<P> The quake coincidentally occurred on the 32nd anniversary of the 1985 Mexico City earthquake, which killed around 10,000 people . The 1985 quake was commemorated, and a national earthquake drill was held, at 11 a.m. local time, just two hours before the 2017 earthquake . Twelve days earlier, the even larger 2017 Chiapas earthquake struck 650 km (400 mi) away, off the coast of the state of Chiapas . </P> <P> Mexico is one of the world's most seismically active regions, sitting atop several intersecting tectonic plates . The border between the Cocos Plate and North American Plate, along the Pacific Coast of Mexico, creates a subduction zone that generates large seismic events . Activity along the edges of the Rivera and Caribbean plates also generate seismic events . All together, these seismic forces cause an average of 40 earthquakes a day in Mexico . </P> <P> Mexico City is built on a dry lakebed with soft soil made up of sand and clay, which amplifies the destruction that major earthquakes cause . Loose sediments near the surface slow the shockwaves' speed from 1.5 miles (2,414 m) per second (8690.4 km / h) to roughly 150 feet (45, 72m) per second (164.592 km / h). This increases the shockwaves' amplitude, which causes more violent shaking . Deeper and denser soil layers increase amplified shockwaves' destructive duration . </P> <P> Less than two weeks before the Puebla earthquake, Mexico had been struck by an earthquake in Chiapas on 7 September, which killed almost 100 people . Despite its close timing, the Puebla earthquake was not an aftershock of the Chiapas event, as the epicenters were 650 km (400 mi) apart . </P>

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