Tsunami Generation from the 2004 M=9.2 Sumatra-Andaman Earthquake

Seismological Aspects of Tsunami Generation

Several seismological aspects of the 2004 Sumatra-Andaman earthquake are important in explaining the tsunami that was generated:

• Magnitude,
• Mechanism, and
• Centroid.

Magnitude

The magnitude (M) of a submarine earthquake is, in most cases, the most important factor that determines the size of a tsunami. Because of the size and complexity of this earthquake, it has been difficult to assign a precise magnitude. A magnitude of M=9.2 seems to be most appropriate for tsunami studies as ascertained from analysis of seismograms and geodetic data (Banerjee et al., 2007; Chlieh et al., 2007). See also the NEIC FAQ page regarding magnitude for this event.

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.

Earthquake magnitude versus local tsunami intensity for subduction zone earthquakes from 1896-2005.

Near the earthquake source, local tsunami size increases with the magnitude of the earthquake, although there is significant variability in this relationship. The term tsunami earthquake refers to anomalous earthquakes, in which the tsunami is larger than expected from the magnitude of the earthquake. These earthquakes tend to rupture the interplate thrust near where it approaches the sea floor at the trench. Although the local tsunamis from tsunami earthquakes (solid dots above) are larger than would generally be expected from earthquake magnitude alone, local tsunami intensity trends linearly with magnitude. In contrast, the intensity of local tsunamis from normal tsunamigenic earthquakes do not trend exactly with magnitude.

The size of 2004 Indian Ocean local tsunami is consistent with the size of local tsunamis generated by other earthquakes of similar magnitude, for example the 1964 Great Alaska earthquake and tsunami. The Earthquake Research Institute (University of Tokyo) has calculated the tsunami magnitude for the 2004 Indian Ocean tsunami and has shown that it is consistent with such a high magnitude earthquake. Though local tsunami runups are among the highest ever recorded (nearly 32 m), this earthquake does not appear to be an anomalous tsunami earthquake.

Mechanism

Consistent with the decoupled tectonics of the Sumatra subduction zone described in the previous tectonics section, the focal mechanism for the 2004 Sumatra-Andaman earthquake shows that rupture occurred on the interplate thrust with very little oblique motion.

Focal mechanism for 2004 Sumatra-Andaman earthquake

This focal mechanism is associated with the southern part of fault rupture near the epicenter. However, this was a very long rupture extending from the epicenter northward to the Andaman Islands, as indicated by the analysis of seismic array data. A more complete analysis of the focal mechanicsm by Tsai et al. (2005) indicates that oblique motion occurred on the northern part of the fault rupture.

When nearly all of an earthquake's energy is released in a thrust motion, as in the 2004 Sumatra-Andaman earthquake, a large tsunami is generated. In contrast, large strike-slip earthquakes, involving more horizontal than vertical displacement of the sea floor, are not efficient tsunami generators.

Aftershocks occurred on both the interplate thrust and faults in the overriding plate, including the transcurrent fault.

Centroid

The centroid of an earthquake is the location of the center of energy release. The location of the 2004 Sumatra-Andaman centroid, as 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. This is discussed in our comparison of the tsunamis generated by the 2004 Sumatra-Andaman earthquake and the March 28, 2005 northern Sumatra earthquake.

Base map of the Sumatra subduction zone showing seismicity associated with the 2004 Sumatra-Andaman earthquake. Figure is taken from the USGS Earthquake Summary Poster. (see a larger version of this image, 244 kb)

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.

Details of Tsunami Generation (238 kb)