New Delhi, Sept. 20 -- Every Sept. 19, residents of Mexico City ask themselves an unsettling question: "Is the ground shaking?"

On that day 40 years ago, at 7:19 a.m., a 8.1-magnitude earthquake and its aftershocks left the Mexican capital devastated. Official counts put the death toll around 12,000, but the real number remains unknown.

The earthquake was a watershed moment for the city. A new culture of civil defense evolved, better warning systems developed, building codes changed and, since 2004, there have been annual earthquake drills held on that day.

Then, on that very same day in 2017, things changed again. Barely two hours after the annual drill, a 7.1-magnitude temblor began shaking the ground; its epicenter was so close to t...