Mexico earthquake

The extent of damage and the death toll from thoday’s 7.1 earthquake in Mexico won’t become clear for a day or two, but it looks very bad, with widespread damage reported and over 200 deaths recorded so far. With many collapsed buildings it will get worse.

Guardian:  At least 217 dead after powerful earthquake hits central Mexico

The 7.1 magnitude earthquake is deadliest to hit country in more than 30 years and has brought down buildings in the capital, Mexico City

Emergency crews and ordinary people are digging through rubble with their bare hands in search of trapped survivors after a powerful earthquake stuck central Mexico on Tuesday afternoon, toppling dozens of buildings and killing at least 217 people

The magnitude 7.1 quake – the deadliest to hit the nation since 1985 – struck shortly after 1pm local time, causing violent, prolonged shaking which flattened buildings and sent masonry tumbling onto streets, crushing cars and people in the capital, Mexico City, and surrounding areas.

CNN Seismologist: What caused Mexico’s latest earthquake

Plate tectonics was the engine behind the shaking, as is true for all major earthquakes. Along the coast of Mexico, the Cocos Plate slides underneath the North American Plate, moving about three inches per year. Tuesday’s earthquake, however, was caused by crumpling arising from the downward bending of the sinking Cocos Plate, rather than directly by slippage between plates.

A similarly deep but much larger magnitude 8.1-earthquake struck two weeks ago, also from the crumpling of the Cocos Plate. It struck Mexico 400 miles to the southeast and offshore, not far from Guatemala, and killed dozens of people.

In the history of Mexican earthquakes, Tuesday’s was coincidentally the 32nd anniversary of the 1985 Michoacan earthquake, which resulted in thousands of deaths. The 1985 earthquake was the more typical great earthquake that breaks the boundary between the plates, and caused great devastation in Mexico City despite being more than 100 miles distant.

Mexico City’s downtown area is notoriously vulnerable to earthquakes because of the very soft and wet ground underneath. Its soil amplifies shaking like Jell-O on a plate, and is prone to liquefaction, which is the ability to transform dirt into a dense liquid when sufficiently churned. In the 1985 earthquake, many large buildings were destroyed, and Tuesday’s quake was another blow mostly to the older, less solidly constructed structures.

We’ve learnt a lot about building on soft ground and the risks of liquefaction after Christchurch.

 

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