Mexican earthquakes

Mexico has been hit by its second deadly earthquake in less than two weeks. Are the two events related, and could they indicate more tremors are on the way?

Both quakes occurred on the Cocos tectonic plate, which runs along the western coast of Mexico, and is sliding beneath the neighbouring North American tectonic plate to the north-east at a rate of about three inches per year.

The 7.1 magnitude quake, which struck shortly after 6pm local time on Tuesday, occurred 120km south-east of Mexico City. This came just 11 days after a magnitude 8.1 quake off the coast of southern Mexico.


Most of the victims died in the capital, according to government figures:

Mexico City: 94 dead
Morelos state: 71 dead
Puebla state: 43 dead
Mexico state: 12 dead
Guerrero: 4 dead
Oaxaca: 1


In each case, the tremors originated from within the Cocos plate, deep beneath the surface, rather than being caused by friction at the interface. Tuesday’s quake occurred at 50km depth and the earlier quake was even deeper, at 70km.

As the Cocos plate is forced downwards, it deforms – causing the structure to kink and crumple. But this process is not smooth and incremental. Instead, stress builds up inside the plate over months or years until a threshold is reached and it is suddenly released in a giant tremor.


“What happened yesterday was most likely a tearing motion in the subducting Cocos plate,” says Prof David Rothery of the Open University.


A similar mechanism is thought to be responsible for the earlier recent quake, but seismologists do not think that one led to the other. Stephen Hicks, of the University of Southampton, said: “It’s quite a long way for them to be directly linked. It might have slightly increased the stress, but if it did it’s a tiny amount and the fault must have been close to rupturing anyway.”

Mexico City is particularly vulnerable because it sits on an ancient lake bed that is filled with deep layers of sediment, which can magnify the shaking. “Once the seismic wave enters that bowl, it reverberates around,” said Hicks. “It behaves like a bowl of jelly.”

Unlike some natural disasters, scientists have yet to devise a reliable way to predict when earthquakes will occur. However, planning for the worst can hugely reduce the devastation caused and death toll of future quakes.

The most recent quakes in Mexico will, once again, raise questions about whether the appropriate building codes were adhered to. “The Enrique Rébsamen elementary school where many children died looks like a modern building, and ought to have had inbuilt earthquake resilience,” said Rothery. “Had it been properly constructed it should not have collapsed, and I expect questions will be asked”.