A strategy to rapidly determine the magnitude of great earthquakes

In the initial hours following the origin of the Sumatra‐Andaman Islands earthquake at 0058:53 GMT on 26 December 2004, the event was widely reported as having a magnitude of about 8. Thus, its potential for generating a damaging teletsunami (ocean‐crossing tsunami) was considered minimal. The event's size later was shown to be approximately 10 times larger, but only after more than four and a half hours had passed, when a moment estimate based on 2.5 hours of data became available from Harvard University's Centroid‐Moment Tensor (CMT) Project (M.Netties and G. Ekstrom, Quick CMT of the 2004 Sumatra‐Andaman Islands earthquake, Seismoware FID: BR345, e‐mailed announcement, 26 December 2004). This estimate placed its magnitude at Mw ≈9.0, in the range capable of generating a damaging teletsunami. Actually, the earthquake had caused a teletsunami, one that by that time had already killed more than a hundred thousand people. The magnitude estimate has been subsequently revised to at least 9.3 (Stein and Okal, http://www.earth.northwestern.edu/people/∼seth/research/sumatra.html), with the exact magnitude of the event likely to be a subject of further research in the coming years.