This December 12, three microseisms were recorded in CDMX, in some areas the seismic alert was activated, while in others it was not. But the question is what these movements were. According to the UNAM Institute of Geophysics, a microseism can be defined as “small ruptures inside the earth, close to the surface.”
These are brief telluric movements of small magnitude, but with high intensity for the epicenter regions. The vast majority are not perceived, however, it usually happens that citizens of a localized area perceive them with great intensity, while for the rest of the city they go unnoticed.
What is a microseism?
In recent months, Mexico City has been experiencing microearthquakes, leading to the question of whether earthquakes are increasing or not.. The Institute of Geophysics of the National Autonomous University of Mexico explains what microseisms are and if they are really increasing.
Víctor Manuel Cruz, head of the Seismology Department of the Institute of Geophysics of the UNAM, says that a microseism can be defined as small ruptures inside the earth, which occur close to the surface. They are brief earthquakes of very low magnitude that occur in localized areas of Mexico City; however, when they occur near the surface, they are felt with great intensity in the epicenter region.
Normally when earthquakes occur in the Valley of Mexico, the seismic waves usually come from the Pacific coasts of states such as Guerrero, Michoacán, Oaxaca or Chiapas. This type of seismicity is felt throughout the valley and
Instead, Microseisms have their epicenter within Mexico City itself and although their magnitude is low, when they occur near the surface they are felt as an intense vibration for a short period of time. But also, the clearest characteristic of microseisms is that they are felt only in the regions closest to the epicenter, this is the reason why the citizens of some municipalities feel them, while the rest of the Valley of Mexico is not able to perceive them. .
According to UNAM, microseisms can be caused by various causes such as nearby explosions, subsidence or movements of faults as a result of intense rains, but it clarifies that in most cases microseisms are products of nature.
Are microseisms increasing?
The recent microseisms lead to, however, Manuel Cruz clarifies that these are not unusual events. There are experts specialized in the topic of microseismicity, who report that they have been recorded for decades.
Manuel Cruz added that, in fact, microseisms are not so frequent despite the fact that in the surroundings of the basin, in a relatively small radius of about 150 kilometers, many geological faults have been identified in the Mexican Transvolcanic Belt, which is comprised of the Popocatépetl, Iztaccíhuatl and Nevado de Toluca volcanoes.
According to calculations by the UNAM Institute of Geophysics, microseisms of magnitude 2.5 represent a rupture of a fault measuring approximately 200 meters in a tenth of a second. This is why they feel like an intense but brief vibration, since the slip that the fault suffers is sudden, although it does not pose any danger and is normal, the experts explain.
The Basin of Mexico has a complex geological and tectonic structure, so microseisms are not unusual events and the damage caused is usually low to none.
Why doesn’t the seismic alert sound during microearthquakes?
The Seismic Warning System depends on seismic sensors that are placed in different parts of the Pacific coasts. These sensors are responsible for detecting important movements in the Guerrero Gap. and before the seismic waves reach the Valley of Mexico, the seismic alert system is turned on 50 seconds in advance.
But when it comes to microseisms, the scenario is completely different, since the epicenter is generated just below Mexico City, which leaves no warning time. In addition, these are low-magnitude movements that go unnoticed for a large part of the basin.