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The geological scar left by the devastating earthquake off the coast of Sumatra in December 2004 has healed more quickly than expected.
Satellite measurements of Earth's gravitational field taken just after the quake show it left a depression 8 millimetres deep in the crust and shallow mantle. While this does not seem like much, the shifting mass jolted Earth's axis of rotation enough to move the poles by 10 centimetres.
In under a year, however, the depression had nearly vanished - something that surprises geologists, because according to models of how rocks in the mantle move it should have taken 20 years. "It's almost impossible for rocks to move, that quickly," says Kosuke Heki of Hokkaido University in Sapporo, Japan.
So Heki's team developed a new model to show how Earth could have healed itself in as little as seven months. The key, he says, is that the mantle beneath the 1200-kilometre fault has more water than usual - about 1% of the rock by weight. As the water is under intense heat and pressure, it behaves like a gas and can move through kilometres of solid rock in a short time.
In his model, water flows from rocks compressed by the quake into those that expanded as it released their stresses. The influx causes the de-stressed rocks to return to their original state faster. The model also suggests that the extent of permanent shifts in Earth's rotational axis due to strong quakes would be less than expected.
Journal reference: Geophysical Research Letters (vol 34, p L06313)
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Water helps earthquake-ravaged Earth to heal
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