Poster of the Southern Sumatra, Indonesia Earthquake of 25 July 2004 - Magnitude 7.3
The earthquake occurred deep beneath the island of Sumatra, which is one of many islands in the large Indonesian island-arc system. Off the southwest coast of Sumatra, the Australia plate* is being subducted, toward the north-northeast, beneath the Sunda plate at a rate of about 7 cm/yr (see the Plate Tectonic Setting map). At the earth's surface, the boundary between the plates is marked by the 3400 km long Sunda (Java) Trench. The island arc is delineated by the Sunda Trench axis, a parallel line of volcanoes, and zones of intense seismicity. The subducted Australia plate has previously been known to be seismically active to a depth of about 650 km beneath Java, but the 25 July 2004 magnitude 7.3 earthquake is one of the few to occur below 300 km beneath the island of Sumatra. Due to the earthquake's great depth it was felt over a large area, but caused little damage.
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