Earthquakes in Mexico: What science says about frequent events in September

And it happened again. As if it were something that only happens in fiction, Mexico was hit again by a strong earthquake in September.

What makes this fact even more incredible is that this Monday’s earthquake occurred on September 19, a date well remembered in Mexico for the occurrence of earthquakes in previous years.

The bitter memory of this date began with the 8.1 magnitude earthquake of September 19, 1985, which affected the center, south and west of the country, leaving approximately 10,000 dead.

This earthquake was what led to the creation of Mexico’s National Civil Protection System.

The earthquake of September 19, 2017 followed. Due to the events of 1985, on this day a mega simulation is carried out in the country as a preventive measure; 2017 was around noon. Approximately one hour after the exercise, central Mexico was hit by a magnitude 7.1 earthquake.

Now, 32 and 5 years after the 1985 and 2017 earthquakes respectively, another magnitude 7 earthquake strikes the country. On this occasion, it also happened almost an hour after the macrodrill; its magnitude was 7.6 according to the United States Geological Survey (USGS) and 7.7 according to the National Seismological Service (SSN) of Mexico, which is part of the National Autonomous University of Mexico (UNAM).

And to add to the incredibleness of the situation, there were other earthquakes in September that lead to the same bad memory due to the proximity of the 19th: on September 7, 2017, before the 1985 anniversary and the new earthquake, a magnitude 8 earthquake occurred. ,2 which greatly affected the southwest of the country; and four years later, on September 7, 2021, another was recorded, now of magnitude 7.1, with an epicenter off the coast of Acapulco.

To these is added the magnitude 6.8 earthquake, according to the USGS, registered in the early hours of Thursday, September 22, with its epicenter in the state of Michoacán. This was just days after the one recorded on September 19.

What does science say about earthquakes in Mexico?

Due to the occurrence of several major earthquakes in September and on similar dates (and even at similar times for the last two of the 19th), doubts have arisen in Mexico as to whether earthquakes occur more frequently this month.

And it’s a reasonable doubt. Who would have thought that this situation would occur so precisely? It looks very incredible. But science is categorical about it: it’s a coincidence.

Víctor Hugo Espíndola Castro, an investigator for the SSN, told a press conference on Monday that it was “an unpleasant coincidence”.

“This earthquake (on September 19, 2022) occurred at 1:05 pm (local time). There will be those who say that the one in 2017 also took place at the same time (it was at 1:14 pm). Also remember that a year ago there was one on September 7 in the Acapulco region, with a magnitude of 7.1. But it’s just a coincidence, an unpleasant coincidence,” Espíndola Castro said.

What is the probability that large earthquakes will occur on the same date in a given year? The probability is very low, added the expert, but it is not impossible.

So when we talk about probability, we are referring to the odds in numbers of something happening, and even if the numbers are low, there could be a scenario where an event becomes a reality.

“If we look at the entire seismic catalog that we have since the early 1900s, we see that earthquakes larger than 7 have recurred in different months. For example, we see several in the month of December, several in every month. So certainly the probability of happening on the same day is very low, but just because a probability is small doesn’t mean it can’t happen.

“(The probability of) an earthquake occurring again on September 19 may be very small, but it is not impossible. That is, the probabilities only tell us that, about what is most likely to happen”, Espíndola explained.

To clarify the issue of probability, the SSN investigator mentioned that the coincidence of the September 19 earthquakes “is like winning the lottery three times in a row, or once, if you prefer, every four years.” In short, there is a low probability of this happening, but it is not impossible.

Likewise, he added some data that also comes from probabilities: for example, that statistically every three years there is a magnitude 7 earthquake off the Mexican coast, or that every six years there is a magnitude 8 earthquake.

“This is a prediction, but basically it’s still statistics and it doesn’t give us a lot of information to use these prediction techniques, which is very different,” Espíndola Castro said.

For his part, Luis Quintanar Robles, academic secretary of the Institute of Geophysics at UNAM, told the conference that so far there is no scientific certainty that guarantees that the earthquakes in Mexico occur on a specific date.

“What we can say is that earthquakes usually trigger when there is a release of stress produced in a neighboring area. So, if we consider that, for example, on September 14 there was a magnitude 5 earthquake in the Guerrero area, it is possible that this earthquake triggered some efforts that facilitated the movement between the oceanic and North American tectonic plates”, which could have resulted in Monday’s earthquake, he explained.

However, this tells us that the force of a previous earthquake can help rub the surrounding plates to trigger another earthquake, but not that the tremors occur on the same day in September.

“Obviously, at this point the only thing I can say is that it’s a coincidence. However, this question is open (…) and I believe it is possible to study the possible origin of what is called an earthquake anniversary, which occur on the same day of the year. But at this point we can’t say anything convincingly about it,” Quintanar Robles said.