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Earthquake Hazards Program

Tectonic Summary

Magnitude 9.1 OFF THE WEST COAST OF NORTHERN SUMATRA
Sunday, December 26, 2004 at 00:58:53 UTC

The devastating earthquake of 26 December 2004 occurred as thrust-faulting on the interface of the India plate and the Burma plate. 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 plate.

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. In the region of northern Sumatra and the Nicobar Islands, most of the relative motion of India/Australia and the Eurasia plate is accommodated at the Sunda trench and within several hundred kilometers to the east of the Sunda trench, on the boundaries of the Burma plate. The direction in which India/Australia converges toward Eurasia is oblique to the trend of the Sunda trench. The oblique motion is partitioned into thrust-faulting and strike-slip faulting. The thrust faulting occurs on the interface between the India plate and the western margin of the Burma plate and involves slip directed at a large angle to the orientation of the trench. The strike-slip faulting occurs on the eastern boundary of the Burma plate and involves slip directed approximately parallel to the trench. The 26 December main shock occurred as the result of thrust faulting on the western Burma-plate boundary, but many strike-slip faulting aftershocks occurred on the eastern plate boundary.

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 width of the earthquake rupture, measured perpendicular to the Sunda trench, is estimated to have been about 150 kilometers and the maximum displacement on the fault plane 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 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 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 one on 26 December, 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 far exceed those caused by tsunamis associated with the earlier earthquakes.

 


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