Two earthquakes have hit Mexico within two weeks, both occurring on the Cocos tectonic plate. But are they related, and could Mexico face more tremors?

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



Two days after the second earthquake in Mexico, large quakes struck the Pacific island of Vanuatu and off the north-east coast of Japan.

Both Mexican 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 1pm 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.

Mexico: coastal earthquake The location of the 8.1 magnitude earthquake, 7 September.

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.

Earthquake south-east of Mexico City. The 7.1 magnitude earthquake, 19 September.

“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.”

Experts have dismissed the possibility of any causal relationship between the Mexico quakes and those in other locations along the seismically active Pacific “ring of fire”.

A 6.4 magnitude quake struck Vanuatu’s Erromango island early on Thursday local time, but caused no damage, according to the US Geological Survey and local authorities.

Hours earlier, a 6.1 magnitude earthquake was recorded 283 kilometres off the coast of north-east Japan, the same region that was devastated by a tsunami following a 9.0 magnitude quake in March 2011. The disaster killed more than 18,500 people and triggered a triple meltdown at the Fukushima Daiichi nuclear power plant.

But Hongfeng Yang, an assistant professor in the Earth System Sciences Programme at the Chinese University of Hong Kong, said the fact that this week’s three big quakes had all occurred within such a short timeframe was a coincidence.



“They are really far away from each other so that physical mechanisms to link them together are weak,” Yang told the Guardian.

“There is a chance, however, to trigger distant earthquakes by seismic waves generated by prior earthquakes located far away. But the likelihood of that triggering earthquakes larger than magnitude 5 is nearly zero.”

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”.

• This article was amended on 21 September 2017. An earlier version said the 7.1 magnitude quake hit shortly after 6pm local time. This has been corrected to 1pm.

