Mexico City - Mexico City has updated its 911 emergency app to send earthquake alerts to residents' smartphones following September's magnitude 7.1 temblor that killed 369 people, including 228 in the capital, authorities announced last week.

Mayor Miguel Angel Mancera said users of the free 911 CDMX app can get sound and vibration alerts for any quake strong enough to threaten damage in the city.

It was developed by the governmental center known as C5, for Command, Control Computing, Communications and Contact, and is available for both iOS and Android.

With nerves still raw from the Sept. 19 earthquake that collapsed 38 buildings in Mexico City, Mancera said there would be no demonstration of the system to avoid causing unnecessary alarm.

"We are not interested in having anyone hear it who does not know the context in which it is being presented," the mayor said at a news conference.

More than 20 million people live in the capital and surrounding suburbs, much of which is built on a former lake bed. Its soil can amplify the effects of earthquakes that strike far away and whose shock waves arrive in the sprawling metropolis some time later.

An even stronger earthquake Sept. 7, whose magnitude was recently adjusted upward from 8.1 to 8.2 by the U.S. Geological Survey, was centered hundreds of miles away, off the country's southern coast, but was still felt strongly by many in Mexico City.

The capital already has a system of loudspeakers that blare alarms when a significant temblor is detected, but C5 general coordinator Idris Rodriguez Zapata urged residents to download the app. He also said they should heed quake protocols "without hesitation at the moment [the alarm] is heard through the system of speakers or on cell phones."

Last month, Mancera said there had been reports of people setting their cell phone intones to the sound of the seismic alarm and urged them to remove it so as not to provoke panic.

Other 911 app functions let users view tweets about seismic activity, contact a 911 operator, or register their blood type and medical history.