Hidden Fault Amplified 2004 Tsunami

Five years ago, the Earth shuddered, and nearly a quarter million people lost their lives in the tsunami the rippled across the Indian Ocean.

Scientists thought the culprit behind the deadly waves was clear: the magnitude 9.2 Sumatra-Andaman earthquake, one of the most powerful ever seen.

It turns out the quake had help from a fault few scientists even knew existed. According to several new studies, the much smaller fracture spawned a separate tremor that sent a 100-foot-high wall of water barreling into Indonesia's Aceh province.

If true, the discovery would shed light on what really spawned the 2004 disaster. The main earthquake broke along a 1,600-kilometer-long section of fault where the India tectonic plate grinds underneath the Sunda plate. The fault is thought to have slipped 20 to 25 meters (65.6 to 82 feet) almost instantaneously.

The capital of Aceh province, Banda Aceh, lies near a portion of the fault that didn't move at all, and yet the area was literally wiped off the map. By some counts, 130,000 people were killed there.

How could the city have suffered such disproportionate damage? Scientists now think they know why: a previously ignored fault nearer to the Sumatra coast that broke almost at the same time as its much larger cousin.

Felix Waldhauser of Columbia University and a team of researchers analyzed thousands of aftershocks in the area since 2004. Epicenters from the small quakes lined up with this unusual fault, suggesting that it, and not the main fault, has been active in this area since the disaster.

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Crucially, this fault, which the team calls a "splay" fault, slices through the Sunda plate much closer to Sumatra's west coast, and at a much steeper angle to the ocean floor than the main fault. This means that whenever this splay fault breaks, it pushes the ocean floor upward more forcefully, causing a larger tsunami.

"Observations of the earthquake along the (main fault) were not enough to generate a tsunami that big," Waldhauser said. He presented the team's finding at the annual meeting of the American Geophysical Union earlier this month.

Another team of researchers led by Satish Singh of the Institut de Physique du Globe de Paris in France showed in computer models that the splay fault only had to slip 5 meters (16.4 feet) in order to generate the massive tsunami that engulfed Banda Aceh -- the equivalent of a magnitude 7.8 or 7.9 earthquake.

Disturbingly, this massive event was lost amid the chaos of events on December 26, 2004. Though the smaller quake had only 2 percent the energy of the Sumatra-Andaman rupture, it focused its wrath on Banda Aceh in the form of 100-foot-high surge of water.

"Our worry is that such a small event can produce such a devastating effect, and it will not get noticed," Singh said.

The smaller magnitude of the event suggest it's likely to recur more frequently than larger earthquakes, perhaps once every 100 to 200 years.

Even now, Singh has his eyes on similar splay fault that is building up stress offshore of Padang, a city south of Banda Aceh with a population of 750,000. He says it could break in the next decade.