Chilean Earthquake Caused by Offshore Subduction Zone

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Location of the April 2012 M6.7 earthquake (image from USGS)

An earthquake which struck off the coast of Chile near Valparaiso has been reported by the United States Geological Survey as having a magnitude of 6.7 (M6.7). Although an earthquake of this scale would be significant in many parts of the world it is in fact remarkably small for the Pacific coast of South America, where major tremors (greater than M8.0) are by no means unknown.

Tectonic Setting: The Andean Margin

The margins of the earth’s tectonic plates are vulnerable to seismic and volcanic activity, and the edges of the plates which surround the Pacific Ocean are particularly notorious, having acquired the nickname, “Pacific Ring of Fire.” Here, plates moving together directly and obliquely, along with zones where new ocean crust is being created, are responsible for many of the planet’s major earth tremors.

The tectonic setting for the offshore earthquake of April 2012 is relatively straightforward. The Nazca Plate, of dense oceanic crust, is moving westwards against the more buoyant continental crust of the South American plate at a relatively rapid rate of approximately 80mm per year. Where the two plates collide the denser plate is forced downwards (subducted) creating an ocean trench: earthquakes occur where tension builds up and is released.

Mountain Building and the Chilean Triple Junction

Such subduction is responsible not only for the repeated pattern of earthquakes along the plane where the two plates meet (the depth of the earthquakes increases as the subducting plate is forced downwards) but is also the mechanism underlying the formation of the Andes mountains, which are formed as heat and pressure melts rocks in the depths of the Earth, and causes molten rock to rise, reaching the surface as volcanoes.

To the south, the pattern is complicated by the existence of a triple junction: where a constructive margin between the Nazca and Antarctic plates (caused as plates move apart) is being subducted below the South American plate, effectively terminating the offshore trench. It should be noted, however, that the majority of Chile’s significant earthquakes, including that of April 2012, occur to the north of this junction.

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