Analysis: Mexico has borne the brunt of the most brutal of quakes

It was a devastating event that flattened hospitals, schools, factories, high-rise buildings and much else: Mexico City’s 8.0-magnitude earthquake of 1985 left an estimated 10,000 dead.

This month history has repeated itself, albeit with fewer deaths.

An 8.2-magnitude quake in the south of the country earlier this month - Mexico’s the strongest for a century - was followed, 32 years to the day after the 1985 quake, by a 7.1-magnitude event on Tuesday with an epicentre nearer the capital.

Mexico’s vulnerability stems from its location beside the Pacific 'Ring of Fire', where tectonic plates join. The country mostly lies directly above the North American plate, but to the south-west is the join to the Cocos plate, while at its north-west Mexico skirts the Pacific Plate. The Cocos Plate is moving towards the North American Plate and is sinking beneath it, a process called subduction that generates stresses which cause earthquakes.

Seismologist Dr Stephen Hicks, from the University of Southampton in the United Kingdom, said both this month’s earthquakes were caused by the bending of the Cocos Plate.

“Once [the Cocos Plate] starts sinking, it bends under its own weight. The bending pulls apart some pre-existing fractures within the plate,” he said.

The 8.1-magnitude earthquake earlier this month was in an area where the Cocos Plate “gradually bends” as it is pulled under a thin layer of the North American plate. The 7.1-magnitude quake this week happened at an area with a sharper bend in the Cocos Plate which, again, at this point sits beneath the North American plate.

Given the epicentres were about 600km apart, Dr Hicks said it was unlikely the first quake directly led to the second.

“It’s possible there was a slight increase in the stress, but it wouldn’t have been the main factor causing the second quake,” he said.

Local factors mean Mexico City is especially badly affected by seismic events, as the city is built on an area once covered by a chain of lakes.

It is, said Professor Andreas Kappos, co-author of the book Earthquake-Resistant Concrete Structures, among the "very few cases" where a major city was built on "very soft alluvium", the sand, clay and silt deposits left by water.

Which buildings are most at risk depends, he said, on the frequency of the waves generated in the area by the earthquake. This, in turn, is determined by how far away the epicentre is. With a more distant epicentre, the waves are lower frequency and affect taller buildings more. A closer epicentre is linked to higher frequency waves, which are more likely to damage shorter buildings.

With this week’s quake, which had an epicentre about 120km from Mexico City, the frequency peaked at 0.3 to 0.6 seconds, which Prof Kappos said would affect buildings three to six storeys the most.

In 1985 the epicentre was further away at a distance of 400km, which resulted in waves with a peak frequency of two to three seconds, which made buildings of 20 to 30 storeys most vulnerable.

“If its a longer period event, it will affect the high-rise buildings,” said Prof Kappos, who is director of the Research Centre for Civil Engineering Structures at City, University of London.