Great earthquakes in history repeat themselves

The seismologist with the most academic citations in Latin America and emeritus researcher at the Institute of Geophysics (IGf) of the UNAM, Shri Krishna Singh, revealed the finding of repeated tremors in the cities of Oaxaca and Acapulco in the last 100 years.

According to advances of his research presented during the Institutional Seminar "Repetition of great tremors: the importance of historical seismograms", in that academic entity, the also visiting professor of the universities of Rome, Paris, Wisconsin, Nevada, and Bergen asked himself:

Do these great tremors of history repeat themselves? And he answered: "They can only be considered in this way if the location and geometry of the earthquakes are the same, and if the shapes of the waves are similar. This helps to know what is the state of frictional coupling on the interface (of the plates)".

The specialist in the repetitive behavior of seismic activity in the so-called Mexican subduction zone (Pacific) explained that Oaxaca is an ideal place to study them, "for three reasons: because the sources of its telluric movements are very simple in general, because if they were complex it would be very complicated to expect repetitions; they are characteristic because they have almost the same magnitude; that is, they register little variation".

Also, because the period of occurrence of their great tremors is relatively short, in such a way that we have the possibility, from the records (of the last 100 years), of verifying when these repetitions occur (from three to five decades).

Krishna Singh, a scientist from Azamgarh, India, pointed out that fault segments in the area tend to generate phenomena of the same magnitude. That state has a peculiar earthquake statistic.

In 1928, he explained, four occurred in the Oaxacan capital: March 22, June 17, August 4, and October 9. According to the seismograms analyzed, they are similar to those presented with another one that occurred in 1965 and the most recent one in 2020.

"The amplitudes are real, the magnitudes are very close, yes they are repetitions, most likely at this magnitude level (above seven)." It is an unusual situation: from 1928 to 1965 was 37 years; and from 1965 to 2020, 55 years away, he specified.

Regarding Acapulco, Guerrero, also a specialist in the field of seismic engineering and risk mitigation, analyzed the movement that occurred on September 8, 2021, and that of May 11, 1962.

"Looking at their characteristics, it is likely that the most recent one is the repetition of the previous one, with 59 years of difference. I am not talking about the seismic gap of Guerrero, nor the northeastern and southeastern part," he clarified.

Based on the seismograms, in the case of Oaxaca, there are three similar events, and it is an almost unique observation. "I have not seen this type of phenomenon elsewhere, which does not mean that they have not occurred during the period of record," he stressed.

The also member of the National System of Researchers level III, commented that the fact that they are repeated identically means that the regions and the friction properties (of the tectonic plates) are maintained during several centuries of tremors.

"Why Acapulco and Oaxaca? Because other places have not been reported; their tremors are simple and compact, and that favors repetitions and characteristic tremors, which is such a clear thing that is seen in few places, and the frequency period is relatively short (a century)," he referred.