Quake Struck in Hot, Sleepy Geological Zone

The precise location of Sunday’s massive earthquake off Indonesia surprised geologists because it hit an area that had been quiet for more than a century.

The whole region is a geological hotbed where two major tectonic plates crash together, “but no one ever thought much about earthquakes along this section,” said seismologist Kerry Sieh of Caltech, one of the few researchers who have studied the region.

The region had earthquakes similar to Sunday’s magnitude 9 temblor in 1797 and in 1833, but since then the segment where the latest quake occurred had been relatively quiet. “We had thought about going up [north of Sumatra] and working on it, but it was too difficult to get there,” Sieh said.

The Indian Ocean is marked by what geologists call a subduction zone -- a region where two of the massive plates that make up the Earth’s crust meet, and one slides under the other. In this case, the India tectonic plate to the west is slowly sliding under the Burma plate to the east.

The subduction zone arcs from Burma, now Myanmar, around Sumatra nearly all the way to Western Australia. “It is one of the great subduction zones of the world,” Sieh said.

The plates move at a rate equivalent to the length of a fingernail every year, but the motion is not constant because of friction. The plates stay locked together until sufficient stress builds up to cause them to slip over one another, a process that releases large amounts of energy. Only this kind of slippage is sufficient to create a quake with a magnitude of 8.5 or higher, Sieh said.

In Sunday’s quake, a section of the subduction zone up to 600 miles long -- from India’s Andaman Islands in the north to Indonesia’s Simeulue Island in the south -- broke free.

The Burma plate jumped about 60 feet west, climbing atop the India plate and raising the land level about 3 feet. In the process, the Andaman and Nicobar islands probably moved about 60 feet farther from the coast of the Indonesian island of Sumatra, Sieh said. “But there are no GPS stations out there, as far as I know, so it’s hard to know exactly what happened,” he added.

As the ocean floor off the coast rose, he said, the land behind it would stretch and sink. In the Indonesian province of Aceh in northern Sumatra, “the western coast ... has probably sunk by a meter or two” -- 3 to 6 feet, he said.

The provincial capital, Banda Aceh, probably sank a little below sea level, he said. “Television pictures showed people in the capital walking around in knee-deep water” after the waves had receded, he said, indicating that the land was lower than it was before.

A massive quake like this one would affect both the Earth’s rotation and its wobble about its axis, experts said Monday, but only the most sensitive instruments would detect it. “The question is how much and can it be detected,” said seismologist Hiroo Kanamori of Caltech.

A large quake “moves mass from one point to another. Ocean water also moves around, and that will affect” the rotation, he said. “But it will be very, very small.”