Saturday 5 March 2016

Magnitude 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 beneath the Indian Ocean, roughly 660 km to the southwest of the coast of Sumatra, slightly before 7.50 am Western Indonesian Time (slighrtly before 12.50 pm GMT) on Wednesday 2 March 2016. While a quake of this size would be likely to be cause significant damage if it occurred on land, this far from shore it did not present significant threat, though it was felt as far away as Singapore and Negeri Sembilan Sate in Malaysia. Off more concern was the potential for an offshore quake of this size to cause a tsunami, though on this occasion only a small event was recorded, with waves 60 cm in height recorded in the Cocos Islands (about 850 km to the south) and on Simeulue Island (off the coast of Sumatra).

The approximate location of the 2 March 2016 Indian Ocean 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.

Earthquakes along subductive margins are particularly prone to causing tsunamis, since these often occur when the overlying plate has stuck to the underlying plate, being pulled out of shape by its movement.. Eventually the pressure builds up tp far and the overlying plate snaps back, causing an Earthquake and a tsunami. 

Simplified graphic showing tsunami generation along a convergent margin.NASA/JPL/CalTech.

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... on Mount Sinabung.                 Residents of villages close to Mount Sinabung on North Sumatra have been forced to evacuate their homes following two eruptions on the volcano on Thursday 25 February 2016. The first eruption occured at about... cancelled to and from Lombok and Bali following eruption on Mount Rinjani. Authorities in Indonesia have been forced to close airports in Lombok and Bali following a series of eruptions on Mount Rinjani, an active volcano... confirmed following Magnitude 6.0 Earthquake in Sabah State, Malaysian Borneo.                                                         The United States Geological Survey recorded a Magnitude 6.0...

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