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Seismological Aspects of Tsunami Generation
Three general aspects of the 2004 Sumatra earthquake are important in explaining the tsunami that was generated:
The magnitude of the earthquake is, in most cases, the most important factor that determines the size of a tsunami. The figure below shows local tsunami intensity (a function of maximum tsunami runup) plotted against the moment magnitude of the earthquake (Mw) for a number of tsunamis that occurred in the past century.
Near the earthquake source, local tsunami size increases with the magnitude of the earthquake, though there is significant variability in this relationship (Geist, 2002). The term tsunami earthquake refers to anomalous earthquakes, in which the tsunami is larger than expected from the magnitude of the earthquake. Tsunami earthquakes also tend to rupture the shallowest part of the interplate thrust near the trench. In contrast to normal tsunamigenic earthquakes, tsunami earthquakes follow a well-defined trend (black line above).
The size of 2004 Sumatra local tsunami is consistent with the size of tsunamis generated by other earthquakes of similar magnitude. The Earthquake Research Institute (University of Tokyo) has calculated the tsunami magnitude for the event and shown that it is consistent with its earthquake moment magnitude. Though tsunami runups from this earthquake are among the highest ever recorded (nearly 32 m), this earthquake does not appear to be an anomalous tsunami earthquake.
Consistent with the decoupled tectonics of the Sumatra subduction zone described in the previous section, the focal mechanism for the 2004 Sumatra main shock shows that rupture occurred on the interplate thrust with very little oblique motion. Aftershocks occurred on both the interplate thrust and faults in the overriding plate, including the transcurrent fault.
Focal Mechanism for 2004 Sumatra earthquake
When nearly all of an earthquake's energy is released in a thrust motion, as in the 2004 Sumatra earthquake, a large tsunami is generated. In contrast, large strike-slip earthquakes, such as the 1906 San Francisco earthquake which occurred on the San Andreas fault, are not efficient tsunami generators.
The centroid of an earthquake can be simply defined as the location of the center of energy release. The location of the 2004 Sumatra centroid, determined by Harvard University, is near the trench (black circle with yellow plus sign on map below). This centroid location indicates that most of the energy release from the earthquake took place in deep water. This generally results in an initial tsunami with larger potential energy than a tsunami generated by a similar rupture located closer to shore beneath shallower water (Geist, 2002).
Base map of the Sumatra subduction zone showing seismicity associated with the 2004 Sumatra earthquake. Figure is taken from the USGS Earthquake Summary Poster. (see a larger version of this image, 244 kb)
Other Factors
In the Sumatra earthquake, the rupture started at the epicenter (represented by the star on the map) and spread throughout the region indicated by the finite fault model derived from global recordings of the earthquake. This rupture most likely extended to the Sunda Trench and broke through the sea floor as surface faulting. Holding all other parameters of the earthquake constant, a tsunami generated by a sea-floor rupture is greater than one generated by an earthquake that does not rupture the sea floor (i.e., imbedded faulting). The graph below shows a comparison of initial tsunami wave profiles for the two cases (Geist and Dmowska, 1999).
For additional information regarding the earthquake and tsunami, see Roger Bilham's site at the Cooperative Institute for Research in Environmental Sciences (CIRES) and the Disaster Prevention Research Institute site at Kyoto University.
Tsunami Generation Modeling (58 kb)
Contents:
Tectonics of Sumatra-Andaman Islands (81 kb)
Seismological Aspects of Tsunami Generation (70 kb)
Tsunami Generation Modeling (58 kb)
References
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