The City has been the epicenter of 191 microseisms in 10 years. The reason? #What we know

In the last decade, according to data from the Earthquake Catalog, between January 2013 and this week, there have been 191 telluric movements with an epicenter in Mexico City. The strongest was felt on July 27, 2021, when an earthquake of magnitude 3.8 was registered six kilometers south of San Andrés Mixquic at 10:35 in the morning.

Mexico City, February 11 (However).– The Mexico City has been accustomed for several generations to the habitual earthquakes that shake houses, buildings and neighborhoods in the capital, but now the citizens have experienced them in a particular way: they are telluric movements who have their epicenter in the same cityand not that they come from other states.

In the last year, between January 2022 and February 2023, the National Seismological Service (SSN) has registered 31 of these so-called “microseisms”, due to their low magnitude, although always with an epicenter in some area of ​​the city. The largest in that period was that of August 23 of last year, of 3.2 magnitude, southwest of San Pablo Oztotepec. The mildest, just 0.3 magnitude, occurred on February 22, also last year, five kilometers southwest of San Lorenzo Acopilco.

Of those 31 movements, three have been registered recently: the first occurred on January 29, with magnitude 1.3, and was perceived by citizens who turned to social networks to share the facts. According to the SSN, the epicenter was located three kilometers southwest of the Benito Juárez City Hall in the capital, at 12:49 p.m. that Sunday.

The second was registered on Tuesday, February 7, at the Coyoacán Mayor’s Office, in the south of the CdMx. It was a magnitude 1.5 earthquake at 8:46 a.m., located just three kilometers southwest of the demarcation. Finally, on Wednesday, February 8, the SSN detected a magnitude 1.2 earthquake in the Álvaro Obregón Mayor’s Office.

“Although the magnitude of the earthquakes is low, they were felt in various areas of Mexico City due to the proximity to the place of the epicenter and its shallow depth. We must remember that the closer to the epicenter, the seismic waves are less attenuated and the earthquake feels stronger, as was the case with these events,” the SSN explained in a report published this week.

“In seismological terms, microseism is not used, but it is a colloquial word that is understood and can be used that way, there is no major problem,” he explained to However Víctor Hugo Espíndola, head of Analysis of the SSN.

SEISMIC ACTIVITY IS OF LOW MAGNITUDE AND ACTIVITY

The Seismological explains that the seismic activity of the Basin of Mexico and its vicinity “is few in number and of low magnitude.” However, “several earthquakes are recorded per year” and there are indications that “most of the local seismic activity is concentrated on the margins of the Basin of Mexico”, although with historical exceptions, such as a movement of close magnitude at 6.9 in 1912, 111 years ago, in the municipality of Acambay, State of Mexico, about 100 kilometers northwest of the CdMx.

The particularities of the Basin of Mexico, where the CdMx is located, is that it is located on one of the most important physiographic features of the country, details the SSN, referring to the so-called “Trans-Mexican Volcanic Belt” (FVTM), the which extends from the Gulf of Mexico to the Pacific Ocean from east to west and is one of the most important topographical prominences in Mexico. “In the FVTM there are buildings and volcanic remnants among which are the highest peaks and the most active volcanoes in Mexico,” the Service completes.

“The Basin of Mexico is located in the central part of the FVTM and is completely surrounded by mountains dominated by the most important volcanic edifices such as: Popocatépetl (currently active), Iztaccíhuatl, Ajusco, and in the State of Mexico, the Nevado de Toluca”, he adds.

In the last decade, according to data from the Earthquake Catalog, between January 2013 and this week, there have been 191 telluric movements with an epicenter in Mexico City. The strongest was felt on July 27, 2021, when an earthquake of magnitude 3.8 was registered six kilometers south of San Andrés Mixquic at 10:35 in the morning.

Of the total of 191, up to 13 earthquakes greater than 3.0 of magnitude have shaken the Mexican capital between 2018 and 2022. Among the 10 least powerful they have varied from 0.3 to 1 of magnitude, and almost all of them occurred in 2022 or 2023, with the exception of three that occurred in 2019.

WHY DO THESE EARTHQUAKES HAPPEN?

According to the SSN, this type of microseisms in the city presumably occur “due to the reactivation of old faults.” “It is also considered that these events may occur as a result of the accumulation of regional tension or that the subsidence of the Valley of Mexico could cause tensions that, although they do not properly generate earthquakes, could trigger them,” the Service indicated.

In addition, another hypothesis cited by the SSN is that “the large earthquakes generated on the coast could give rise to unbalanced conditions and trigger local earthquakes.” “The Basin of Mexico has complex geology and tectonics, so it is not surprising that small earthquakes occur in the area,” they conclude.

The Service mentions recent studies of seismicity in the south of the CdMx. A 2007 study that cataloged local earthquakes with epicenters in the Basin of Mexico recorded 218 movements with magnitudes between 0.8 and 4.4, the latter being the largest calculated for earthquakes in this area.

