Prof David Rothery, Professor of Planetary Geosciences, The Open University, said:
“The epicentre of yesterday’s magnitude 7.1 earthquake in Mexico was about 500 km from the magnitude 8.1 quake of 8 September. There is no direct connection between the two, although they are both consequences of the convergence of the Pacific floor (here belonging to the Cocos Plate) and the North American Plate on which Mexico sits. The plates don’t slip smoothly past each other. Instead the deformation builds up by flexing and bending until enough stress has been built up to release the blockage allowing many year’s worth of movement to occur in a few seconds. What happened yesterday was most likely a tearing motion in the subducting Cocos Plate. Fortunately this occurred deep down (about 50 km) so the shaking at ground level was not as bad as it might have been.
“On the other hand, parts of Mexico City are built on a former lake bed, where the nature of the soil tends to magnify the shaking, as happened in the 1985 earthquake (also 19 Sept).
“It is all too common for schools to collapse during earthquakes. The Enrique Rébsamen elementary school where many children died looks like a modern building, and ought to have had in-built earthquake resilience. Had it been properly constructed it should not have collapsed, and I expect questions will be asked about whether the appropriate building codes were adhered to.”