Slightly before 1.10 am local time on Monday 2 January 2012 (slightly before 6.10 pm on Sunday 1 January, GMT) an Earthquake occurred in the interior of Northern Sumatra, roughly 163 km southeast of Sumatra. The quake was recorded as having a magnitude of 5.3 on the Richter Scale and occurring at a depth of 1 km by the United States Geological Survey and a magnitude of 5.5 on the Richter Scale and occurring at a depth of 10 km by the Indonesian Meteorological, Climatological and Geophysical Agency (Baden Meteorologi, Klimatologi, dan Geophysica or BMKG), making it a powerful, shallow, quake that would have caused serious problems in a densely populated area. Fortunately, despite Sumatra having a population in excess of 50 million, the northern interior is still largely forested and sparsely populated, so there have been no reports of any serious damage or casualties.
Map of the quake location from the United States Geological Survey.
Sumatra lies on the Sunda Plate, close to its boundary with the Indo-Australian Plate. To the southeast of the island the Sunda Trench marks where the Indo-Australian Plate is being subducted, passing beneath the island, causing Earthquakes as the plates rub against one-another and feeding Sumatra's 32 volcanoes with rock from the Indo-Australian Plate, which partially melts as it sinks into the Earth's interior.
The subduction of the Indo-Australian Plate beneath Sumatra.
The Indo-Australian plate is also moving northward with regard to the Sunda Plate, and as it does so it drags the westernmost part of the Sunda Plate, the bit that overlies the subducting plate, with it. This leads to a transform fault along the centre of Sumatra, with the west of the island moving northward with regard to the east; this is known as the Sumatran Fault. Most of the island's volcanoes lie along this fault, and it was on this fault that the 2 January Earthquake occurred.
A transform fault occurs where two bodies of rock move past one-another horizontally.