Pacific Coastal and Marine Science Center
Tsunamis and Earthquakes
Tsunami Generation from the 2004 M=9.1 Sumatra-Andaman Earthquake
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.
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, from the USGS Earthquake Hazards Program Earthquake Summary Poster, shows:
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).
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.