Kerry Sieh, a US professor who has studied the fault for more than 10 years, likened the seam 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?"

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 give an exact prediction.

The fault line is the seam in the earth where the Eurasian and Pacific tectonic plates 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 200 kilometers (125 miles) offshore.

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

"People did not really care because such a tremor is nothing new," Erwin, a receptionist at a hotel 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 Asian tsunami, a magnitude 8.6 quake further down the seam killed 1,000 people, while in September last year, an 8.7 quake opposite Bengkulu province damaged thousands of homes, killed about 25 people and sent a 3-meter (10-foot) tsunami crashing into nearby coastlines.

The most recent events have occurred opposite Bengkulu.

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 (euros) on establishing a nationwide tsunami warning system, but there are still few warning sirens on beaches, including on Sumatra's threatened western coast.

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