The earthquake that shook northern Iran yesterday and killed an estimated 10,000 people occurred in one of the most geologically active regions in the world, where the northward-marching landmass of Arabia, carried upon the back of a tectonic plate, smashes into the supercontinent of Eurasia.

The epicenter of the temblor, which occurred 12:31 a.m. yesterday (5:01 p.m. Wednesday EDT), was under the Caspian Sea several miles north of the port city of Rasht.

Scientists with the U.S. Geological Survey in Colorado reported the Iranian earthquake measured 7.7 on the Richter scale for ground motion and was the largest ever recorded in that area of Iran.

"A 7.7 is just a really, really large earthquake," said Gerald Wieczorek, a geological engineer with the USGS Office of Earthquakes in Reston, who was examining geological maps of the region and trying to piece together what happened.

The temblor that killed about 25,000 people in Soviet Armenia in 1988 measured 6.9; many of the deaths in Armenia were attributed to faulty building construction.

Yesterday's earthquake in Iran was located in a geologically unstable region that is being squashed between the north-advancing Arabian plate and the Eurasian plate.

The Earth is composed of about a dozen great tectonic plates that drift around the planet carrying the continents and oceans on their surfaces. Wherever actively moving plates meet, such as in Iran and Soviet Armenia, there is the potential for earthquakes.

In the United States, the San Andreas fault in California is the boundary between the North American plate and the Pacific plate.

Because of its location between two plates, Iran has suffered severe earthquake damage. In the last 30 years, there have been 54 temblors in Iran in which there were deaths or significant injuries. A quake in eastern Iran measuring 7.7 on the Richter scale killed 25,000 people in 1978.

The site of yesterday's temblor in Iran is a region sandwiched between two well-defined fault lines that cross to the north and south. Called the Northern Seismic Zone by scientists, it is a geologically complex area laced with dozens of smaller faults.

Based on preliminary information, geologists in the United States say yesterday's earthquake was one in which a section of the Earth's crust was thrust below another on the other side of one of many fault lines running through the Northern Seismic Zone.

Geologists said the Iranian quake could have been magnified by the region's mountainous landscape, which triggered landslides and hampered rescue efforts.

Wieczorek also said that Rasht and other cities bordering along the Caspian Sea and in valleys could have suffered from a phenomenon called seismic amplification, due to the fact that the valley floors and coastal towns may have been built upon loose soil, rather than bedrock.