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Tsunamis and Earthquakes

Tsunami Generation from the 2004 M=9.1 Sumatra-Andaman Earthquake

Tectonics of Sumatra-Andaman Islands

2004 tsunami wavefield at 1 hour
Tsunami wave field in the Bay of Bengal one hour after the earthquake. View to the north west.

The December 26, 2004 M=9.1 Sumatra-Andaman earthquake occurred along a tectonic subduction zone in which the India Plate, an oceanic plate, is being subducted beneath the Burma micro-plate, part of the larger Sunda plate.

oceanic subduction zone
General diagram of an oceanic subduction zone.
Sumatra and the Andaman Islands are part of an island arc.
(Figure is taken from online edition of This Dynamic Earth)

The interface between the two plates results in a large fault, termed an interplate thrust or megathrust. This fault lies below the southwestern part of Sumatra and the Andaman Islands. Where the interplate thrust intersects the sea floor is marked by the Sunda trench that can traced along an arc from Burma in the north to Java in the south. The figure below, based on info from the USGS Earthquake Hazards Program, shows:

  • the interplate thrust where it interesects the sea floor along the Sunda trench,
  • the epicenter of the December 26, 2004 mainshock, and
  • major strike slip faults in the overriding plate of the subduction zone.

tectonic base map of the Sumatra subduction zone showing major faults
Tectonic base map of the Sumatra subduction zone showing major faults. Map based on info from USGS Earthquake Hazards Program

The direction of convergence of the India Plate relative to the Sunda plate (thick arrows on map above) is oriented oblique to the orientation of the interplate thrust (i.e., trench axis). For oblique subduction zones such as this, movement between the two plates can be accomodated one of two ways as shown in the figure below (Michael, 1990).

diagram showing the difference between oblique faulting and decoupled faulting

As described in a classic paper by Fitch (1972), the Sumatra subduction zone is characterized by decoupled faulting, as in (b) above. In this case, nearly pure thrust faulting occurs along the interplate thrust and strike-slip faulting occurs in the overriding plate, most notably along the Great Sumatran fault.

An example of oblique faulting, as in (a) above, occurs in the northern Puerto Rico subduction zone.

Next page, Seismological Aspects of Tsunami GenerationSeismological Aspects of Tsunami Generation


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