For decades, scientists say, tiny islands off the west coast of Sumatra have been sinking, an ominous sign that strain was building toward yesterday's 9. 0 magnitude earthquake, which...

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SAN JOSE, Calif. — For decades, scientists say, tiny islands off the west coast of Sumatra have been sinking, an ominous sign that strain was building toward yesterday’s 9.0 magnitude earthquake, which set off tidal waves that killed thousands of people.

The earthquake occurred in an area about 125 miles off the west coast of Sumatra that is marked by a deep trench.

Based on clues found among the area’s coral reefs and along its beaches, scientists know this is the fourth giant earthquake to strike the same fault zone, known as the Sumatran subduction zone, since 1797.

Here one giant plate of the Earth’s crust is diving beneath another, pushing the ocean bottom underneath Sumatra at a rate of about 2 inches per year. The plates can become stuck, accumulate strain for decades or centuries and then suddenly spring loose, producing some of the biggest earthquakes ever recorded.

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Under Sumatra, much of the subduction zone had been locked and building strain since the last giant earthquake in the 19th century, said Kerry Sieh, a paleoseismologist at the California Institute of Technology in Pasadena, Calif., who has been working in the region.

The necklace of little islands off the west coast of Sumatra sits on the plate that is diving. Dragged downward, they have been sinking by about half an inch a year. “The villagers know this,” Sieh said. “They can see their boardwalks and harbors sinking.”

During yesterday’s quake, he said, it’s likely that the two plates suddenly slipped past each other by about 40 feet along the fault zone. This released the downward pressure on the ocean bottom, allowing it — and the little islands it carries — to spring up about 6 feet. At the same time, the west coast of Sumatra would have dropped by about a yard, enough to flood low-lying villages during high tide.

All this is speculation, based entirely on what is known about the last major earthquake in 1833, Sieh said. Communications in the area of the earthquake are down, and travel in the regions closest to the epicenter is dangerous because of an ongoing political insurgency.