Earthquakes in Mexico City

Mexico City faces a very high earthquake risk, made especially dangerous by the ground it is built on. While the largest earthquakes are generated hundreds of kilometers away along the Pacific coast, where the Cocos plate dives beneath the North American plate, the capital sits on the soft clay sediments of a drained ancient lake, Lake Texcoco. These waterlogged sediments dramatically amplify and prolong seismic waves, so distant quakes that would be modest elsewhere can shake the city violently. This unusual geology means Mexico City can suffer severe damage even from earthquakes whose epicentres lie far from the metropolis.

The defining disaster was the 1985 Michoacan earthquake, a magnitude 8.0 event off the coast some 350 kilometers away. Amplified by the lakebed, it devastated central Mexico City, collapsing hundreds of buildings and killing an estimated 5,000 to 10,000 or more people, one of the deadliest quakes in the Americas. Tragically, on the 32nd anniversary in September 2017, a magnitude 7.1 quake nearer the city struck again, killing hundreds and collapsing dozens of buildings. Since 1985 the city has adopted much stricter building codes and operates a seismic early-warning system that can give tens of seconds of notice, but its soft-soil foundation and dense population keep the earthquake threat among the most serious of any major world capital.

Mexico City's soft lakebed clay is the crucial variable: seismologists estimate the sediments can amplify shaking by a factor of five to fifty compared with solid bedrock, an effect nicknamed the "jello effect." That is why the 1985 Michoacan earthquake, whose epicentre lay roughly 350 km away on the Pacific coast, still collapsed hundreds of buildings and killed an estimated 5,000 to 10,000 people or more in the capital. In response, Mexico built SASMEX, the Sistema de Alerta Sísmica Mexicano, the world's first public earthquake early-warning network. Sensors along the Pacific subduction zone detect fast-moving P-waves and broadcast an alert to inland cities, often giving Mexico City residents 30 to 60 seconds of warning before the slower, destructive S-waves hit.

That warning window does not cover every threat. The magnitude 7.1 earthquake of 19 September 2017 struck much closer to the capital, an intraslab rupture inside the subducting Cocos plate beneath Puebla state, and its alert activated only seconds before shaking began, too late to help. It killed hundreds and collapsed dozens of buildings, arriving on the exact 32nd anniversary of the 1985 disaster. Since then, Mexico City has tightened building codes further and expanded its network of public speakers, but the lakebed geology means even moderate, distant earthquakes can still produce dangerous local shaking.