Between the September 19, 1985, and 2017 earthquakes, there are major differences beyond temporality: the epicenter of the second event was 230 kilometers closer to Mexico City; however, the first was 30 times more destructive, with a magnitude of 8.1, due to its occurrence in different tectonic environments.
Ral Valenzuela Wong, the researcher at the Department of Seismology of the Institute of Geophysics of the UNAM, explains the above about the earthquakes that occurred 37 years ago and five years ago, respectively.
For the expert, the actions to be implemented in social, academic, and public policy matters should be more focused on "prevention than a prediction; it is imperative to know how we can guarantee that our homes and buildings are well built".
Progress has been made in construction regulations and we must comply with them to be better prepared for the occurrence of the next phenomenon of this nature, which undoubtedly represents a problem that is also of an economic and social nature due to the cost of building the buildings in a better way.
A graduate of the Department of Earth and Planetary Sciences of the University of Washington, Valenzuela Wong mentions that for some researchers the reference had been that of 1985, "but five years ago the lessons and teachings continued, even though we are better prepared in certain aspects, but it clearly shows us that we still have a long way to go."
Ideally, in 2017, no building would have collapsed, which perhaps indicates that there were or are old buildings that have not been reinforced, or that do not comply with the most current construction standards, he warns.
The occurrence of these earth tremors in Mexico is not as frequent as in other latitudes, "although perhaps it is more common than we would like."
"For an earthquake to be repeated in the same place, many years, decades, sometimes hundreds of years have to pass, it is a process that will take time, and there will be generations of seismologists who will continue working on this issue". In this sense, he rules out that in Mexico, the month of September is considered a season of high seismicity.
We have earthquakes of great magnitude that have not happened in September; one of these was felt strongly in Mexico City in July 1957, known as the "Angel" earthquake; another one in March 1979, the "Ibero" earthquake, which left the facilities of the Universidad Iberoamericana in the Campestre Churubusco neighborhood badly damaged, argues the university professor.
There have been four magnitudes 8 earthquakes in the twentieth century and so far in the twenty-first century: one in 1932 with an epicenter in Colima and Jalisco; one in 1985 with an epicenter in Michoacán's coasts; and one in 1995 with an epicenter in Colima and Jalisco.
The one on September 7, 2017, was 8.2, with an epicenter in Oaxaca, a magnitude not seen since 1932; "85 years had passed, it is important to put it in context if we think of others of magnitude 8 or greater in Mexico," Valenzuela Wong emphasizes.
"The largest earthquakes are normally associated with movements that occur in subduction zones in the contact between two plates, what we call tectonic compression environments." These are the cases of those that occurred in 1932, 1985, and 1995. The one in 2017, of 8.2, was different because it happened within a single tectonic plate, within the Cocos Plate that is below the North American Plate, and occurred in a tectonic environment of extension, "he indicates.
The probability of earthquakes in Mexico
Wong explains that telluric movements occur at 10, 15, 50, or more kilometers deep, where there are really few possibilities to measure before they occur.
"The seismometers are the instruments with which we register these tremors, but once they have already occurred, it is very difficult to undertake observations of the Earth from its interior and that this anticipates that one may occur," argues the specialist.
If we know that in a certain part of the world or country there was an earthquake of whatever magnitude, we can expect that, after a certain time, several years, because of the way energy accumulates, because of the way tectonic plates move, there could be another one of the same magnitudes in that area, he says.
As it depends on the region, the number of years that must pass is variable; in some areas, it is shorter. In the case of the Guerrero Gap, which is the part of the coast between Acapulco and Zihuatanejo, we are talking about something that happened in December 1911.
"The last one that occurred there was 111 years ago, and we have not had a tremor of that magnitude again." That is the reason that makes us think that, at some given moment, there may be an important movement in that area. We do not know. That is what we can say," he concludes.