Mexico has recently reminded us of the latent seismic threat in most Latin American countries, due to the occurrence of several earthquakes during the month of September 2017. The most striking thing is that one of these events occurred on the very day that marked the 32nd anniversary of the great earthquake of September 19, 1985. 

* This article was published in Geociencias SURA Journal | Issue 3 | March 2018.

In September 2017, Mexico was hit by two major earthquakes within a span of 12 days. The sequence began on September 7 with a magnitude 8.1 (Mw) earthquake on the Pacific coast, near the state of Chiapas. Twelve days later, on September 19, a magnitude 7.1 (Mw) earthquake struck Puebla, causing the collapse of 44 buildings in Mexico City. 

As put by Ph.D. Ross Stein, professor of geophysics at Stanford University, USGS scientist emeritus and CEO of Temblor Inc.: “A disturbing question arises after the occurrence of these two earthquakes: Are these events related to each other?” 

The proximity of these two events in time and space and the nature of their tectonic origin could lead one to assume that there is indeed a chain reaction. To answer this interesting question, Ph.D. Ross Stein and the team at Temblor Inc. studied the two earthquakes in order to estimate the possible relationship between them. 

To do this, they evaluated the potential impact that the Chiapas earthquake could have had on the Puebla earthquake, through a stress transfer analysis. The results of this analysis allowed them to conclude that the 8.1 magnitude (Mw) earthquake recorded on September 7 in Chiapas did not generate stress on the fault that gave rise to the earthquake recorded in Puebla 12 days later.. 

As Ph.D. Ross Stein puts it in his publication on this analysis: “When calculating the stresses generated by the Mw = 8.1 earthquake in Chiapas on the fault that generated the Mw = 7.1 earthquake in Puebla, we found that the stresses experienced in it are so small that they are even smaller than those generated by rubbing the fingers of the hand. 

Could the recorded seismicity show any relationship between the 7S and 19S earthquakes of 2017?

The Temblor Inc. team analyzed the catalog of the National Seismological Service (UNAM) and found that the aftershocks of the Chiapas earthquake presented a pattern consistent with the distribution of estimated stress increases (Coulomb analysis). 

Another very compelling finding raised by Ph.D. Ross Stein is that when reviewing the distribution of the points where the aftershocks of the 7S earthquake in Chiapas originated (Mw = 8.1), no aftershocks were found near the region where the 19S earthquake in Puebla occurred (Mw = 7.1). 

From this analysis it is concluded that The stresses transmitted by the 7S earthquake in Chiapas on the fault that gave rise to the 19S earthquake in Puebla are negligible, and in that sense, there is no relationship between these two earthquakes.

What is the probability that these two events are independent? 

According to the analysis carried out by the Temblor Inc. team, it is possible to infer that the Chiapas earthquake had no impact on the occurrence of the Puebla earthquake, but, ¿What is the probability that these two events are independent, considering the fact that the Puebla earthquake occurred only 11 days after the Chiapas earthquake, and that their epicenters were 600 km away from each other?? 

According to estimates by Ph.D. Ross Stein and his team, this probability corresponds to 1 in 30.000: “You could say that a probability of 1 in 30.000 is too remote to think that the earthquakes in Chiapas and Puebla are not related. 

But before we consider that 1 in 30.000 is too small a number to consider both earthquakes a coincidence, it is good to ask ourselves this question: What is the probability that the Puebla earthquake (Mw = 7.1) occurred just 2 hours after the commemorative drill for the 32nd anniversary of the 1985 earthquake in Mexico City? This definitely has to be a coincidence, right? This probability corresponds to 1 in 900.000. Almost one in a million! 

So, in the words of Ph.D. Ross Stein, “Extreme coincidences can happen in our lifetimes and after seismological analysis, this is the best explanation we have.” 

“Extreme coincidences can occur in our lives and after seismological analysis, this is the best explanation we have.”

Ph.D. Ross Stein.

Tectonic environment and historical seismicity of Mexico

Mexico is located in an area of ​​the American continent where five tectonic plates interact (the Pacific, Riviera, Cocos, North American and Caribbean plates), which makes this region present a significant seismic threat. As stated by the National Seismological Service of Mexico, Although there is no current method or technology to predict earthquakes, the tectonic context of Mexico includes regions where large earthquakes have occurred and where they may occur in the future. This conviction has led this Latin American country to seek, through the development of knowledge, a path towards seismic resilience.

Fonts

• David Jacobson. Bachelor of Science in Geology from Whitman College in Walla Walla, Washington, and M.Sc. in Geology from the University of Canterbury in Christchurch, New Zealand. 
• Ross S. Stein. CEO of Temblor.net, Professor of Geophysics at Stanford University.