Poster of the Sumatra-Andaman Islands Earthquake of 26 December 2004 - Magnitude 9.1

Tectonic Summary

The devastating earthquake of 26 December 2004 occurred as thrust-faulting on the interface of the India plate and the Burma microplate. In a period of minutes, the faulting released elastic strains that had accumulated for centuries from ongoing subduction of the India plate beneath the overriding Burma microplate.

In a broad sense, the India and Australian plates move toward the north-northeast with respect to the interior of the Eurasia plate with velocities of about 60 mm/y in the region of the earthquake. This results in oblique convergence at the Sunda trench. The oblique motion is partitioned into thrust-faulting, which occurs on the interface between the India plate and the Burma microplate and involves slip directed at a large angle to the orientation of the trench, and strike-slip faulting, which occurs on the eastern boundary of the Burma microplate and involves slip directed approximately parallel to the trench. Details of the velocity of the Burma microplate remain to be determined and may, in fact, be clarified by further analysis of the December main shock and its aftershocks.

Currently available models of the 26 December main-shock fault displacement differ in many interesting details, but are consistent in implying that fault-rupture propagated to the northwest from the epicenter and that substantial fault-rupture occurred hundreds of kilometers northwest of the epicenter. The data upon which the modeling is based do not permit confident resolution of the extent of rupture beyond about 500 km northwest of the main-shock epicenter. The finite fault model shown here implies that the width of the earthquake rupture, measured perpendicular to the Sunda trench, was about 150 kilometers, and that the maximum displacement on the fault plane was about 20 meters. The sea floor overlying the thrust fault would have been uplifted by several meters as a result of the earthquake.

The zone of aftershocks to the 26 December earthquake is over 1300 km long. Because aftershocks occur on and very near the fault planes of main shocks, the length of the aftershock zone suggests that main-shock fault-rupture may have extended north of epicenter by an amount significantly larger than 500 km. However, a great earthquake may also trigger earthquake activity on faults that are distinct from the main-shock fault plane and separated from it by tens or even hundreds of kilometers. We will not know until further analysis how much of the 26 December aftershock zone may correspond to activity in the immediate vicinity of the main-shock rupture, and how much may correspond to activity remote from the main-shock rupture.

Since 1900, earthquakes similarly sized or larger than the 26 December earthquake have been the magnitude 9.0 1952 Kamchatka earthquake, the magnitude 9.1 1957 Andreanof Islands, Alaska, earthquake, the magnitude 9.5 1960 Chile earthquake, and the magnitude 9.2 1964 Prince William Sound, Alaska, earthquake. All of these earthquakes, like the 26 December earthquake, were mega-thrust events, occurring where one tectonic plate subducts beneath another. All produced destructive tsunamis, although deaths and damage from the 26 December tsunami have far exceeded those caused by tsunamis associated with the earlier earthquakes.


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