Mexico City remembers quake 25 years ago: 10,000 dead (Feature)

By Felix Meschede Sep 16, 2010, 3:32 GMT

Mexico City - The earth shook under Mexico City in the early morning of September 19, 1985.

It happened fast. Within a few seconds some 450 buildings in the capital collapsed as if built of cards. Thousands of homes were damaged by the quake.

Close to 10,000 people lost their lives in the rubble, according to official figures. Humanitarian organizations say the actual death toll was much higher.

The quake, measuring 8.1 on the Richter scale, had its epicentre around 350 kilometres away from the city, in the southwestern Mexican state of Michoacan.

'Usually, quakes that far away have less devastating effects,' says Carlos Valdes Gonzalez, director of the National Quake Service at the National Autonomous University of Mexico (UNAM). 'And yet on that day it was the capital which suffered the worst damage in the country.'

The reasons for this reach back in history and far underground. In the 14th century the Aztecs laid the foundations for the current Mexican capital on an island amidst a large lake and marsh landscape. The Spanish occupiers, after defeating the indigenous communities, drained the water and built up the areas.

When the quake it, the sandy sediment became so unstable with the vibration that the foundations of many buildings just sagged. This chain of unfortunate circumstances would later come to be known as the Mexico City effect, that is, the shocks of the 1985 quake were virtually unhindered by the loose and humid sediment under the capital.

'The several-storey buildings of the 1960s and 1970s were particularly affected,' says architect Jose Avila Mendez, who has studied the effects of the quake at the UNAM. 'The vibrations carried up the tall buildings, and the resonance eventually caused them to collapse.'

Many city centre historic buildings from colonial times were left askew, but their interlocked and relatively simple construction left them better equipped for the quake than the high- rise buildings of the newer neighbourhoods.

A pleasant surprise came from the 45-floor Torre Latinoamericana, only a few blocks away from the cathedral, which withstood the quake. It had already braved a quake with little damage soon before it opened in 1956, a situation which earned the building praise from architects.

'Its construction style was an example for the buildings that went up after 1985,' says Avila Mendez. 'Nowadays, the damage in the old city would be a lot less.'

The list of possible preventive measures is however limited, and ideas about drying up the humid sandy underbelly were rejected.

'The ground would only sag even further,' says Valdes Gonzalez. 'In any case the city sinks 30 centimetres per year.'

The quake risks in Mexico City remain high. Along the Americas' Pacific coast there are quakes almost on a daily basis. Tectonic plate movement along the 16,000-kilometre-long break line regularly scare people from San Francisco to Santiago de Chile.

For quake experts, the west coast of the Americas is one of the world's most tectonically-active regions.

'But these quakes are usually harmless for people,' Valdes Gonzalez stresses.

They happen out at sea and have relatively low magnitudes on the Richter scale, he adds.

In fact, it was fortunate that the 1985 quake in Mexico City happened early in the morning, a little after 7 am. Many people were on their way to work, and office towers collapsed while they were largely empty.

Had the quake happened an hour later, the catastrophe would have been even worse.