At 2.38 pm local time (8.38 am GMT) on Wednesday 11 April 2012 the north-east Indian Ocean was shaken by an Earthquake 435 km southwest of the northern tip of Sumatra, recorded by the United States Geological Survey as measuring 8.6 on the Richter Scale and occurring at a depth of 22.9 km. This was followed by three major aftershocks, the first measuring 6.0 on the Richter Scale and happening at 9.28 am, a second at 10.01 which measured 5.3 on the Richter Scale, then at 10.24 am a quake measuring 5.4. Then at 10.43 the area was shaken by a second major quake, measuring 8.2 on the Richter Scale, at a depth of 16.4 km. Since then there have been (at the time of writing, eight hours after the initial shock) 14 quakes in the area measuring in excess of 5.0 on the Richter Scale.
Map showing the area of the 11 April quakes. Squares represent the location of Earthquakes, larger squares for larger quakes, red more recent than blue, yellow square represent quakes earlier in the week. The red line is the Sunda Trench, the large island at the bottom right is Sumatra. USGS.
There are not any reports of major damage or casualties at this time, but there have been reports of power outages and panic in Sumatra, and minor damage as far away as Kolkata (Calcutta). If no casualties do emerge then the initial quake may have been the most powerful quake ever recorded that did not cause any casualties. A tsunami warning was issued for the nations of the Indian Ocean, but in the event no major wave occurred; the network of buoys used to monitor such waves in the Ocean did record a small tsunami event, but the wave reached only 80 cm in height when it reached Sumatra.
The Indo-Australian Plate is being subducted beneath the Sunda Plate (a breakaway part of the Eurasian Plate) in the Sunda Trench, passing under Sumatra, where friction between the two plates can cause Earthquakes. As the Indo-Australian Plate sinks further into the Earth it is partially melted and some of the melted material rises through the overlying Sunda Plate as magma, fueling the volcanoes of Sumatra.
Diagram showing the passage of the Indo-Australian Plate beneath Sumatra. Virtual Upper Mantle of the Earth.
This does not happen at a 90° angle, as occurs in the subduction zones along the western margins of North and South America, but at a steeply oblique angle. This means that as well as the subduction of the Indo-Australian plate beneath the Sunda, the two plates are also moving part one-another. This causes rifting within the plates, as parts of each plate become stuck to the other, and are dragged along in the opposing plate's direction. The most obvious example of this is the Sumatran Fault, which runs the length of Sumatra, with the two halves of the island moving independently of one-another. This fault is the cause of most of the quakes on the island, and most of the island's volcanoes lie on it.
The Indo-Australian Plate encounters the Sunda Plate at an oblique angle, passing it as well as being subducted beneath it.
The 11 April quakes occurred considerably to the west of the Sunda Fault, and appears to have been caused by two parts of the Indo-Australian Plate moving past one-another horizontally. Such a quake would not generally cause a tsunami event, as there is little vertical movement to generate waves. Most major tsunamis are caused by subduction movement, or submarine landslides, which do cause considerable vertical displacement.
See also Massive Earthquake off Northern Sumatra, 11 January 2012, Large Earthquake in Northern Sumatra, Sumatra shaken by series of Earthquake. 3-4 August 2011 and Earthquakes on Sciency Thoughts YouTube.
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