Short answer. Mexico's Seismic Alert System (SASMEX), operated by CIRES, detects an earthquake that has already started and broadcasts a warning before strong shaking reaches distant cities 1. The government's Sistema Nacional de Alertas also sends alerts to mobile phones by cell broadcast, with no app, internet or credit needed: tested in Mexico City in April 2025 3, then nationwide on 19 September 2025 2.
What is SASMEX?
The Sistema de Alerta Sísmica Mexicano (SASMEX) is developed and operated by the Centro de Instrumentación y Registro Sísmico (CIRES). Its sensors sit mainly along the Pacific coast, from Jalisco to Chiapas 1. The principle, in CENAPRED's words: "information travels faster than seismic waves" 1. Once sensors detect an earthquake that has started, the warning is relayed before the strong shaking arrives in more distant cities.
How much warning will you get?
It depends mainly on your distance from the epicentre. Close to the epicentre, the shaking can be felt at the same time as the alert, or even before it 1. The alert is a head start, not a guarantee: practise what to do so you can act in seconds. Our earthquake preparedness guide explains the protective actions agencies recommend.
How does the alert reach you?
| Channel | How it works | Source |
|---|---|---|
| SASMEX receivers | In schools, offices and public spaces | 1 |
| Radio and television | Broadcast of the seismic alert | 1 |
| Street loudspeakers | In the states that have them; 11 states for the 19 September 2025 drill | 2 |
| Mobile phones (cell broadcast) | Text message with a distinctive sound and vibration | 23 |
The seismic alert has one distinctive sound 1. On phones, the message may display as "alerta máxima" or "alerta extrema" 2.
Do you need an app for phone alerts?
No. The phone alert "does not require internet, credit or installing apps: you only need cell signal and wireless alerts switched on" 2. The first nationwide test was announced for 19 September 2025 at 12:00 central time, aimed at more than 80 million active phones 2; an earlier test limited to Mexico City ran during the April 2025 drill 3.
If the alert did not reach your phone, the Digital Transformation and Telecommunications Agency (ATDT) says to check that emergency alerts are enabled in your settings, or to call the CABI line, 079 3.
What should you do when you hear the alert?
"If we hear the seismic alert, we must take it seriously and act" 1. Act even before you feel movement: stay calm, move away from windows and objects that can fall, protect yourself in the lowest-risk area and follow the authorities' instructions 1. Identify that area in advance, as part of your family plan: see our family plan guide.
When are the alerts tested?
During the National Drill (Simulacro Nacional), held each year and sometimes twice: in 2025, on 29 April and 19 September 23. The 19 September 2025 drill included the first nationwide phone alert 2.
Frequently asked questions
Does SASMEX warn of tsunamis or hurricanes?
SASMEX is a seismic alert system: it detects earthquakes that have already started 1. For coastal hazards, see our guides on tsunamis and hurricanes.
Why did I feel the earthquake before the alert?
Warning time depends on distance from the epicentre; close to it, shaking can arrive at the same time as the alert or before 1.
Will the phone alert work without mobile data?
Yes. Cell broadcast needs only cell signal and wireless alerts switched on 2.
Whom do I contact if my phone did not receive the test?
Check your alert settings or call 079 (CABI line) 3. Other numbers are on our Mexico emergency contacts page.
Sources
- Centro Nacional de Prevención de Desastres (CENAPRED) — ¿Cómo funciona la alerta sísmica? La ciencia detrás de esos segundos de prevención — https://www.gob.mx/cenapred/articulos/como-funciona-la-alerta-sismica-la-ciencia-detras-de-esos-segundos-de-prevencion (accessed 2026-10-20)
- Secretaría de Seguridad y Protección Ciudadana (SSPC) — SEGUNDO SIMULACRO NACIONAL 2025: SE REALIZARÁ PRIMER ALERTAMIENTO POR TELEFONÍA CELULAR — https://www.gob.mx/sspc/prensa/segundo-simulacro-nacional-2025-se-realizara-primer-alertamiento-por-telefonia-celular (accessed 2026-10-20)
- Agencia de Transformación Digital y Telecomunicaciones (ATDT) — Se activa alerta en teléfonos celulares durante el Simulacro Nacional — https://www.gob.mx/atdt/comunicacion/se-activa-alerta-en-telefonos-celulares-durante-el-simulacro-nacional (accessed 2026-10-20)
Turn this into action
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Sources
- Centro Nacional de Prevención de Desastres (CENAPRED) · ¿Cómo funciona la alerta sísmica? La ciencia detrás de esos segundos de prevención checked 2026-10-20
- Secretaría de Seguridad y Protección Ciudadana (SSPC) · SEGUNDO SIMULACRO NACIONAL 2025: SE REALIZARÁ PRIMER ALERTAMIENTO POR TELEFONÍA CELULAR checked 2026-10-20
- Agencia de Transformación Digital y Telecomunicaciones (ATDT) · Se activa alerta en teléfonos celulares durante el Simulacro Nacional checked 2026-10-20