An Indonesian looks at his tsunami-hit house

The shock waves from December’s giant earthquake set off devastating tsunamis. They also set off a series of temblors nearly 7,000 miles from the epicenter in the Indian Ocean. One hour after the earthquake struck off the Indonesian island of Sumatra on December 26, seismometers that keep watch over Mount Wrangell in Alaska recorded an unusual pattern: 14 earthquakes over 11 minutes.

The small quakes, up to magnitude 2.0, coincided with a train of shock waves from Sumatra. When the ground pulled apart slightly, Mount Wrangell shuddered.

“Pulsing, if you will, in sync with the waves from Sumatra,” said Michael E. West, a seismologist at the Alaska Volcano Observatory.

Mount Wrangell is a huge 14,000-foot-high volcano ? “It has little pimples on the side of it that are the size of Mount St. Helens,” West said ? in southern Alaska, and while it has not erupted in historic times, it regularly spews out steam, and small earthquakes are common.

The findings by West and his colleagues appear in a recent issue of the journal Science, part of a package of scientific papers describing the earthquake, which ruptured more than 800 miles of the sea floor in the eastern Indian Ocean.

The earthquake started about 180 miles south of Banda Aceh, the hard-hit city on Sumatra, and the fault broke to the north-northwest. At first, the fault broke relatively slowly for an earthquake: 2,000 miles per hour. At a bend in the fault to the west of Banda Aceh, the breakage sped up to 5,000 to 6,000 mph and continued to the Nicobar Islands. The shaking lasted 10 minutes.

North of the Nicobar Islands, hundreds of miles of the fault also slipped, but the slippage occurred so slowly, an hour or longer, that it did not generate seismic waves as it tilted and lifted the Andaman Islands.

“That is something we have not observed before,” said Dr Thorne Lay, a professor of earth sciences at the University of California, Santa Cruz, and the lead author of one of the articles in Science. “It tells us there is frictional behaviour on these faults that we don’t understand.”

That slow slippage also contributed to the tsunamis, although less than if the fault had broken quickly. That is also partly why the earthquake, initially measured at magnitude 9.0, is now regarded as having been between magnitude 9.1 and 9.3. The largest earthquake ever recorded was a magnitude 9.5 quake in Chile in 1960. A magnitude 9.2 earthquake occurred in Alaska in 1964. But the Sumatra earthquake “is the longest rupture we’ve ever seen,” said Roger Bilham, a professor of geological sciences at the University of Colorado who wrote an article in Science summarising the other papers.

Bilham calculated that the movement, pushing the Indo-Australian tectonic plate up to 50 feet beneath the Eurasian plate, reduced the size of the Indian Ocean enough to raise sea level by 0.1 millimetre, and that the quake released the energy equal to a billion tons of TNT.

Kenneth Chang/NYTNS