Sunday 25 December 2016

Magnitude 7.6 Earthquake on the south coast of Chiloé Island, Chile.

The United States Geological Survey recorded a Magnitude 7.6 Earthquake at a depth of 34.6 km on the south coast of Chiloé Island in southern Chile, slightly after 11.20 am local time (slightly after 2.20 pm GMT) on Sunday 25  December 2916 There are no reports of any injuries associated with this event, but people have reported feeling it across much of Chile and eastern Argentina, and damage has been reported to a number of roads and at least one bridge on Chiloé Island, as well as cutting power to about 21 000 homes. Due to the location of the event a tsunami warning was issued, leading to the evacuation of around 5000 people from low-lying areas in the region, though this was lifted after about three hours when no significant wave appeared.

Damage to a road at Tarahuin on Chiloé Island, following the 25 December 2016 Earthquake. Alvaro Vidal/EPA.

Chile is located on the west coast of South America, which is also the convergent margin between the Nazca and South American Plates. The Nazca Plate is being subducted beneath the South American Plate and is sinking beneath the South American Plate. This is not a smooth process, the rocks of the two plates continuously stick together then, as the pressure builds up, break apart again, causing Earthquakes. As the Nazca Plate sinks deeper it is partially melted by the heat of the Earth's interior. Some of the melted material then rises up through the overlying South American Plate as magma, fueling the volcanoes of the Chilean Andes.
The subduction of the Nazca Plate beneath the South American Plate, and how it causes Earthquakes and volcanoes. Pacific Earthquake Engineering Research Center.
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.
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