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EARTHQUAKES 2006 IRIS 5-YEAR PROPOSAL IRIS Data Allowed Rapid Public Information About Sumatra Earthquake and Tsunami Seth Stein, Emile A. Okal • Northwestern University The devastation caused by the December 26, 2004 Sumatra earthquake and resultant tsunami caused worldwide public interest and hence provided a “teachable moment” to inform the public about a variety of scientific and hazard issues. However, until recently, such opportunities after major earthquakes were limited because the time required to acquire and analyze seismic data was longer than the period of intense public interest. As a result, although earthquake locations, magnitudes, and mechanisms are available during this “news window”, the most interesting scientific information about the underlying tectonics and physics of major earthquakes often emerges after public interest waned. As a result, the public often gets little feel for how earthquake studies are addressing exciting and unresolved science questions which have great societal import. The availability of IRIS GSN data immediately following the earthquake made it possible for exciting science to be reported by investigators worldwide. For example, within six weeks (February 7) of the earthquake we reported publically an analysis of ultra-long period normal mode data showing that the earthquake was even bigger than first appeared.These results were disseminated on the WWW and reported worldwide and drew great interest. Thus we were able to explain through the media a number of surprisingly sophisticated concepts. We explained the often-confusing concepts of different magnitude scales yielding different results at different periods. We further explained that these results indicated that slip occurred along the entire rupture zone suggested by aftershocks, a much larger area than previously inferred. We were then able to explain that these seemingly-arcane seismological issues had important implications. Comparison of predicted tsunami amplitudes assuming the entire fault ruptured or only the southern segment did. The largest tsunami waves would have missed Sri Lanka if only the southern segment of the fault had ruptured. First, the long rupture played a key role in generating the devastating tsunami. In particular, as shown here, the large tsunami amplitudes in Sri Lanka and India result from rupture on the northern, north-trending, segment because tsunami amplitudes are largest perpendicular to the fault. Second, because the entire rupture zone slipped, strain accumulated from subduction of the Indian plate beneath the Burma microplate has been released, leaving no immediate danger of a large tsunami being generated by slip on this segment of the plate boundary. However, we explained that the danger of a large tsunami resulting from a great earthquake on segments to the south remained. When such an earthquake occurred in March, in the week our paper was published, the media again responded with interest. We used this opportunity to explain both the concept that the first earthquake may have loaded the segment to the south, and that the March earthquake, though large enough to generate a major tsunami, did not due so because the rupture did not extend updip to the sea floor. Throughout this processes we were gratified by the often-sophisticated questions we were able to explore with the media. S. Stein and E. A. Okal, Speed and size of the Sumatra earthquake, Nature, 434, 581-582, 2005. 20
2006 IRIS 5-YEAR PROPOSAL EARTHQUAKES Multiple Source Analysis of the 2004 Sumatra Earthquake Victor C. Tsai, Meredith Nettles, Göran Ekström, Adam M. Dziewonski • Harvard University While it is agreed that the great Sumatra earthquake of December 26, 2004, was among the largest earthquakes of the past century, there has been disagreement on how large it was, which part of the fault ruptured, and how the rupture took place. We carried out a centroid-moment-tensor (CMT) analysis of the earthquake in which multiple point sources were used in the inversion to mimic a propagating slip pulse. Moment-tensor elements and centroid locations for a set of five sources were determined simultaneously. We used waveforms from the IRIS Global Seismographic Network (GSN) filtered in the pass band of 200 to 500 s in the inversion. The multiplesource inversion resulted in a 56% improvement in the fit to the waveforms compared with the original CMT analysis. The strong directivity towards the northwest observed in the data is successfully modeled by the five sources. The total moment for the five sources is 1.17*10 30 dyne-cm, which corresponds to a moment magnitude Mw = 9.3, significantly larger than the Mw = 9.0 obtained in the standard single-source CMT analysis, but in agreement with reported long-period normal-mode amplitudes. The figure shows the locations and focal mechanisms of the five sources. The southernmost source ruptured first, and the remaining four sources ruptured in northward succession with a total duration for the earthquake of approximately 500 s. The focal mechanisms of the five sources change systematically from south to north. The strikes of the mechanisms (indicated by the needles) rotate clockwise, in agreement with the geometry of the subduction zone, and the slip vectors (indicated by black arrows) rotate from nearly pure thrust to oblique slip with a large strike-slip component. Most of the moment release occurred in the southern portion of the fault. The numbers in the figure give the individual seismic moments of each source in units of 10 29 dyne-cm. The seismic data analyzed in this study do not appear to require a slow component of slip for the Sumatra earthquake. 21
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