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 Sunday’s great earthquake, which set off tidal waves that killed thousands of people.

Researchers don’t yet know exactly what happened. But if previous giant quakes in the region are any guide, the one Sunday would have suddenly pushed up the ocean bottom by a yard or two, raising a bulge of water on the surface that rippled out in all directions.

The resulting waves, or tsunamis, would have been only a foot or so tall as they raced across the ocean at 500 to 600 mph.

But when they hit shallower water near shore, they piled up into walls of water up to 30 feet high, depending on the shape of the coastline and the contours of the ocean bottom.

The tsunamis might look like angry waves or rapidly rising tides. As the first one receded, exposing the ocean bottom, people would have been tempted to go gather up any fish left behind, only to be trapped by a second wave as powerful as the first, arriving 10 minutes to an hour later. Four or five tsunami waves may hit in succession, traveling thousands of miles to menace distant shores.

Sunday’s 9.0-magnitude 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 two inches per year. The plates can become stuck, accumulate strain for decades or centuries and then suddenly spring loose in an earthquake.

This process has produced the biggest earthquakes ever recorded.

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 who has been working in the region.

The necklace of little islands off the west coast of Sumatra sit near 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 Sunday’s quake, he said, it’s likely that the two plates suddenly slipped past each other by about 40 feet along 600 miles of 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 know 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.

There was never any danger of tsunamis from Sunday’s earthquake hitting the West Coast, said Paul Whitmore, scientist in charge of the West Coast and Alaska Tsunami Warning Center in Palmer, Alaska. There are simply too many pieces of land between here and there.

The center gets advance warning of tsunamis from six instruments in the Pacific Ocean operated by the National Weather Service. Located at depths of 10,000 feet or more, they look for small increases in the weight of the overlying water caused by the bulge of a passing tsunami. The information is transmitted to buoys on the surface and then to the warning center, which notifies state authorities.

However, there were no waves sensors in the region of Sunday’s quake, authorities said. And India and Sri Lanka, where thousands of deaths were reported, are not part of the international tsunami warning system.

Although tsunamis are rare in California, they are a potential danger here. Tsunamis from the 1964 Good Friday earthquake in Alaska killed 11 people in Crescent City and did the equivalent of more than $2 billion in damage in today’s dollars in California. Waves seven to 21 feet high were reported from Crescent City to Monterey Bay.

One of the biggest potential hazards here is the Cascadia Subduction Zone, which stretches more than 700 miles from Vancouver Island in British Columbia to California’s Cape Mendocino.

The last great earthquake along that zone struck in 1700. It was as powerful as the one in Sumatra, with an estimated magnitude of 9.0, and generated tsunamis along the West Coast and in Japan. The fault has been locked ever since, and is considered ripe for another big one.

Other faults off the West Coast could also generate tsunamis – either by abruptly raising or dropping the ocean bottom, as happened off Sumatra, or by triggering undersea landslides in places like Monterey Canyon.

A recent study found 51 well-documented tsunamis in San Francisco Bay since 1850. The biggest one, generated by the 1964 Alaskan earthquake, measured 3.7 feet at the Presidio and caused $177,000 in damage to boats and floating structures.

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Caltech’s Tectonic Observatory has posted information on the Sumatran subduction zone at www.sumatran.catharinestebbins.com/.