Public Earthquake Warning System for Mexico City
Mexico City's warning system began operation in 1991 making it the first public earthquake early warning system anywhere in the world.
The system uses seismometers along the Guerrero coastline to detect large earthquakes. Warnings are then broadcast electronically to Mexico City where alarms sound. Given the distance of the city from the earthquake source zone, warnings are usually more than one minute.
For a summary of the history of the Mexican system see
Espinosa-Aranda et al. 2009
Map showing geographic distribution of earthquakes, sensors, and Mexico City. From Espinosa-Aranda, 2011.