A magnitude 7.1 earthquake occurred on September 19, 2017 at 20:14 pm Italian time (13:14 pm local time) in central Mexico.
Location of the magnitude 7.1 earthquake that occurred on September 19, 2017 at 20:14 Italian time in central Mexico.
The data received in real time in the seismic monitoring room of the National Institute of Geophysics and Volcanology (INGV) made it possible to locate the event: epicenter about 55 km from Puebla and 120 km from Mexico City, with geographical coordinates (lat , lon) 18.61, 98.43 at a depth greater than 50 km. It should be remembered that for earthquakes of this magnitude, the representation of the event with a point (the hypocenter) is not adequate, as it is a fault of approximately 50 km x 20 km that moves. Since this fault is located about 200 kilometers from the coast, a tsunami warning has not been issued for this earthquake, unlike the one of magnitude 8.0 on September 8, 2017.
Seismogram of the 7.1 magnitude earthquake that occurred on 19 September 2017 at 20:14 Italian time in Mexico, recorded by three seismic stations of the National Seismic Network.
Located within three major tectonic plates, Mexico is one of the most seismically active regions in the world. The relative motion of these crustal plates causes frequent earthquakes and occasionally volcanic eruptions. In fact, most of the Mexican mainland is located on the westward-moving North American plate.
Yesterday's earthquake occurred in a seismically very active region, where earthquakes are frequent due to the sliding of the oceanic Cocos plate under the North American and Caribbean plates.
Mexico has a long history of destructive earthquakes and volcanic eruptions. On September 19, 1985, a magnitude 8.0 earthquake claimed more than 9.500 lives in Mexico City.
Location of the epicenter of the September 19, 2017 earthquake and previous seismicity (source: USGS)
The earthquake of September 19, 2017 most likely occurred within the Cocos plate, where it deepens and flexes under the North American continent. Most of the strong earthquakes in the Mexican region occur along the Pacific coasts, including that of September 19, 1985 which caused thousands of victims, but there are also important precedents in the continental region, such as the event of June 15, 1999, magnitude 7.0.
Scheme of the "subduction" process of the Cocos plate beneath Mexico. Note the earthquake hypocenters within the plate where it flexes and deepens to the east (source: USGS)
The capital of Mexico, despite being located over a hundred kilometers away from the epicenter of the earthquake of September 19, 2017, suffered serious damage and total collapse of some buildings. The city is, in fact, built on an ancient lake basin whose sediments tend to amplify the duration and amplitude of the shaking, producing a sort of "pudding" effect. This is why the 1985 earthquake caused so many collapses and casualties in the capital despite being about 400 kilometers away from the epicenter.
Link to the INGVterremoti Blog:
https://ingvterremoti.wordpress.com/2017/09/20/terremoto-m-7-1-in-messico-del-19-settembre-ore-2014-italiane/