Don Blakeman, an analyst for the USGS, said the earthquake was felt strongly in
Mexico City because the epicentre was quite shallow and the ground under
the capital, built on a former lake bed can intensify shock waves.

Al Jazeera's Franc Contreras said that the quake's timing, coinciding with widespread fear over the spread of a deadly swine flu virus which is suspected of killing up to 149 people in the country, could not have been worse for authorities.

Many of those forced to leave buildings in Mexico City stood in the streets still wearing blue face masks in a bid to ward off infection.

Jose Angel Cordova, Mexico's health minister, was also in a news conference about the spread of swine flu when the earthquake hit.