“For his part, Bello Segura (2013) analyzed the mechanisms of earthquakes that occurred in the Basin of Mexico between 2008 and 2012 and reports normal-type mechanisms and some compounds. The tendency in the course of the mechanisms is varied depending on the region, the depths on average are eight kilometers, which indicates the existence of shallow faults, which is important from the point of view of seismic risk”, explained the SSN.

In addition to the earthquakes registered this week in the CdMx, there is the so-called “seismic sequence” of 2019, when between July 12 and 18 of that year there were 20 earthquakes with epicenters in the Miguel Hidalgo mayor’s office. The highest magnitude in this series was 3.0.

“The 2019 earthquakes were due to a sequence, it was through this region of the Miguel Hidalgo Mayor’s Office and further south, they were attributed to the fact that they were doing works in the Metro Observatory, but nothing to do with it, they were due to landslides of small faults in that region, in the Basin there is also a large amount of faulting, a large amount originates from that landslide. Those of February are also associated with parallel faults,” Espíndola explained to this medium.

HOW LONG DO THESE EARTHQUAKES LAST? ARE THERE REPLICAS?

The number of aftershocks, says the SSN, can vary from a few to hundreds of events in the next few days or weeks after the main tremor occurred. “However, the earthquakes that have their epicenter in the geographical area that includes the Basin of Mexico, due to their size, do not always present aftershocks,” he clarified.

And it is that, when an earthquake of considerable magnitude occurs, “the rocks that are near the rupture zone suffer a rearrangement, which generates a series of tremors in the area that are called aftershocks”, which is more difficult for it to happen with magnitudes such as those registered in the capital.

The experts from the Seismological Service also stressed that to date “there are no scientific techniques in any part of the world that can determine when an earthquake will occur” nor can it be known how big it will be.

With regard to the duration of these telluric movements, the SSN also explained that there are several concepts: one is the one perceived by the human being and another the one marked by the instrumental record, “which can range from several minutes to hours.” Another concept is the time that the movement of the fault that caused the earthquake lasted, “which can be a few seconds”

People usually feel the most intense part of the earthquake. “The person will report a movement time shorter than the seismometer will report, because the person only feels the most intense part of the ground movement, while the seismometer perceives even the most insignificant movement that occurs just when the earthquake starts and when it ends”, detailed the Service.

The duration of these movements, concludes the SSN, is variable: “When an earthquake occurs, people who live in different places do not perceive the same duration, they experience different times. There are three main factors that intervene in the duration of the movement: the distance to the epicenter, the type of terrain and the type of construction where we are at that moment”.

Asked if there could be earthquakes of a higher magnitude, Espíndola assured that it is possible, but the earthquakes that have been seen “are characteristic of these tiny magnitudes of this type of faulting.” “Yes, one of 4.5 or 5.0 can occur, taking into account the magnitude. Physically, geologically, let’s hope no more, taking the dimensions of the faults as a parameter, let’s say that it gives for that”.

WHY IS THE SEISMIC ALERT NOT SOUNDING?

On Tuesday, the Head of Government of Mexico City, Claudia Sheinbaum, assured at a press conference that there is a working group, coordinated by the capital’s Secretary of Education and in which different universities in the country participate, which will meet again, as in 2019, to discuss the topic of microseisms.

“This working group continues to function. The thing about microseisms has to do with normal movements of local faults, so to speak, and that is why they are of such a slight magnitude, the issue is that they are felt because the epicenter is here, there is no major problem, but they are going to quote this group of experts to hear their opinion,” he said.

In addition, he recalled why the seismic alert does not sound in this type of telluric movements: “The alarms do not sound because there is a very low magnitude.”

to convene a meeting of this group to publicize the progress in research and monitoring (2/2)

— Dr. Claudia Sheinbaum (@Claudiashein) February 8, 2023

The seismic alert is not operated by the SSN, but by the Mexican Seismic Alert System (Sasmex), which is part of the Seismic Instrumentation and Recording Center, AC (Cires).

The sensors monitor seismic danger zones and recognize earthquakes within a radius of close to 90 kilometers. They estimate in the first seconds of the detection of the earthquake its possible magnitude. With the data sent by the sensors, each city determines the distance between the earthquake to decide the dissemination of the seismic alert notice.

For the alarm to be activated, one of these conditions must be met: if in the first seconds of the seismic detection, at least two stations exceed the pre-established energy levels; depending on the estimate of energy of the earthquake; and depending on the estimated magnitude of the earthquake and the distance to the city to be alerted: if the magnitude is greater than 5.0, it should occur no more than 170 km away; with magnitude greater than 6.0 that occurs more than 350 km; and with magnitude greater than 5.5 that occurs at no more than 350 km.

“An earthquake does NOT MERIT an alert when it occurs far from the SASMEX detection coverage area, it is too far from the city to alert, or when the estimates of the earthquake’s energy do not exceed the established levels,” the agency explains.