Monday 11 July 2016

Magnitude 5,1 Earthquake beneath West Sumatra.

The United States Geological Survey recorded a Magnitude 5.1 Earthquake at a depth of 13 km, about 49 km to the northwest of the city of Bukittinggi in West Sumatra Province, Indonesia, slightly after 4.30 pm Western Indonesian Time (slightly after 9,30 am GMT) on Sunday 10 July 2016. The event was felt across much of West Sumatra, but there are no reports of any damage or casualties.

The approximate location of the 10 July 2016 West Sumatra 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.

 The Subduction zone beneath Sumatra. NASA/Earth Observatory.

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.

The movement of the tectonic plates around Sumatra. NASA/Earth Observatory.

Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organization Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.

See also... flow kills at least seven on Mount Sinabung, North Sumatra.                          Seven people have been confirmed dead and an unknown number are still missing following a pyroclastic flow (avalanche of hot rock, ash and gas) that swept through farms and villages on the slopes of... students confirmed dead after flash flood and landslide at popular Sumatran tourist spot.                                          Seventeen students have been confirmed dead and another... 7.8 Earthquake to the southwest of Sumatra triggers small tsunami.              The United States Geological Survey recorded a Magnitude 7.8 Earthquake at a depth of 10 km...
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