Just when we were in the final celebrations of Mother’s Day, a 3 magnitude earthquake with epicenter in Mexico City without seismic alert. To be specific, in the Magdalena Contreras mayor’s office with a depth of 1 kilometer.
The thing is that we Chilangos feel it intensely and the seismic alert did not go off… why? is it a glitch? Not really, but let’s go in parts.
Why didn’t the seismic alert sound with the CDMX tremor?
Not long ago we visited the Seismic Recording and Instrumentation Center (CHERRY TREE), the center that created, operates and maintains the Seismic Alert System in Mexico.
The director, engineer Juan Manuel Espinosa Aranda, explained to us what the conditions are for the Mexico City seismic alert to go off. During the first seconds of the earthquake, sensors estimate its possible magnitude and the information is communicated at the speed of the újule to determine if it represents a threat. The alert will turn on depending on three factors:
- If in the first seconds of the earthquake at least 2 stations exceed preset energy levels (was not fulfilled)
- Depending on the estimated energy of the earthquake
- Depending on the estimated magnitude of the earthquake and the distance in which the city is to alert (since the epicenter was in CDMX, there is no distance in the city to alert)
At this last point, for example, it will be activated when the magnitude is greater than 5 and that the city to be alerted is no more than 170 kilometers from the epicenter. Similarly, when the magnitude is greater than 6 and the city is more than 350 kilometers and when the magnitude is greater than 5.5 and occurs at no more than 350 kilometers.
So, when the epicenter is in Mexico City, the seismic alert system will not turn on because there is no opportunity to alert.
To begin with, it was not an earthquake with a high magnitude (greater than 5.5), so the pre-established energy levels were not exceeded. And then, the epicenter was CDMX, it is impossible for the alert to go off in the same city where the epicenter is.
Because? The idea of the seismic alert is to detect a dangerous earthquake at a certain distance from a city and to warn it with advantage. The lead time is the time it takes for the movement to reach the city where they are fired. If the epicenter is in CDMX, the movement is already there, it does not travel, there is no advantage time and there is no warning opportunity.
“But it did feel very strong!” Yes, because the epicenter was in Mexico City, in the same city and it was very shallow, with a depth of only 1 kilometer.
