February 17, 2005

Sumatra quake three times larger than originally thought

Emile Okal (left) and Seth Stein, professors of geological sciences, examine data from the Sumatra earthquake.

Bob Black / Courtesy of the Chicago Sun-Times

By Megan Fellman

Northwestern seismologists have determined that the Dec. 26 Sumatra earthquake that set off a deadly tsunami throughout the Indian Ocean was three times larger than originally thought, making it the second largest earthquake ever instrumentally recorded and explaining why the tsunami was so destructive.

By analyzing seismograms from the earthquake, Seth Stein and Emile Okal, both professors of geological sciences in the Weinberg College of Arts and Sciences, calculated that the earthquake’s magnitude measured 9.3, not 9.0, and thus was three times larger. These results have implications for why Sri Lanka suffered such a great impact and also indicate that the chances of similar large tsumanis occurring in the same area are reduced.

“The rupture zone was much larger than previously thought,” said Stein. “The initial calculations that it was a 9.0 earthquake did not take into account what we call slow slip, where the fault, delineated by aftershocks, shifted more slowly. The additional energy released by slow slip along the 1,200-kilometer long fault played a key role in generating the devastating tsunami.”

The large tsunami amplitudes that occurred in Sri Lanka and India, said tsunami expert Okal, result from rupture on the northern, north-trending segment of the fault — the area of slow slip — because tsunami amplitudes are largest perpendicular to the fault.

Because the entire rupture zone slipped (both fast and slow slip fault areas), strain accumulated from subduction of the Indian plate beneath the Burma microplate has been released, leaving no immediate danger of a comparable ocean-wide tsunami being generated on this segment of the plate boundary.