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The magnitude-8.1 earthquake on 7 September 2017 offshore of Chiapas, Mexico was a magnitude-8.1 event at a depth of 70 km that occurred at 11:49 pm local time or just before midnight. The location was 87 km southwest of Pijijiapan, Mexico. At the time of this writing, more than 96 deaths have been confirmed, however many people are still unaccounted and the number of fatalities could rise significantly.

The number of fatalities is less than what can be expected from such a large event. This is in part due to the earthquake being offshore and not near population centers. However, much credit is due to the Mexico’s seismic alert system (CIRES), implemented after the disastrous 1985 Mexico City earthquake, which killed at least 5,000 people in collapsed buildings. The system relies on sensors placed along the country’s Pacific coast and allowed residents to make it out of their residences and to the street. Such a system has also been implemented in Japan and is under consideration in California.

Many buildings have collapsed or suffered damage in the Oaxaca, Chiapas and Tabasco federal districts and the event was felt as far as Mexico City and Guatemala City. It is estimated that more than 50 million people felt this earthquake and nearly two million homes lost electricity or water.

Officials report many structures in the three affected districts were damaged or collapsed. The out-of-plane failure of unreinforced masonry walls observed in the past earthquakes, also is noted in this event. The damage was not limited to houses as schools, hospitals, hotels and commercial buildings also were severely damaged or collapsed. Irregular structural configuration and lack of ductile detailing likely contributed to some of the failures. Damage also extended to Guatemala, where infrastructure damage was reported in Antigua, the onetime colonial capital of Guatemala and a designated UNESCO World Heritage site.  In Mexico City 700 km away, buildings and iconic monuments swayed and there was damage to glass doors at the airport. In Juchitan, a city of more than 70,000 people in Oaxaca, the 19th-century city hall (Palacio Municipal) partially collapsed.

The epicenter occurred close to the subduction of the Cocos plate beneath Central America at the Middle America Trench. The location, depth and normal-faulting mechanism of this earthquake indicate that it is likely an intraplate event within the Cocos slab. In the past 100 years, eight other M-7+ earthquakes have occurred within 250 km of the hypocenter of this earthquake. The 2012 M-7.4 earthquake offshore near Guatemala resulted in an estimated 50 fatalities and more than 150 injuries and caused significant damage. The 1931 M-7.8 and 1980 Oaxaca M-7.0 earthquakes resulted in 114 and 300 fatalities. The M-5.1 earthquakes of 14 June 2017 and M-6.8 earthquake of 22 June 2017 are the most recent events in this area.

An engineering team from Miyamoto International is in contact with local authorities and people on the ground to assess the situation and provide assistance.

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