Seth Stein and Emile A. Okal of Northwestern University analyzed the low-frequency seismological signal that the December 26, 2004, Sumatra-Andaman quake generated. They determined that it was 2.5 times stronger than initial estimates and measured 9.3 on the Richter scale. Scientists based the initial 9.0 magnitude estimate on surface waves. The authors posit that slow slip between the plates that was not detectable in the surface waves accounts for their larger measurement. A second analysis published in the same issue of Nature indicates that the December earthquake also involved a longer section of the fault than previously believed. Sidao Ni of the University of Science and Technology of China and colleagues determined that the quake resulted in a rupture length of 1,200 kilometers--about twice as long as had been inferred from analyses performed soon after the event.
Scientists will continue to try to piece together just how and why the earth moves below the Bay of Bengal region. Kerry Sieh of the California Institute of Technology notes in an accompanying commentary that "over the next year or two, figuring out what happened will be a showcase both of what modern observations and analysis can do and of the multidisciplinary nature of modern earthquake science."