Poster of the Nicobars Islands Earthquake of 24 July 2005 - Magnitude 7.3

Tectonic Summary

This earthquake occurred near the western boundary of the aftershock zone of the devastating Sumatra-Andaman Islands earthquake of 26 December 2004. The aftershock zone of the 26 December earthquake continues to be active and forms a crescent-shaped region extending about 1200 km north of Sumatra, Indonesia. The recent event was a strike-slip earthquake, probably in the India plate below its shallow-dipping boundary with the Burma plate. The earthquake resulted from horizontal displacement of the ground across a nearly vertical fault plane. This is unlike the thrust-fault motion that produced the 26 December earthquake and that occurred on the interface of the eastward-subducting India plate and the overriding Burma plate. The recent event therefore occurred on a different fault than the fault that caused the Great Sumatra-Andaman Islands earthquake. However, the two earthquakes are almost certainly related. A great earthquake can 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. A magnitude 7.3 earthquake is considered a major earthquake, but the size of this recent earthquake pales in comparison with the 26 December earthquake. A M7.3 strike-slip earthquake typically ruptures a 15 km by 80 km patch along a fault, whereas the 26 December 2004 earthquake ruptured a fault patch about 1200 km long and 200 km wide.

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