Mexico sits atop several intersecting tectonic plates. Along the Pacific coast, the Cocos plate dives beneath the North American plate in a subduction zone that has generated some of the hemisphere's most powerful earthquakes. Mexico City's particular vulnerability is geological: the capital was built on the drained bed of ancient Lake Texcoco, where loose sediments slow incoming shockwaves from about 2.4 kilometers per second to roughly 45 meters per second. That deceleration amplifies the waves' height, causing far more violent shaking than the same quake would produce on solid rock. Twelve days before September 19, the even larger 2017 Chiapas earthquake — magnitude 8.2 — had struck 650 kilometers away. Scientists investigated whether the two events were linked through static stress transfer, but the distance between them exceeded the roughly 400-kilometer threshold for such a connection.

Mexico operates one of the world's most sophisticated seismic early warning systems, SASMEX, which detects earthquakes near the coast and transmits alerts to Mexico City before the shaking arrives. On September 19, the system failed to provide advance warning. The epicenter was too close. Unlike coastal subduction earthquakes that originate hundreds of kilometers from the capital, this quake began only 120 kilometers away, deep within the subducted Cocos plate. The seismic waves reached the city before the alert could outrun them. Residents reported that the alarm sounded only after the shaking had already begun — or not at all. The coincidence with the anniversary drill added a surreal quality to those first moments: people who had just rehearsed an evacuation suddenly found themselves in the real emergency, unsure at first whether the shaking was part of the exercise.

Within hours, thousands of volunteers flooded into collapsed neighborhoods. Images from Colonia Roma, Colonia Obrera, and Colonia del Valle showed ordinary citizens forming human chains to clear debris with their bare hands. Rescue teams worked through the night at the Colegio Enrique Rebsamen, a private school in southern Mexico City where the building's collapse killed 26 people, including 19 children. The owner, Monica Garcia Villegas, was later found guilty of culpable homicide for ignoring safety regulations and sentenced by a court in September 2020. Across the country, more than 6,000 people were injured. International rescue teams from Israel, Japan, the United States, and elsewhere joined the effort. The casualties included eight foreign nationals: four Taiwanese women, a Korean man, a Spanish citizen, and others working or studying in the capital.

An investigation published in October 2017 revealed that since 2012, more than 6,000 complaints about construction violations had been filed in Mexico City, with no public record of follow-up. Many of the buildings named in those complaints collapsed on September 19. The Urban Development and Housing Secretariat did not respond to information requests after the earthquake. Local activists described the construction regulatory system as 'totally corrupt,' alleging that developers routinely circumvented building codes and city authorities frequently ignored reports. The earthquake exposed a gap between Mexico's seismic engineering knowledge — among the most advanced in Latin America, forged in the aftermath of 1985 — and the enforcement mechanisms meant to turn that knowledge into safe buildings. The regulations existed. The will to enforce them had not.

In Jojutla, Morelos, near the epicenter, 652 homes were destroyed and 1,157 damaged, along with schools and the Palacio Municipal. Two years later, in January 2020, residents were still waiting for reconstruction to begin. By January 2021, more than three years after the quake, Mexico City's reconstruction commissioner reported that only 53 percent of the 13,945 damaged buildings had initiated rebuilding. Work on another 30 percent had not yet started, and plans for 4,601 homes remained undefined. Fifty-nine buildings had been demolished; eleven more were scheduled for demolition but untouched. The government budgeted 5.3 billion Mexican pesos for reconstruction from January 2019 through December 2020 and spent 3.8 billion. The numbers tell a story of a city where the earthquake's aftermath lasted far longer than its twenty seconds of shaking.