Western Sumatra, a particularly earthquake-prone area of the earth’s crust, is regularly the location of major seismic events which include some of the largest earthquakes on record. January 11th 2012 saw the latest in a series of major tremors with the occurrence of a magnitude 7.3 event of the west coast of northern Sumatra.
The Sumatra Earthquake of January 2011
Initial data published by the United States Geological Survey show that Sumatra’s most recent major event occurred at a relatively shallow depth (around 18 miles) over 200 miles offshore. Its magnitude, and the fact that it occurred beneath the sea rather than on land, raised initial concerns about a local tsunami, although the early warning issued by the NOAA’s Pacific Tsunami System was later withdrawn.
Causes of the January 2012 Quake: Tectonic Setting of Sumatra
Earthquakes are an inevitable outcome of the relative movements of the earth’s tectonic plates. Sumatra is positioned where the Indo-Australian (or Australian) plate is moving inexorably north-north eastwards, bringing it into contact with the many microplates. Thes microplates, jostling for position, form a buffer zone between the Indo-Australian Plate and the Eurasian Plate on the far side of the Indonesian archipelago, of which the island is a part.
In typical subduction-zone earthquakes, at the point of contact the former plate is subducted (forced downwards) below the archipelago, and friction between the subduction and over-riding surfaces increases until the upper of the two plates rebounds and energy is released in the form of an earthquake.
The situation in Sumatra is slightly different. Because the angle at which the Indo-Australian plate is being subducted is relatively shallow, it is not only moving downwards but also has a component of lateral, or strike-slip, movement. Today’s earthquake occurred on a strike-slip fault, some distance to the west of the subduction zone itself.
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