The large earthquake that shook central Mexico on September 7th, 2021, has left the country’s seismologists on alert due to the epicenter’s proximity to the Guerrero seismic gap, a tectonic belt that has not recorded a major earthquake for more than a century. More violent earthquakes have occurred in the area of the Mexican state of Guerrero, but on Tuesday, September 7th, the quake began at a point on the coast closest to Mexico City, which is why it was felt so strongly in the capital.
In addition, the earthquake began in an area of the country where a magnitude 8+ earthquake has been expected for over 100 years, explained Víctor Manuel Cruz-Atienza, a researcher at the Geophysics Institute of the National Autonomous University of Mexico (UNAM).
Every year, Mexico records an average of about 30,000 earthquakes, and Guerrero state is hit by about 25% of all national seismic activity. For this reason, the Guerrero seismic gap is something of an anomaly in not experiencing a 7+ magnitude earthquake for more than 110 years.
Cruz-Atienza explained that if the seismic gap were to rupture, it would cause a major earthquake, potentially stronger than 8.2 in magnitude. Tuesday’s earthquake could have repercussions on Guerrero’s seismic gap, he added, which continues to accumulate unreleased energy.
“It was a small earthquake of low to moderate magnitude, which tells us there are certain portions of the gap that are ready to break, but these are insufficient to trigger a larger earthquake yet,” Cruz-Atienza said.
The theory of plate tectonics suggests that the earth’s crust or lithosphere is from 5 to 65 km (3 to 40 miles) thick and divided into about a dozen large tectonic plates, tabular blocks that drift across the Earth in different directions and at various speeds (up to a few centimeters or inches per year), probably as a result of thermal convection currents in the Earth’s molten mantle. Most plates consist of a combination of both ocean floor and continent, though some are entirely ocean floor.
Each tectonic plate is moving relative to other plates. The movements are not independent because the plates smash into and scrape against one another. Areas in the center of tectonic plates, far from the boundaries, have relatively little seismic activity, but the boundaries between plates are highly seismically active, creating earthquakes and volcanoes. The level of seismic activity depends on the relative speed and direction of the plates at the boundary.
Almost all of Mexico sits atop the southwest corner of the massive North American plate (see map). Immediately to the south is the much smaller Caribbean plate. The North American plate extends westwards from the Mid-Atlantic Ridge, which runs through Iceland and down the middle of the Atlantic Ocean, to the western edge of North America. In a north-south direction, it extends from close to the North Pole as far south as the Caribbean.