A series of powerful earthquakes has terrorized residents in western Indonesia — including one that triggered a tsunami warning Friday — leaving thousands sleeping on plastic sheets in the hills. Seismologists warn the worst may yet to come.

Kerry Sieh, from the California Institute of Technology, has spent decades studying the volatile fault line. He is one of several experts predicting a repeat of the massive earthquake that triggered the 2004 Asian tsunami, which killed more than 230,000 people in a dozen Indian Ocean nations.

"No one can say whether it will be in 30 seconds or 30 years," he said. "But what happened the other day, I think is quite possibly a sequence of smaller earthquakes leading up to the bigger one."

An 8.4-magnitude quake that shook Southeast Asia on Wednesday was followed by dozens of strong aftershocks — including one measuring a magnitude of 7.8 and another 7.1 — that killed 13 people, damaged hundreds of houses and spawned a 10-foot-high tsunami.

On Friday, a 6.4-magnitude temblor hit the area again, according to the U.S. Geological Survey, triggering the latest in a string of tsunami warnings that was later lifted.

On Wednesday, a wall of water slammed into several fishing villages on the island of Sumatra and swept away nearly a dozen houses, but caused no deaths. The massive quakes have also heightened experts' concerns.

The fault, which runs the length of the west coast of Sumatra about 125 miles offshore, is the meeting point of the Eurasian and Pacific tectonic plates, which have been pushing against each other for millions of years. This can cause huge stresses to build up.

"There is a strong indication this foreshadows the big one," said Danny Hillman, an earthquake specialist at the Indonesian Institute of Science. "We all agree there is an 8.5 or stronger earthquake waiting to happen."

That's exactly what residents along Sumatra's western coast, which is expected to bear the brunt of the next disaster, are worried about. The island was hardest hit by the 2004 tsunami, accounting for nearly two-thirds of the deaths.

In the tiny fishing village of Sungai Pisang, just south of the badly damaged city of Padang, hundreds of people were too scared to return home after the recent tremors sent a large wave washing into their bay.

At a camp pitched on a muddy hillside cemetery, they have been sleeping atop plastic sheets or on the cold ground between graves. A small generator powers a light bulb, hung over branches in the thick tropical undergrowth, but there is little else.

"I am very afraid of another tsunami," said Dasima, a 50-year-old rice farmer who fled with her 7-year-old grandson, Rolin. "We only cook our rice in the town and then return here to eat and sleep. We will stay here until we feel it is safe."

Indonesia, the world's largest archipelago, with a population of 235 million people, is prone to seismic upheaval due to its location on the "Ring of Fire," an arc of volcanos and fault lines encircling the Pacific Basin.