The testimony of the mother of a victim of the 2017 earthquake in Mexico 3:46

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In Mexico City, earthquakes have been recorded in which the seismic alert has not sounded and some residents wonder why. The truth is that this alert is not activated with each tremor, but rather depends on multiple variables. We 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, emits a signal that uses radio waves, which alerts the cities in which it has coverage. with a variable lead time.

The National Autonomous University of Mexico (UNAM) points out that the seismic alert provides warning between 20 and up to 120 seconds before the arrival of an earthquake, depending on the distance from the epicenter and the first detections of the seismic stations.

SASMEX is managed by the Seismic Instrumentation and Registration Center and is made up of a network of 96 sensors installed in Guerrero, Oaxaca, Michoacán, Colima, Jalisco and Puebla, as well as 8 centers that receive the alert: Mexico City, Oaxaca, Acapulco, Chilpancingo, Morelia, Puebla, Cuernavaca and Colima.

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According to SASMEX, the earthquake is evaluated by up to three algorithms to determine its possible magnitude and the system must evaluate the earthquake in more than two stations. In the event that the tremor exceeds the pre-established warning threshold in both, a Seismic Alert notice will be issued regionally.

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There are two main factors to take into account: the estimated magnitude and the distance from the city where the alert would be activated.

Seismic alert activation

The seismic alert is activated when the magnitude of the earthquake is greater than 5 and is recorded no more than 170 kilometers from the city to be alerted; in a magnitude 6 earthquake with an epicenter located more than 350 kilometers away and when the magnitude of the earthquake is greater than 5.5 and its epicenter is less than 350 kilometers away.

The information from the sensors near the site of the earthquake allows the automatic issuance of alert warnings via radio wave that anticipate the effects of a strong earthquake. This occurs because radio waves are faster than seismic waves.

“This alert seeks to protect the most densely populated areas with the greatest infrastructure vulnerable to tremors, such as the Valley of Mexico, Oaxaca, Colima and Guadalajara. This prevention tool is not in the entire Republic because in certain places, instead of alerting, it could cause alarm, since there are localities where seismicity is low and the tremors are almost imperceptible," explains Víctor Hugo Espíndola Castro, researcher at the National Seismological Service, in an article from UNAM.

Furthermore, the academic points out that it is a myth that this system must be activated every time there is an earthquake, since in Mexico City alone there are about 70 tremors daily and that does not mean it should ring 70 times. "What they do at CIRES is to calculate the number of maximum accelerations of certain stations near the epicenter to make sure if it is necessary to detonate the alert," he adds.

Researchers from the National Autonomous University of Mexico (UNAM) previously reported that, from December 3, 2023 to January 10, 2024, 23 earthquakes with magnitudes ranging from 1.1 to 3.2 occurred in the Magdalena municipalities. Contreras and Álvaro Obregón, with depths less than 1.4 kilometers. They attributed the earthquakes in that area of ​​the country's capital to the newly discovered Plateros-Mixcoac fault, which, the researchers explained, is due to "the accumulation of tension in the region, and possibly also to the subsidence of the Valley of Mexico and the recharge of the aquifer in the Sierra de las Cruces, which feeds the metropolitan area".

How are earthquakes measured?

The Government of Mexico points out that earthquakes are measured based on their intensity, which refers to the effect that a tremor produces on the ground, buildings and humans. The value of the intensity depends on the place where it is measured.

Its magnitude is also taken into account, which is a measure of the amount of energy released during the tremor and, unlike intensity, has a globally shared scale.

For this reason the perception of earthquakes can be variable. For example, the earthquake of September 19, 2017 in Mexico City was of magnitude 7.1, and its perception was higher than that of the 1985 earthquake, which was 8.1.

According to a UNAM bulletin, this is explained because in 1985 the epicenter was very distant 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 away. kilometers south of Mexico City; Furthermore, the rupture of the September 19, 2017 earthquake occurred within the Cocos oceanic plate.

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When is the seismic alert not activated?

According to CIRES, an earthquake does not warrant an alert when it occurs far from the SASMEX detection coverage area or when its energy estimates do not exceed defined levels.

An alert can be activated for certain cities and not for others, depending on the energy estimate and the distance where the earthquake originates.

Jonatán Arreola Manzano, deputy director of Seismic Risk at the National Center for Disaster Prevention (CENAPRED), explains in an article from UNAM: “The system is efficient when these phenomena occur more than 400 kilometers away because there will be around 60 seconds for the population to protect itself and take appropriate precautions. If the earthquake occurs below the point to be alerted, technologically it is not possible for it to sound because seismic waves travel at four kilometers per second, therefore we feel it almost at the same time as the alert.”

Manzano adds that, if the algorithm does not estimate that it is a potentially dangerous earthquake, or if the tremor occurs in an area without coverage, the alert will not be issued. "The further away the site that you want to alert is, the more time there will be to safeguard ourselves; but that is not known until the event occurs," he details.

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