Major Earthquake, Central Mexico

A major earthquake struck central Mexico early this morning, New Zealand time, causing widespread damage and deaths in several Mexican provinces.

Wednesday, 20th September 2017

A magnitude 7.1 earthquake struck central Mexico at 6:15 this morning, New Zealand time. The earthquake was centred 4.5 km east-north-east of the town of Raboso at a depth of 51 km.

The earthquake has had devastating consequences for central Mexico and occurred 32 years to the day after a magnitude 8.0 earthquake killed tens of thousands in central Mexico in 1985. Many residents of Mexico had participated in earthquake drills earlier in their day to remember and plan for the consequences of another 1985 quake occurring in this area.

Today’s earthquake struck at 13:15 local time in Mexico, centred 4.5 km east-north-east of the town of Raboso at a depth of 51km. The epicentre was close to major population centres in Mexico, being 55 km south-south-west of Puebla (population 1.5 million) and 120 km south-east of Mexico City (population 22 million).

There has been considerable loss of life, but not as serious as in the 1985 event. Early estimates put the death toll at more than 150. The highest death toll seems to have been in the province of Morelos where 64 deaths have been reported.

In Mexico City, it is thought that 44 buildings have been severely damaged or destroyed.

After the 1985 earthquake, Mexico put in place an early warning system to try to limit the deaths caused by these large earthquakes. Mexico City has been built on the bed of an ancient lake which amplifies the shaking of large earthquakes and, with its population density, relies on early warning for its residents to take appropriate action when a large earthquake occurs.

Today the early warning system, SASMEX, would have had about 13 seconds to detect and issue a warning for the citizens of Mexico City. It is clear from video evidence that SASMEX alarms were sounding in Mexico City just seconds before the shaking began today, giving some residents the chance to move away from dangerous structures if they were in the open, or take shelter inside buildings.

Two weeks ago, on September 12th, Mexico City had two minutes’ warning that shaking would occur from the magnitude 8.1 earthquake off the coast of Oaxaca. Today, that warning would have been just seconds.

Nevertheless, video shows that some residents were able to take evasive action if they heard the SASMEX alert in time.

This first video shows people in the open in Mexico City as the SASMEX alert sounds. They were able to record traffic stopping and the shaking beginning – the stresses on the eight-storey building during the quake are staggering. But it didn’t collapse.

This video taken in a Mexico City call centre shows a television screen relaying the SASMEX alert on a television screen as the office workers scramble – they only had seconds to react to the warning.

This video is a composite of images, but it shows how violent the ground movement was in Mexico City. It shows the near collapse of a building in the earthquake, with masonry peeling off its façade, and one of the gas leaks in the city. Again, residents had only seconds to react to the SASMEX warning siren to record these events.

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