Plate ... until it bends

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 600 kilometres 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, because the city is built on an area once covered by a chain of lakes.

It is, said Prof 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 120 kilometres from Mexico City, the frequency peaked at 0.3 to 0.6 seconds, which Prof Kappos said would most affect buildings of three-to-six storeys.

In 1985 the epicentre was farther 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 it’s 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.