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(CNN)- Seismic alerts are not activated for every earthquake recorded in Mexico, as it depends on several factors. Let us explain it to you.
According to the Government of Mexico, the Mexican Seismic Alert System (SASMEX) is a network of sensors that, when detecting a strong earthquake, emit a signal that uses radio waves, alerting the cities in which it has coverage. It happens. Timing with a variable lead.
The National Autonomous University of Mexico (UNAM) states that seismic alerts provide warning 20 to 120 seconds before an earthquake occurs, depending on the distance from the epicenter and the first detection of seismic stations.
SASMEX is managed by the Center for Seismic Instrumentation and Registration and is composed of a network of 96 sensors installed Warrior, oaxaca, michoacan, Kolyma, Jalisco and Puebla, as well as 8 plants that received alerts: Mexico City, oaxaca, Acapulco, chilpancingo, Morelia, Puebla, Cuernavaca and Colima.
According to SASMAX, earthquakes are evaluated by three algorithms to determine the probable intensity of an earthquake and the system must evaluate earthquakes in more than two stations. In the event that earthquakes exceed pre-established warning limits in both, a seismic warning notice will be issued at the regional level.
There are two main factors to take into account: the estimated magnitude and the distance from the city where the alert will be activated.
seismic alert activation
The seismic alert is activated when an earthquake has a magnitude greater than 5 and is recorded no more than 170 kilometers away from the city to be alerted; In an earthquake of magnitude 6 whose epicenter is located more than 350 kilometers away and when the intensity of the earthquake is more than 5.5 and its epicenter is located less than 350 kilometers away.
Information received from sensors located near the earthquake site allows warnings to be automatically issued via radio waves that anticipate the effects of a strong earthquake. This happens because radio waves are faster than seismic waves.
“This alert seeks to protect the most densely populated areas, such as the Valley of Mexico, Oaxaca, Colima and Guadalajara, where the largest infrastructure is most vulnerable to earthquakes. This prevention device is not in place throughout the Republic because in some places, instead of warning, it can cause alarm, as there are areas where seismicity is low and tremors are almost invisible,” explains Victor Hugo Espíndola Castro, researcher at the National Seismological Institute. Service, in an article by UNAM.
Furthermore, academics point out that it is a myth that the system should be activated every time an earthquake occurs, since there are about 70 tremors per day in Mexico City alone and this does not mean that it should sound 70 times. He further added, “In CIRES they calculate the number of maximum accelerations of certain stations near the epicenter of the earthquake to determine whether it is necessary to trigger an alert or not.”
Researchers from the National Autonomous University of Mexico (UNAM) previously reported that, from December 3, 2023, to January 10, 2024, 23 earthquakes ranging in magnitude from 1.1 to 3.2 occurred in Magdalena municipalities. Contreras and Álvaro Obregón, with a depth of less than 1.4 kilometers. They blamed the earthquakes in that area of the country’s capital on the newly discovered Plateros-Mixcoac Fault, which the researchers said “has caused the accumulation of stress in the area and possibly even the collapse of the valley.” “Recharging the aquifers in Mexico and the Sierra de las Cruces that feed the metropolitan area”.
How are earthquakes measured?
The Mexican government explains that earthquakes are measured based on their intensity, which refers to the effect that a shock produces on the ground, buildings and humans. The value of intensity depends on the place where it is measured.
Also taken into account is intensity, which is a measure of the amount of energy released during a shock and, unlike intensity, has a globally shared scale.
For this reason the perception of earthquake can be variable. For example, the September 19, 2017 earthquake in Mexico City had a magnitude of 7.1, and its magnitude was greater than that of the 1985 earthquake, which was 8.1.
According to the UNAM bulletin, this is explained because the epicenter of the 1985 earthquake was much further away and under the coast of the state of Michoacán, more than 400 kilometers from the country’s capital, while the 7.1 occurred 120 kilometers south of Mexico City; Additionally, the rupture of the September 19, 2017 earthquake occurred within the Cocos Oceanic Plate.
When does the seismic alert not activate?
According to CIRES, an alert is not required when an earthquake occurs too far from SASMEX’s detection coverage area or When its energy estimate does not exceed the prescribed level.
Depending on the energy estimate and distance to the earthquake’s origin, alerts may be activated for some cities and not others.
Jonathan Arreola Manzano, deputy director of seismic risk at the National Center for Disaster Prevention (CENAPRED), explains in an article for UNAM: “The system is efficient when these events occur more than 400 kilometers away because it takes about 60 seconds. There will be time for the population to protect themselves and take appropriate precautions. If an earthquake falls below the warning point, it is technically not possible to hear it because seismic waves travel at a speed of four kilometers per second, so we can hear it approximately. Feel it only at the time of warning.
Manzano says that, if the algorithm does not predict that it is a potentially dangerous earthquake, or if the earthquake occurs in an area with no coverage, an alert will not be issued. “The further away the site you want to alert is, the more time you have to protect yourself; but the incident may not be known until it happens,” he explained.
