Magnitude 6.5 Earthquake in Guerrero, Mexico

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Location and tectonic setting of the 2011 Mexico earthquake: Image by USGS

An earthquake with a magnitude of 6.5 was recorded in the western Mexican state of Guerrero on the evening of 10 December 2011.  Data published by the United States Geological Survey (USGS) indicate that the tremor took place at a depth of around 40 miles, and that its epicentre was around 100 miles SSW of the country’s capital, Mexico City.

The tremor was much smaller than the major event which devastated the country in 1985, with the loss of an estimated 9,500 people, and the country appears to have escaped major damage. At the time of writing it was reported by the BBC that at least two people had been killed in the tremor.

2011 Mexico Earthquake: Tectonic Setting  and Causes

Most major earthquakes are caused by the relative movement of the earth’s tectonic plates – huge slabs of crust which move slowly across the surface of the globe. Mexico is located in a tectonically complex part of the world, where several plate movements interact, and is part of the seismically and volcanically active zone surrounding the Pacific Ocean known as the Pacific Ring of Fire.

At a regional scale, several plates are involved. A simple summary of the situation is that the relatively small Cocos plate is moving westward: in the south it is forced against the largely static Caribbean plate, and in the north it converges with the North American plate. Still further north, the Pacific and North American plates are moving in broadly the same direction but obliquely and at different rates.

The 2011 Mexico earthquake occurred at the boundary between the relatively dense oceanic crust of the Cocos plate and the more buoyant North American plate. Here, the convergence forces the Cocos plate below the North American plate and at this zone – called a destructive margin – friction builds up until earthquakes occur.

Historic Mexican Earthquakes

Damage caused by the 1985 Mexico earthquake (USGS)

Damage caused by the 1985 Mexico earthquake (USGS)

Such zones are characterised by major earthquakes and the 2011 event, though significant, is by no means the largest to have occurred in Mexico. In fact the USGS historic data indicate that the country has suffered 21 earthquakes of greater than M6.5 since 1887 – 16 of them M7.0 or greater, including tremors of M8.0 and M8.1. Most recently, an M7.2 earthquake further north killed two people in April 2010.

Mexico Earthquake

Mexico’s largest recorded earthquake, in 1932, killed relatively few. The most notorious tremor was that of 1985, which took place further west but within the same tectonic context. That earthquake is believed to have caused around 9,500 deaths and 30,000 injuries, many of them in Mexico City itself.

Sources

BBC News online. Mexico earthquake kills two in Guerrero state. Accessed December 11, 2011.

Smithsonian Institution, USGS and US Naval Research Laboratory. This Dynamic Planet, World Map of Volcanoes, Earthquakes, Impact Craters, and Plate Tectonics. Accessed December 11, 2011.

USGS. Historic EarthquakesAccessed December 11, 2011.

USGS. Earthquake Hazard Program Magnitude 6.5 – Guerrero, Mexico. Accessed December 11, 2011.

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