Tectonic Summary

The devastating M 9.1 earthquake off the west coast of northern Sumatra on December 26, 2004, occurred as the result of 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 Australia plates move towards the north-northeast with respect to the interior of the Eurasia plate with velocities of about 60 mm/yr in the region of the earthquake. This results in oblique convergence at the Sunda Trench, about 200 km to the southwest of the December 26th earthquake. 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 mainshock and its aftershocks.

Early models of the December 26th mainshock fault displacement differ in many 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 mainshock epicenter. The width of the earthquake rupture, measured perpendicular to the Sunda Trench, was about 150 km, and maximum displacement on the fault plane was upwards of 20 m. The sea floor overlying the thrust fault was uplifted by several meters as a result of the earthquake, causing the ensuing tsunami that devastated shores around the Indian Ocean.

The zone of aftershocks of the December 26th earthquake is more than 1,300 km long. Because aftershocks occur on and very near the fault planes of mainshocks, the length of the aftershock zone suggests that mainshock fault rupture may have extended north of the 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 mainshock fault plane and separated from it by tens or even hundreds of kilometers. Detailed modeling of this earthquake over the months and years following the event, using teleseismic data (that the models discussed here were based on) and data from regional global positioning system (GPS) stations, indicates that significant slip extended over a region up to about 1,500 km to the north of the epicenter, and that rupture lasted for more than 500 seconds.

This is the third largest earthquake in the world since 1900 and is the largest since the 1964 Prince William Sound, Alaska, earthquake. Since 1900, earthquakes similarly sized or larger than the December 26th earthquake have been the M 9.0 1952 Kamchatka earthquake, the M 9.5 1960 Chile earthquake, and the M 9.2 1964 Prince William Sound, Alaska, earthquake. All of these earthquakes, like the December 26th earthquake, were megathrust events, occurring where one tectonic plate subducts beneath another. All produced destructive tsunamis, although fatalities and damage from the December 26th tsunami have far exceeded those caused by tsunamis associated with the earlier earthquakes and was documented nearly worldwide on tide gages.

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)

Comparison of Fault Rupture Area to Size of California

map showing rupture area is half the size of California

Event Details

Event Map