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Mexico City sits on an earthquake time bomb

Hundreds have died and many more were injured in the 7.1 magnitude earthquake that hit Mexico City on Tuesday, September 19. The terrible irony for the residents is that it struck 32 years to the day after the deadly earthquake of 1985 which killed more than five thousand.

The epicentre in the latest quake was located 37km from Puebla, a city in the south central region, according to the US Geological Survey, and around 80km south of Mexico City. The area is one of the most active geological zones in the world, which is bad news for the nearly 9-million inhabitants of the Mexican capital.

Disaster relief teams are on the ground trying to pull people from the rubble while reports of the extent of the destruction begins to pour in.

The region has been hit by many earthquakes over the past 100 years owing to an unfortunate geological characteristic.

Lying to the East, around 350km southwest of Mexico City, is the Acapulco Trench, where the Cocos Plate slides below the North American Plate. And directly west is the Rivera Plate, which is also sliding beneath the North American Plate. Both of these plate interactive zones lead directly to volcanic and earthquake activity.

The main reason for massive damage in the city area is the geological structure of the Mexico Basin where earthquake waves, which emanate from the Trench, move rapidly through the Mexican volcanic belt. When they reach Mexico City they are trapped in a basin-shaped formation, which then causes the waves to deflect back and forth.

Worse still, most of Mexico City is based on clay-filled soils, which liquefy when shaken in an earthquake. This means building foundations begin to shift and even sink, causing buildings to crack and crumble.

The September 19 1985 magnitude 8.1 quake saw these waves radiate back and forth in the Mexico City basin, destroying parts of the city in a four-minute period. Low-rise buildings and those built in the 1700s were only slightly damaged, although some slid into the liquefied clay.

Five thousand people died in that earthquake and despite warnings, city officials have allowed multi-storey apartment blocks to be built again. Citizens in Mexico City say they will query whether these buildings conformed to the earthquake construction policy.

Less than two weeks before the latest quake, a magnitude 8.4 earthquake was reported off Mexico's southern coast, which President Enrique Peña Nieto said was the most powerful in 100 years. It was reported off shore and in spite of tsunami warnings, the waves generated were small.

There are reports of fires and explosions from parts of Mexico City as gas lines were ruptured. Water and sewerage facilities have also been damaged and international relief efforts have begun in order to send aid to the country.


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