Disaster Events

The Mexico Analysis | Two Earthquakes and Multiple Aftershocks

Mexico Earthquake Backdrop

Late in the evening of September 7, 2017, an 8.1 magnitude earthquake struck Mexico’s southern coast, which Mexico declared as its strongest earthquake in over hundred years. The September 7th earthquake in Mexico is the strongest among a series of earthquakes and aftershocks that have been occurring in the region around Mexico in September 2017. The 8.1 magnitude earthquake in Mexico’s southern coast led to almost 100 deaths in Mexico and triggered a small tsunami with waves rising to up to 0.7 m in the Pacific Ocean as per the Pacific Tsunami Warning Centre, with the sea retreating about 50 m before the tsunami (L. Shaffer, 2017). Mexico was successful in evacuating some coastal areas at the time.

According to the US Geological Survey (USGS), the epicentre of the 8.1 magnitude earthquake was located at a depth of about 69 km on the floor of the Pacific Ocean some 87 km southwest from Pijijiapan town in southern Mexico. The main earthquake, as per the USGS, was shortly followed by five aftershocks, whose magnitudes ranged between 4.9 and 5.7 on the Richter scale.

The tremors from the first large earthquake with an epicentre about 804 km from Mexico’s capital – Mexico City were also felt in the neighbouring countries of Guatemala and Belize.

Fig: Epicentre for the earthquake on September 7th, 2017 Source: USGS

On September 19th, another strong earthquake measuring 7.1 on the Richter scale struck Puebla in southern Mexico, with an epicentre about 650 km from the September 7th earthquake. According to the USGS, the epicentre for this earthquake was located near Raboso town of Puebla, located about 123 km to the southeast of Mexico City.

This second large earthquake in Mexico in a short span led to the collapse of numerous buildings in Mexico City and in towns near the epicentre. As per The Guardian, UK, the two powerful earthquakes in Mexico along with their aftershocks have together led to nearly 400 deaths in Mexico, with many people still missing in the debris. The previous strong earthquake in Mexico was the 1985 Mexico City earthquake, with a magnitude of 8 leading to the deaths of up to 10,000 people (BBC, 2017), and which incidentally also occurred on September 19th.

Fig: Epicentre for the earthquake on September 19th, 2017 Source: Focolare Movement

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Analysis of Earthquakes in Mexico
Paul Earle, a seismologist at the USGS, is reported to have said that the Puebla earthquake in Mexico was located too far from the September 7th earthquake in Mexico to have been merely an aftershock. According to Earle, most aftershocks occur within a radius of about 100 km, and the two epicentres were about 650 km apart. Earle goes on to say that the Puebla earthquake did not occur at the edges of two moving tectonic plates, although it occurred at a known tectonic fault inside the Cocos plate. The Cocos plate, after a radius of about 300 km from the epicentre of the Puebla earthquake in Mexico, goes under the North American plate. The September 19th earthquake in Mexico is most likely to have occurred due to a pulling apart motion among the plates, Earle said.

Ever since these strong earthquakes, there have been many aftershocks and weaker earthquakes in Mexico. On September 23, a tremor measuring 6.1 on the Richter scale struck the state of Oaxaca in Mexico with an epicentre about 18 km southeast from Matias Romero. This was an aftershock of the 8.1 intensity Mexico earthquake that occurred on September 7.

On September 24, the Mexican west coast was struck by an earthquake measuring 5.7 on the Richter scale, further increasing difficulties in rescue efforts after the two strong earthquakes in Mexico and their aftershocks. Its epicentre was located in Chiapas state of Mexico about 80 km southwest of Tonala. This was accompanied with tropical storm Pilar that belted the western coast of Mexico the same day (Associated Press, 2017).

The devastation caused by the recent assortment of earthquakes in Mexico was far less than what could have been, and was definitely far less than the devastation caused in the 1985 Mexico City earthquake, which led to up to 10,000 deaths. The reason has to do less with better building codes adopted than with the nature of the earthquakes itself. Although new building codes have been introduced in a high earthquake-risk zone, implementation is very uneven and inadequate according to experts.

The Mexico earthquake in 1985 had an epicentre that was much shallower than the recent earthquake that struck on September 19. Due to the orientation of the fault, the direction the rocks moved in was more benign to building integrity. Its intensity was also dulled somewhat by having an epicentre 354 km away, as compared to the recent earthquake, originating deep inside the Earth’s crust with an epicentre located just about 123 km from Mexico City.

The Mexico earthquake in 1985 was 30 times more intense than the one that occurred on September 19, 2017, and led to the collapse of a lot of high-rise buildings, increasing the death toll. The recent earthquake however, affected smaller and less populated buildings, decreasing the death toll. Dr. Eduardo Reinoso, a seismic engineer at the National Autonomous University of Mexico, says that the different distances and magnitudes of the two earthquakes produced different seismic activities.

These produced different sorts of shock waves which influenced their effects on buildings, Dr. Reinoso says. Also 71 per cent of Mexico City’s buildings failed to meet compliance with the city’s standards, which could have accentuated the recent earthquake’s effects on some buildings (A. Ahmed, M. Franco & H. Fountain, 2017). A stronger earthquake in proximity of Mexico City like the 1985 earthquake could have a higher bearing on building stability.

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Earthquake Prediction
The prediction of earthquakes is a highly controversial issue, and the scientific community largely believes that earthquakes occur randomly. In terms of validation, no substantial examples exist of earthquakes being predicted successfully in terms of size, schedule and intensity. Many methods that have arisen time after have been discredited, and often the danger instead is one of false public alarm that can have administrative repercussions and cause economic losses.
A last minute warning is possible among seismologists who study rock movements and monitor them along fault lines in terms of the build up of pressure in these regions.

However, this can only provide a time frame of about 30 seconds before an earthquake actually occurs, which is far too less time to organize anything substantial. What would be really useful are long range predictions, yet the best that is possible is the possibility of an earthquake occurring in an area provided by seismologists, which however, is not an indication of an actual earthquake occurrence.

The Mexico earthquake on September 19th, 2017 was not entirely anticipated by scientists. Conventional thinking was to focus on the ruptures near the coastal areas, but this earthquake did not originate from the anticipated area. This earthquake however, originated in the interior mountainous regions about 56 km below the surface. Near this zone, a large part of the tectonic plate has sunk into the molten rock (E. Vance, 2017).

Allen Husker, seismologist, says that although people have tended to focus on the 1985 coastal rupture, this earthquake is not the first time earthquakes in Mexico have originated from deep inside the Earth’s crust. Some scientists even claim that it is more likely for an earthquake to occur near the place they have happened before than in places where earthquakes have not occurred for a long time. For now long term and empirical earthquake prediction is at best a speculative science.

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