More likely in September? What does the science say?

(CNN Spanish) — It happened again. As it happens only in fiction, in September Mexico was again hit by a strong earthquake.

What makes this fact even more incredible is that Earthquake this Monday It happened on September 19, a date well-remembered in Mexico for earthquakes in previous years.

The bitter memory of this date began The magnitude of the earthquake was 8.1 On September 19, 1985, it affected the central, southern and western parts of the country and killed approximately 10,000 people. It was this earthquake that led to the creation of Mexico’s National Civil Defense System.

This was followed by an earthquake on September 19, 2017. Due to the events of 1985, a mega exercise is carried out in the country on this day as a preventive measure; It was noon in 2017. About an hour after the exercise, a magnitude 7.1 earthquake struck central Mexico.

Now, 32 and 5 years after the 1985 and 2017 earthquakes respectively, the country experienced an earthquake of magnitude greater than 7 on the Richter scale. In this case, that too 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 Mexico’s National Seismic Service (SSN), which is part of the National Autonomous University of Mexico (UNAM).

And to add to the implausibility of the situation, there are other earthquakes in September that lead to the same bad memory because of the proximity of the 19th: On September 7, 2017, before the 1985 anniversary and new earthquake, an 8.2-magnitude earthquake hit the southwest of the country; And four years later, On September 7, 2021Another earthquake was reported, now measuring 7.1 on the Richter scale, off the coast of Acapulco.

Along with this, a 6.8-magnitude earthquake struck the state of Michoacán, according to the USGS, recorded early Thursday morning, September 22. This happened a few days after the September 19 filing.

So, does September increase the probability of earthquakes in Mexico? What does science say?

With several large earthquakes occurring in September and several large earthquakes on the same dates (and even in the last two days of the 19th), doubts have been raised about the frequency of earthquakes in Mexico during this month.

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

SSN’s investigator Victor Hugo Espindola Castro said At a press conference This Monday it was nothing more than “an unfortunate coincidence”.

“This earthquake (on September 19, 2022) occurred at 1:05 pm (local time). There will be those who say that the earthquake in 2017 occurred at the same time (it was at 1:14 pm). There was a 7.1 magnitude in the Acapulco area on September 7. But it was nothing more than a coincidence, an unpleasant coincidence,” Espindola Castro said.

What is the probability of large earthquakes occurring on the same date in a given year? The probability is very low, the expert added, but it is not impossible. So, when we talk about probability, we are referring to the chances of something happening, and even if the statistics are low, there may be a situation where an event becomes a reality.

“If we look at the seismological catalogs that we have from the early 1900s, we see that earthquakes greater than 7 occur repeatedly in different months. For example, we see many in December, and many in all months. So, of course the probability of occurrence on the same day is very low, but because a probability is small That doesn’t mean it can’t happen.

“(The probability) of another earthquake on September 19 may be very small, but it is not impossible. In other words, probabilities only tell us about that, about what is more likely to happen,” Espindola explained.

To clarify the probability issue, the SSN investigator noted that the coincidence of the September 19 earthquakes was “like winning the lottery three times in a row, or if you like, once every four years.” In short, it’s unlikely to happen, but it’s not impossible.

Likewise, he added some data that comes from probabilities: for example, an earthquake greater than 7 occurs off the Mexican coast every three years, or a magnitude 8 every six years.

“It’s a prediction, but basically it’s still statistical and doesn’t give us much information to use those techniques to predict, which is very different,” Espindola Castro said.

For his part, Luis Quintanar Robles, Secretary of Education of the UNAM Institute of Geophysics, told the conference that there is no scientific certainty that earthquakes in Mexico occur on a specific date.

“What we can tell is that earthquakes are usually triggered when they release stress that builds up in a neighboring region. So, for example, if we assume that there was a magnitude 5 earthquake in the Guerrero area on September 14th, it’s possible that this earthquake could facilitate movement between the oceanic and North American tectonic plates.” Released efforts,” he explained, which could have caused Monday’s quake.

However, this tells us that the force of the previous earthquake could have helped rub the surrounding plates to trigger another earthquake, but not the tremor on the same day in September.

“Obviously, the only thing I can say at this time is that it is a coincidence. However, this question remains open (…) and I believe that the possible origin of the so-called birthday of someone can be studied. Earthquakes, they occur on the same day of the year, but at this time Nothing can be reliably said about that,” Quintanar Robles said.