Slightly after 12.35 am on 11 January 2012 local time (slightly after 6.35 pm on 10 January GMT) a massive Earthquake occurred roughly 400 km off the southwest coast of Northern Sumatra. This was measured as having a magnitude of 7.3 on the Richter Scale and occurring at a depth of 29.1 km by the United States Geological Survey, and as having a magnitude of 7.1 on the Richter scale and occurring at a depth of 10 km by the Indonesian agency Baden Meteorologi, Klimatologie, dan Geophysica (BMKG) (Meteorological, Climatological and Geophysical Agency). The quake was felt in Sumatra and on the Malay Peninsula, but there are no reports of any casualties.
The location of the quake. Map from the United States Geological Survey.
Due to the size of the Earthquake the BKGM initially issued a tsunami warning, but this has now been lifted. The area is on the boundary between the Indo-Australian Plate to the west, and the Sunda Plate upon which Sumatra lies. The Indo-Australian Plate is being subducted beneath the Sunda Plate, but is also moving north relative to it. This leads to two sorts of Earthquakes in the area; Thrust Earthquakes, in which the Sunda Plate moves abruptly over the Indo-Australian Plate and Transform Quakes, in which the plates move past one-another along a north-south axis. Thrust Faults commonly lead to tsunamis, transform quakes seldom do so. Transform quakes do not always happen on the plate margin, as part of one plate is often dragged along with the other. This quake now appears to have been a transform quake.
How transform faults can lead to tsunamis.
On 26 December 2004 a thrust quake off the west coast of Sumatra caused a tsunami which swept the Indian Ocean. The quake was recorded as having a magnitude of between 9.1 and 9.3 on the Richter Scale, and the resultant tsunami killed over 230 000 people in fourteen countries.
An animation of the progress of the 2004 tsunami, from the US National Oceanographic and Atmospheric Administration.