M7.4 Major Earthquake Strikes Coastal Mexico Near Oaxaco

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The earth tremor which struck close to the Mexican city of Acapulco on March 20, 2012 was the second major earthquake to occur in the region within four months. The M7.4 event followed an M6.5 which occurred in the Guerrero region, though with an epicentre further to the north west, in December 2011 and USGS earthquake data show that it was followed by a number of significant aftershocks.

Impacts of the March 2012 Earthquake

Despite the magnitude and timing of the event (which occurred at mid-day when many people were out and about) and the shallow depth (shallow earthquakes tend to cause more damage than deeper ones) there were no reports of casualties at the time of writing, although extensive damage to buildings was reported near to the epicenter, 100 miles from Oaxaco.

The BBC news site also reported that extensive shaking was felt in Mexico City, some 200 miles away. Built on the site of a former lake bed, the Mexican capital is geologically vulnerable, as sediments amplify shaking. It was this issue that caused widespread damage and loss of life in the 1985 earthquake.

Causes of the Mexico Earthquake

Like many places in the world, Mexico is particularly earthquake-prone because of its location on the planet’s surface. The country’s Pacific coast is coincident with a subduction zone, where one of the earth’s tectonic plates meets another and is forced beneath it. In this case, the north-eastwards moving Cocos plate is forced beneath two other plates – the North American plate and the Caribbean plate.

The tectonic situation in Mexico and the Caribbean is further complicated by the juxtaposition of these plates, and their varying rates and directions of movements. The jostling of these plates for position set up stresses and causes a build-up of friction, which is released as an earthquake when a certain threshold is reached.

Subduction zones generate typical patterns of earthquakes, which occur at greater depths further from the plate boundary, and coastal Mexico is therefore the location of regular, often shallow, tremors of a significant size. The March 2012 event was typical of such subduction events, and the USGS notes that its direct cause was thrust faulting (one plate being forced up over another) at the boundary between the Cocos and North American plates.

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