on auto accident,” said Charles H. Fletcher III, a geologist with West Chester University in Pennsylvania, who has studied the Caspian Sea area.

When two continental plates collide, they are both too thick and too massive to slide under one another, so they grind into each other, creating powerful faults that rupture in major earthquakes. That process created the Himalayas, and on Thursday it produced the earthquake that destroyed a whole series of villages in the gentle countryside of the south Caspian Sea.

Fletcher said in a telephone interview that there are many dynamic processes taking place in that region today, and they may be somehow interlinked. Fletcher will lead a joint American-Soviet expedition to that area this August, under the sponsorship of Earthwatch, to study a mysterious rise in the water level of the Caspian.

The sea has risen nearly five feet in the past decade, threatening to inundate several major industrial ports, and that has added substantial weight to the crust beneath the sea. That, in turn, could be increasing the strain along the faults in a process called “seismic loading.”

“The loading of the Caspian basin might be initiating the seismic behavior,” Fletcher said. But the region is very complex, and its dynamics not well understood, so he quickly added: “But we just don’t know.”

RECENT HISTORY OF DEADLY QUAKES TURKEY Dec. 1939 March 1953 Aug. 1966 March 1970 Sept. 1975 Nov. 1976 Oct. 1983 IRAN June 21, 1990 June 1957 Dec. 1957 Sept. 1962 Aug. 1968 April 1972 Sept. 1978 ARMENIA Dec. 1988 CHINA Dec. 1920 May 1927 Dec. 1932 July 1976 INDIA Jan. 1934 May 1935 Aug. 1950