Saturday 21 December 2013

Magnitude 5.3 Earthquake in northern Sumatra.

The United States Geological Survey recorded a Magnitude 5.3 Earthquake at a depth of 88.3 km, roughly 9 km to the northeast of the city of Meulaboh in the West Aceh Regency in northern Sumatra, slightly after 4.10 am local time on Saturday 21 December 2013 (slightly after 9.10 pm on Friday 20 December, GMT). While a quake of this size at this depth is likely to be felt over a wide area, it is unlikely to cause any significant damage, and there are no reports of any damage or casualties on this occasion.

The approximate location of the 21 December 2013 Meulaboh Earthquake. Google Maps.

The Indo-Australian Plate, which underlies the Indian Ocean to the west of Sumatra, is being subducted beneath the Sunda Plate, a breakaway part of the Eurasian Plate which underlies Sumatra and neighboring Java, along 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.

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 past 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.

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