The September 19, 2017, M 7.1 earthquake in Central Mexico occurred as the result of normal faulting at a depth of approximately 50 km. Focal mechanism solutions indicate that the earthquake occurred on a moderately dipping fault, striking either to the southeast, or to the northwest.
The event is near, but not directly on, the plate boundary between the Cocos and North America plates in the region.
At the location of this event, the Cocos plate converges with North America at a rate of approximately 76 mm/yr, in a northeast direction.
The Cocos plate begins its subduction beneath Central America at the Middle America Trench, about 300 km to the southwest of this earthquake.
The location, depth, and normal-faulting mechanism of this earthquake indicate that it is likely an intraplate event, within the subducting Cocos slab, rather than on the shallower megathrust plate boundary interface.
Several people were killed when the most powerful earthquake to hit Mexico in decades tore through buildings, forced mass evacuations and triggered alerts as far away as Southeast Asia.
While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Normal-faulting events of the size of the September 19th, 2017 earthquake are typically about 50×20 km (length x width).
Over the preceding century, the region within 250 km of the hypocenter of the September 19th, 2017 earthquake has experienced 19 other M 6.5+ earthquakes. Most occurred near the subduction zone interface at the Pacific coast, to the south of the September 19 event. The largest was a M 7.6 earthquake in July 1957, in the Guerrero region, which caused between to 50-160 fatalities, and many more injuries.
In June 1999, a M 7.0 at 70 km depth, just to the southeast of the September 19, 2017 earthquake, caused 14 fatalities, around 200 injuries, and considerable damage in the city of Puebla (MMI VIII).
This earthquake occurred on the anniversary of the devastating 1985 M 8.0 Michoacan earthquake, which caused extensive damage to Mexico City and the surrounding region. That event occurred as the result of thrust faulting on the plate interface between the Cocos and North America plates, about 450 km to the west of the September 19, 2017 earthquake.
This earthquake also occurs 12 days after a M 8.1 earthquake offshore of Chiapas, in southern Mexico. The epicenter of the M 8.1 event is located about 650 km to the southeast of this quake. That earthquake also occurred as the result of normal faulting within the subducting Cocos Plate, at a depth of 50-70 km.
Located atop three of the large tectonic plates, Mexico is one of the world’s most seismically active regions. The relative motion of these crustal plates causes frequent earthquakes and occasional volcanic eruptions.
Most of the Mexican landmass is on the westward moving North American plate. The Pacific Ocean floor south of Mexico is being carried northeastward by the underlying Cocos plate. Because oceanic crust is relatively dense, when the Pacific Ocean floor encounters the lighter continental crust of the Mexican landmass, the ocean floor is subducted beneath the North American plate creating the deep Middle American trench along Mexico’s southern coast.
Also as a result of this convergence, the westward moving Mexico landmass is slowed and crumpled creating the mountain ranges of southern Mexico and earthquakes near Mexico’s southern coast. As the oceanic crust is pulled downward, it melts; the molten material is then forced upward through weaknesses in the overlying continental crust.
This process has created a region of volcanoes across south-central Mexico known as the Cordillera Neovolcánica.
The area west of the Gulf of California, including Mexico’s Baja California Peninsula, is moving northwestward with the Pacific plate at about 50 mm per year. Here, the Pacific and North American plates grind past each other creating strike-slip faulting, the southern extension of California’s San Andreas fault.
In the past, this relative plate motion pulled Baja California away from the coast forming the Gulf of California and is the cause of earthquakes in the Gulf of California region today.
Mexico has a long history of destructive earthquakes and volcanic eruptions. In September 1985, a magnitude 8.0 earthquake killed more than 9,500 people in Mexico City. In southern Mexico, Volcán de Colima and El Chichón erupted in 2005 and 1982, respectively. Paricutín volcano, west of Mexico City, began venting smoke in a cornfield in 1943; a decade later this new volcano had grown to a height of 424 meters.
Popocatépetl and Ixtaccíhuatl volcanos (“smoking mountain” and “white lady”, respectively), southeast of Mexico City, occasionally vent gas that can be clearly seen from the City, a reminder that volcanic activity is ongoing. In 1994 and 2000 Popocatépetl renewed its activity forcing the evacuation of nearby towns, causing seismologists and government officials to be concerned about the effect a large-scale eruption might have on the heavily populated region. Popocatépetl volcano last erupted in 2010.