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Ever since that massive earthquake and tsunami that washed over Southeast Asia in 2004, scientists have been monitoring a fault line in the Indian Ocean. More than 200,000 people died in the last disaster, and now the scientists say they're worried. NPR's Christopher Joyce tells us why.

CHRISTOPHER JOYCE: Geologists don't usually predict earthquakes, but they will make qualified forecasts. And for the past three years, they've been saying, watch out, we don't know exactly where or when, but another big quake, and a tsunami, could happen in the Indian Ocean. Now, scientists from the California Institute of Technology, working with colleagues in Indonesia, say it's coming pretty soon.

Mr. ARON MELTZNER (Ph.D. Candidate, Neotectonics, California Institute of Technology): It'll probably happen in the lifetimes of children who are alive today.

JOYCE: Aron Meltzner is a geologist and an author on a paper in this week's issue of the journal Science. He and his colleagues have used an unusual marker to track the history of quakes: the growth patterns of micro-atolls. These are small coral formations. They look like sombreros a few yards across. They are created when the sea bed moves up and down before or during quakes. Whenever the top of the coral is pushed up out of the water and exposed to air, it stops growing up, but continues to grow out, making the sombrero shape. The growth lines in these coral sombreros tell a 700-year story of the sea floor's rise and fall, punctuated with sudden quakes, and they show a cycle.

Mr. MELTZNER: It's basically this pattern of a century or so of no earthquakes, then a cluster of moderate earthquakes that's anywhere from a few decades to nearly a century long. And every time that we have documented this, which is about four times now, it culminates with kind of the granddaddy of them all, the big magnitude, 8.8 to 9.0.

JOYCE: And the team looked at micro-atolls on islands along the Mentawai fault. It's part of the much longer Sunda trench that runs parallel to the island of Sumatra. A rupture in the northern section of the Sunda trench caused the 2004 disaster. And now the Mentawai section of the fault is acting up. There was a big quake there last September. Meltzer suspects that that was the start of a new cluster in the latest cycle. He says the Mentawai granddaddy could be on the way, and scientists who've studied the fault say it still has a lot of tension in it.

Mr. MELTZNER: So, that fault, even though it just had a major earthquake, is still at a high state of stress, and it's really a ticking bomb, so to speak, on a time scale of several decades.

JOYCE: The government of Indonesia will be better prepared if and when the big one does come. Indonesia inaugurated its own tsunami-warning system last November. Patricio Bernal heads the UN's Intergovernmental Oceanic Commission. He says maps of where another tsunami might hit encouraged Indonesia to invest in the necessary equipment.

Dr. PATRICIO BERNAL (Executive Secretary, UNESCO Intergovernmental Oceanic Commission): People see how - what portion of a given city with hospitals, schools and every - all the infrastructure there will inundated by a tsunami. And then people sort of react and say, oh, we have a problem.

JOYCE: Indonesia has placed undersea detectors in the ocean to warn of a tsunami, and with other Indian Ocean countries, they've put 23 seismometers around the Indian Ocean to detect quakes. Bernal was in Indonesia during a recent earthquake and saw the warning system in action.

Dr. BERNAL: They issued a warning over the night, so that warning did reach the population. So, I'm much more confident today that the systems are slowing evolving to a point where they can be fully effective for warning the population.

JOYCE: Christopher Joyce, NPR News.

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