The seismic alarm was heard, yet those vital seconds were not enough for everyone. Last Tuesday, September 19 , 32 years after the greatest earthquake in Mexico City , a new earthquake hit the country. The epicenter was only 120 kilometers away, between the limits of neighboring states Morelos and Puebla . The 7.1-magnitude earthquake was classified as intraplate and it affected 19 states .
Xyoli Pérez Campos
, Head of the National Seismological Service ( SSN ), explains that earthquakes are classified in two categories: intraplate and interplate . An intraplate earthquake occurs inside a tectonic plate. Precisely, the intraplate earthquake occurred on September 19, it was caused within the Cocos plate . "While the 1985 tremor was an interplate earthquake resulting of the subduction of the Cocos Plate beneath the North American Plate ”.
Generally speaking, tremors of greater magnitude belong to the interplate category and they are mainly originated in the coasts. However, this does not mean that the earthquakes that originate inside a plate do not reach high magnitudes.
Two intraplate quakes had their origin within the Cocos plate in Puebla on June 16, 1999 and October 24, 1980 . The 8.2-magnitude earthquake that shook Mexico on Thursday, September 7 , mainly affecting the states of Oaxaca and Chiapas , also originated in the interior of this plate, but they are independent phenomena.
"Regarding the 7.1-magnitude earthquake, we can say that it happened in a place where the Cocos plate changes its geometry ," she points out, adding that this particular piece of information implied more than a decade of research since they have recently managed to map the plate and to observe how the change of form impacted the seismicity presented at the epicenter.
Xyoli Pérez Campos emphasizes that this phenomenon does not imply that a 7.1-magnitude quake will not occur in the following thirty years, but rather, that the possibilities have decreased . The problem with local earthquakes is that those populations closer to the epicenter have little or even no chance of being alerted, a problem that repeats itself around the world.
The magnitude of a quake is related to the energy released in waveform propagated through the Earth. In order to calculate this energy, mathematical algorithms based on the seismograph records of different stations are performed. The seismic sensor information is what allows the automatic emission of warnings through radio waves.
Currently, the SSN has more than 100 earthquake recording pieces of equipment, organized in different subnets, yet the expert emphasizes that Mexico should have a wider coverage of the seismogenic zones .
"The last event turned out to be so close that the alert only gave a few seconds of advantage, but if the epicenter was closer , we would have no advantage, we could even feel the earthquake before the alert. It is not a problem in the algorithm."
Xyoli Pérez Campos explains that the magnitude depends on the fault that causes it as well. A geological fault is a fracture in the Earth's crust. The specialist says that the faults localized in the Valley of Mexico do not reach a sufficient length for a large earthquake.
However, Luis Quintanar Robles , researcher in the Department of Seismology of the Institute of Geophysics ( IGF ) of the UNAM notes that even though earthquakes with epicenter in Mexico City may sound strange, they are actually common phenomena.
The researcher points out that the main epicentral zones that are registered in the east, are in Iztapalapa , Milpa Alta , and Tlahuac boroughs , as well as in municipalities of the State of Mexico such as Texcoco .
"In the eastern part of the Valley, in addition to the faults, subsidence or land sinking from water extraction is added. This creates cavities that, when collapsed, can produce earthquakes too." Just as the 2.6-magnitude earthquake recorded on Saturday, September 9 originated in the southeast of Tlalpan.
Quintanar explains that although the earthquakes are of a low magnitude, the dense population centers evidence the tremors. "An earthquake cannot be predicted, so in the face of recurring unpredictable scenarios, it is necessary for people to know where they live, " he concluded.
For more information on the subject visit the following link: http://http://www.ssn.unam.mx/english/.
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