Stress Buildup Likely to Trigger another Indian Ocean Earthquake

A buildup of stress on faults in Sumatra is likely to trigger another large earthquake -- and potentially another tsunami -- in the Indian Ocean region, seismologists say.

Researchers at the University of Ulster-Coleraine in Northern Ireland report in Thursday's issue of the journal Nature that stress is building not only in the Sumatra fault, where the magnitude huge tsuanami-causing quake struck Dec. 26, but also on the adjacent fault zone known as the Sunda Trench, located under the sea west of Sumatra.

The Sumatra quake has been put at magnitude 9.3 by one research group, meaning it was three time stronger than the initial estimate of 9.0 (the magnitude scale is logarithmic).

Another rupture could trigger a magnitude 7-7.5 quake on either fault. That would be a significant quake, but far less powerful than the one that unleashed a tsunami that left an estimated 300,000 people dead or missing.

The researchers stopped short of predicting when another large quake would strike the region. But similar events elsewhere in the world have occurred within a few years of each other, or even a few months.

Major earthquakes tend to cluster in areas called subduction zones where two or more plates of the Earth's crust grind and overlap. The Dec. 26 quake occurred where the edge of the deep, flat Indian plate dives below the ragged Burma plate. The nearby Sunda Trench marks where the Indian plate begins its grinding decent into the Earth's hot mantle.

The researchers noted several examples where major earthquakes in subduction zones have been coupled. In Japan, at least five major quakes in the Nankaido fault have been accompanied by similar events on the contiguous Toanakai/Tokai segment within five years -- and three of the subsequent quakes ruptured in the same years as their precursors.

In Turkey, the destructive magnitude 7.4 Izmit quake near Istanbul in 1999 triggered the magnitude 7.1 Duzce quake three months later.

They said stress in the Sumatra fault is confined to its north portion, where there hasn't been a major rupture in 100 years. Stresses along both features are building to levels higher than those measured before the coupled earthquakes in Turkey, they reported.

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