It’s been nearly a year since a 7.1 Richter Scale earthquake hit Mexico City, but what does this actually mean? Many people instinctively use the Richter Scale to evaluate the strength of an earthquake. However, it can be argued that this logarithmic value offers little insight for a truly objective analysis on an earthquake’s impact. “The logarithmic magnitude scale … allows for comparison of earthquakes on relatively the same terms even though their impacts to society and structures … can be quite different,” says geologist, Robert Williams, in an article answering why the Richter Scale is so widely accepted. This suggests that a more pragmatic approach would assess how well the infrastructure withheld the natural disaster or the social impact in terms of human loss. According to an article by Proceso, last year’s earthquake was 10 times weaker than that of 1985. While this statement is scientifically correct, it is worth noting that the Richter Scale only refers to the amount of energy released at an epicenter.

MOVING AWAY FROM THE RICHTER

The distance between Mexico City to the earthquake’s epicenter was 160km in 2017 compared to 400km in 1985. This means that 2017’s epicenter was located 60 percent closer to Mexico City than that of 1985. Bear in mind that:

And that:
acceleration

By analyzing data provided by UNAM, 2017’s earthquake registered a maximum acceleration of 58.89cm/s2 while the maximum acceleration for 1985’s earthquake was 32.58cm/s2. In other words, 2017’s maximum acceleration was nearly double that of 1985.

 

Acceleration is relevant when assessing the structural resistance of any given construction because according to Newton’s second law, the acceleration of an object is directly proportional to the net force acting on it. Hence, a greater acceleration implies a greater force was exerted over Mexico’s City’s buildings in the latest earthquake.

force

If an average 300m², 50-story building has a mass of 250,000 tons, we can exemplify the force required for both earthquakes to exert said acceleration on a skyscraper through the following equations:

 

  • To put these units into perspective, a Space Shuttle Main Engine generates a force of 1.8 megaNewtons at lift-off.

space-shuttle

 

MEXICO’S CONSTRUCTION EFFICIENCY

When referring to the 7.8 Richter Scale earthquake that devastated Nepal in 2015, Hans Rosling, late Chairman of the Gapminder Foundation, explained how the death rate is always higher when a disaster hits a developing country as a result of poorly constructed buildings, underdeveloped infrastructure and poor medical facilities. Even though it is tempting to apply this logic to the Mexican context, the figures tell a different story.

 

Despite the significantly larger force, only 40 buildings were damaged during Mexico’s 2017’s earthquake compared to the 252 buildings that collapsed in 1985. Approximately 370 people died in 2017, while the Red Cross estimates that the death toll in 1985 was around 15,000. The number of victims in 2017 represents only 2 percent of the estimated deaths in 1985.

 

steel-reinforced-concrete

By Jamain [GFDL or CC BY-SA 3.0], from Wikimedia Commons

This can mainly be attributed to the fact that today, civil engineers are aware of the challenges and implications of building in Mexico City. In an interview with Mexico Infrastructure & Sustainability Review, Gabriel Santana, Commercial Director of ITISA Prefabricados, discussed the difficulty in designing and building prefabricated housing facilities en masse because some cities like Mexico City are made up of three seismic zones with distinct needs and specifications for earthquake resistance.

Since September 19, the citizens of Mexico have become much more aware of the importance of infrastructure that can withstand earthquakes. After the 1985 quake, the evidence shows that changes in construction regulations to avoid another disaster of this magnitude may just have worked, evidenced by the drastic reduction in the number of compromised buildings, despite the increased force of the earthquake. “Among the introduced changes, constructions now use steel-reinforced concrete, load distribution in their structural design in addition to better designed evacuation routes,” explains an article by Expansión.  As we approach the one-year anniversary of a disaster that could have claimed many more lives, it is clear that, although regulations must continue evolving, Mexico City is not unprotected.

 

SEPTEMBER 19 BONUS FACT

As September 19 draws nearer, many civilians fear an earthquake may hit again. While it is true that two strong earthquakes happened on the same date 32 years apart, the correlation between dates is a clear example of what statisticians call a gambler’s fallacy. This idea refers to a mistake made in gambling in which people believe that a random event is less or more likely to happen given a previous event. “Our brains do a really bad job of interpreting data,” says Harvey Motulsky, Founder of GraphPad Software. “We see patterns in random data, and tend to be overconfident in our conclusions.” Essentially, probability does not have memory.

 

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