An earthquake of magnitude 6.0 (M6.0) struck Mexico on November 15, 2012. Located around 173 km from Mexico City, the tremor occurred in the early hours of the morning local time at a depth of 61km.
There were no reports of damage or injuries at the time of writing and the USGS’s Pager impact assessment suggested a very low likelihood of either significant damage or fatalities.
Tectonic Setting of the November 2012 Mexico Earthquake
Tectonically, Mexico is part of the so-called Pacific Ring of Fire, a highly seismically and volcanically active belt surrounding the Pacific Ocean. In the eastern Pacific, the boundaries of several of these belts abut against the North and South American continents, where earthquakes are common.
Off the coast of Mexico, the Cocos Plate is moving roughly north-eastwards against the North American and Caribbean plates. Because it is composed of denser oceanic crust, this plate is forced downwards when it comes into contact with the more buoyant continental crust of Central America. The downwards motion of the plate causes friction against the overlying crust and when tension reaches a certain (variable) level, it’s released in the form of an earthquake.
Because the subducting plate slopes downwards, and earthquakes occur along the plane between the two slabs of crust, generally speaking earthquakes increase in depth away from the actual plate boundary, reaching depths of 600km or more. The relatively shallow depth of November 2012 earthquake for its location illustrates an interesting feature of the Mexican subduction zone.
Studies have shown that, rather than continuing to descend, the Cocos Plate levels out beneath Central America, rather like a step. This gives an unusual pattern of depth with many more shallow earthquakes over a longer distance than would be expected elsewhere, although the reasons for this situation are not clearly understood.
Historic Mexican Earthquakes
Because of the setting, the people of Mexico are by no means unaccustomed to earthquake activity: the USGS describes it as “one of the world’s most seismically active regions.” Unsurprisingly, the level of hazard is greatest close to the western coast where the subduction zone lies, and decreases inland. A map of seismicity clearly shows the concentration of larger, shallower earthquakes along the coast, with fewer, generally smaller and shallower, inland.
The M6.0 of November 2012 was only the third largest to strike Mexico this year – a clear illustration of the level of hazard which is to be expected. The country’s largest and most deadly earthquake occurred in 1985, an M8.0 which killed around 9,500 people, many of them in Mexico City. Since then, although the country has experienced 5 tremors of M7.0 or more, deaths and injuries have been relatively few.
California Institute of Technology. The unusual case of the Mexican subduction zone. Accessed November 15, 2012.
United States Geological Survey. M6.0 – 9km E of Tlalchapa, Mexico. Accessed November 15, 2012.