Public earthquake early warning systems already exist in Japan and Mexico City.
Earthquake Early Warning in Japan
In October 2007 Japan turned on the first publicly available nationwide earthquake early warning system. On March 11, 2011 it had its first true test during the M9 Tohoku earthquake off the coast of Sendai. The system issued a warning before the S-wave arrived onshore, a great achievement for the seismological community in Japan.
More about the video above
The user is in Sendai (the closest major city) so the countdown is until the S-wave arrival in Sendai. The radiating circles are the P-wave (blue) which does little damage while providing the information upon which the warning is based, and the S-wave (red) representing the onset of strong shaking. This comes from the Real-time Earthquake Information Consortium. In the coastal areas close to the epicenter there was 15 sec warning prior to the strong shaking.
More about Japan's earthquake early warning system
Japan's nationwide public warning system now makes use of 1100 seismic stations operated by the Japan Meteorological Agency (JMA) and the National Research Institute for Earth Science and Disaster Prevention (NIED). The JMA-operated system issues alerts via TV, radio, cell phones and a plethora of other specialty devices developed by the private sector.
Earthquake early warning in Mexico
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.