Mexico City has made changes to its emergency announcement service known as 911 CDMX.
The service now sends messages to people's smartphones warning them of earthquakes.
The changes come after a magnitude 7.1 earthquake hit Mexico's capital on September 19. The quake killed 369 people and caused 38 buildings to collapse.
Officials announced the new smartphone program last week. Mayor Miguel Angel Mancera said users of the free 911 CDMX program will receive messages for any earthquake strong enough to damage the city.
The program will send sound messages and cause the smartphone to vibrate.
The governmental center known as C5, for Command, Control Computing, Communications and Contact developed the program. It is available for both iOS and Android smartphone operating systems.
Mancera said at a news conference there would be no demonstration of the system to avoid causing any unnecessary worry.
"We are not interested in having anyone hear it who does not know the context in which it is being presented," he said.
More than 20 million people live in the capital and surrounding area. Many buildings there are built on a former lake bottom. Its soil can increase the effects of earthquakes which are centered far away. Shockwaves from such earthquakes may arrive at the city later.
Early in September, an earthquake that was stronger than the one that hit Mexico City was centered hundreds of miles away. The United States Geological survey recently changed the record of its magnitude from 8.1 to 8.2. And many people in the capital felt it strongly.
Mexico City already has a system of loudspeakers that give out a warning whenever a serious earthquake is detected.
Idris Rodriguez Zapata is the general coordinator for C5. He urged residents to put the new 911 CDMX program on their smartphones. He also said they should pay attention to earthquake instructions at the moment they hear announcements on the loudspeakers or their phones.
Mancera said there had been reports of people setting their smartphone ringtones to the sound of the new earthquake warning. He urged them not do so to avoid causing worry.
The 911 CDMX program has other uses. It lets users see messages about earthquake activity on the website Twitter. It also can put them in contact with an emergency worker and lets them register their blood type and medical history.
I'm Pete Musto.