The fault line that spawned the 2004 Indian Ocean tsunami has ruptured nearly 20 times this month, causing three strong earthquakes.

The activity shows the stress the seam is under and could be a harbinger of worse to come, scientists warn.

Kerry Sieh, from the California Institute of Technology, has studied the fault for more than 10 years. He likened it to a length of rope in an imaginary tug of war between a group of men and an elephant.

"One by one, two by two, the men are getting worn out and are letting go of the rope. That puts more stress on each of the remaining men," he wrote in an e-mail Tuesday. "Who knows which one will let go next, or whether they will let go all at once?"

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Sieh and other scientists using Global Positioning System transmitters to measure the uplift of the quakes say another massive temblor sometime in the next 100 years or so is likely, but they cannot predict exactly when that will occur.

The fault line is the boundary between the Eurasian and Pacific tectonic plates that have been pushing against each other for millions of years, causing huge pressure to build up. It runs the length of the west coast of Sumatra about 125 miles offshore.

The steady stream of earthquakes it has produced this month do not seem to be alarming residents much. Witnesses say some have prompted people to flee swaying homes, but few are heeding or are aware of the tsunami warnings that routinely accompany the big jolts.

"People did not really care because such a tremor is nothing new," Erwin, a resident in the coastal town of Padang, said minutes after a powerful quake early Tuesday. "It was just like the one in the afternoon," said Erwin, who like many Indonesians goes by a single name.

The 2004 earthquake off Aceh province in northwest Sumatra had a magnitude of 9.2, making it the most powerful temblor in four decades. It triggered a tsunami that killed 230,000 people in 12 Indian Ocean countries, more than half in Indonesia.

Three months after the tsunami, a magnitude 8.6 quake farther down the fault line killed 1,000 people. Then in September last year, an 8.7 quake opposite Bengkulu province damaged thousands of homes, killed about 25 people and sent a 10-foot tsunami crashing into nearby coastlines.

Last Wednesday, a magnitude 7.4 quake killed three people and damaged scores of houses. Since Sunday, four other events strong enough to prompt tsunami warnings by international agencies have jolted the region.

"They are best seen as part of a chain that began in 2004," said Dr. Fauzi, a top scientist at Indonesia's National Earthquake Center. "The stability of the fault has been disturbed," said Fauzi, who goes by a single name.

Since the Indian Ocean tsunami, Indonesia has spent millions of dollars to establish a nationwide tsunami warning system, but there are still only a few warning sirens in Sumatra's threatened western coast and other beach areas.

Officials and residents of the two most populous cities of Padang and Bengkulu said no sirens sounded following the recent earthquakes despite warnings issued by the country's Meteorology and Geophysics Agency.

"We don't have such equipment," said Suyud, an official at Bengkulu's Meteorology and Geophysics Agency. "If there were tsunami warnings issued it was only government officials who knew that from text messages on their cell phones."