M7.1 Earthquake Shakes Southern Mexico: 7 July 2014 Tremor

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Location of the Mexico earthquake of 7 July 2014. Image credit: USGS

Location of the Mexico earthquake of 7 July 2014. Image credit: USGS

An earthquake of magnitude 7.1 (M7.1) struck southern Mexico in the early hours of 7 July local time.

News reports indicate that the tremor caused power outages over the border in Guatemala, where early reports suggest that at least two people have died.

The tremor occurred onshore, 5 miles to the north east of the town of Puerto Madero and at a depth of 92.2 km.

Early indications from the United States Geological Survey show that shaking was strong in the immediate area and that the tremor was felt as far south as El Salvador and on the Atlantic coast of Mexico itself.

7 July Mexico Earthquake: Tectonic Setting

The Pacific coast of southern and central America is bounded by a series of subduction zones which extend almost unbroken for thousands of kilometres from Tierra del Fuego to the southern Gulf of California, making the Andean margin highly seismically and volcanically active.

In Mexico, the tectonic setting is one in which the Cocos tectonic plate, composed of dense, oceanic crust, is moving (relatively) rapidly north-eastwards against the more buoyant continental crust of the Southern Mexico block, beneath which it sinks. The friction generated by this subduction process causes regular earthquakes. All the available evidence points to this tremor being a classic subduction earthquake, caused by movement at – or very close to – the plate interface.

The trench which marks the plate boundary lies offshore, and many earthquakes, as a consequence, have their epicentres underwater – a feature which, combined with a magnitude of around M7 or more, can contribute to the generation of tsunamis. In this case, the fact that the epicentre was onshore, combined with the depth of the earthquake (shallow earthquakes are more likely to be tsunamigenic than deeper ones) meant that no tsunami occurred.

Earthquake History of Mexico

Damage following Mexico City's earthquake of 1985. Image courtesy USGS

Damage following Mexico City’s earthquake of 1985. Image courtesy USGS

As noted above, earthquakes are so common in Mexico as to be almost a fact of life. The 7 July tremor isn’t even the largest of the year in the country to date (there was an M7.2 in Mexico on 18 April) and the subduction margin of the Cocos plate and Central America has seen a dozen earthquakes of at least M6.0 since the beginning of 2014.

A look further back in time reinforces the point. The USGS historic earthquakes list includes 23 events of at leaf M7.0 since 1900, several of which have led to extensive damage and in some cases to loss of life.

Most notorious of these was the earthquake of September 1985, which killed an estimated 9,500 people – many more than any other recorded tremor in the country’s history.

At M8.0 the 1985 tremor was large, but it is most noteworthy for the fact that much of the damage occurred hundreds of miles from the epicentre, in Mexico City. Built on a former lake bed, the city fell victim to its geology as the soft ground amplified the seismic waves and increased shaking in the city. Combined with poor building standards, this led to devastation and death.

Today’s Mexico Quake

Generally speaking, however, earthquakes in Mexico don’t cause extensive damage. Despite the reported deaths, it’s unlikely that most of the estimated 1.5 million people exposed to shaking will be too much disturbed by this latest event.

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