What are microseisms and why are they felt in Mexico City?

( Spanish) – In the last few hours, several low-intensity earthquakes, known as microseisms, have been recorded in Mexico City, which have been noticeable to the residents of the capital. The majority have had a magnitude of 1 or 2 with the epicenter in mayors of the capital such as Benito Juárez, Miguel Hidalgo and Álvaro Obregón, according to the National Seismological.

Microseisms can be perceived as small “jalones” that last a fraction of a second, says the Gazette of the National Autonomous University of Mexico; and, although they do not cause major damage to the population, they can damage some structures in the city.

“They are small ruptures inside the earth, close to the surface of the earth, in this case about 4 or 5 kilometers deep, and they are not unusual in the Valley of Mexico Basin region,” he said in 2019. Víctor Manuel Cruz, head of the Seismology department of the Institute of Geophysics, to the UNAM Global publicationfrom the National Autonomous University of Mexico.

Mexico is located in an area of ​​high seismicity where five tectonic plates interact: the North American plate, the Cocos plate, the Pacific plate, the Rivera plate and the Caribbean plate. Specifically, in Mexico City there are four notable geological faults: Mixhuca, Santa Catarina, San Lorenzo Tezonco and Copilco, according to a 2017 study by the UNAM Geosciences Center.

However, recently researchers from the Institute of Engineering reported that They located a geological fault in the area from Plateros to Mixcoac, in the west of the capital. According to the UNAM Gazette, a preliminary study indicates that this could be the cause of the telluric movements recorded in that region.

Delia Iresine Bello Segura, from the National Seismology, told Gaceta UNAM It is common for some geological faults to remain stable or “inactive” until they are rearranged by an earthquake. In addition, he explained that after the earthquake of September 19, 2017, the reactivation of some geological faults was detected.

Although it is not possible to predict when or where an earthquake will occur, Mexico has the Mexican Seismic Alert System (SASMEX) that warns the population of Mexico City, Puebla, Acapulco, Chilpancingo, Morelia, Oaxaca, Toluca and Morelos seconds before an earthquake is perceived, so that appropriate civil protection measures can be taken.

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.

That is why microseisms with an epicenter in the municipalities of Mexico City and a magnitude around 2 do not activate the alert.

Some disaster alert applications, like SkyAlertsend a notification to cell phones when they detect these microseisms, as they have monitoring systems independent of SASMEX.