Scientific examinations of historical accounts suggest that up to 40 percent of Mexico’s population lives along a zone that is more seismically active than suspected.
According to the Anales de Tlatelolco, the earth cracked open in central Mexico on February 19, 1575. The ancient codex, composed around the time the Aztec Empire fell to Spanish conquistadors, features a story of a convulsion that lasted for up to five days, creating landslides and opening up a nearly three-mile-long scar in the ground.
This tale of earth-shattering catastrophe is part of a newly uncovered series of scientifically undocumented earthquakes that took place over the past 450 years in Mexico, seismologists report in a recent issue of the journal Tectonics. Specifically, these rumbles happened along a 620-mile-long stretch of volcanoes known as the Trans-Mexican Volcanic Belt, a region of snowy peaks and fiery eruptions that stretches from the Gulf of Mexico to the Pacific Ocean.
Since the dawn of instrumental seismology in the early 20th century, only a handful of powerful quakes have been recorded along this belt, leading many to suspect that it simply wasn’t that seismically hazardous. But if a region’s quake record is like a feature-length film, the era of modern monitoring is just a blip on the screen, says study coauthor Gerardo Suárez of the Universidad Nacional Autónoma de México.
“That hundred years of seismicity is like watching just two or three seconds of the movie,” Suárez says. By diving into historical records, too, scientists can “try to see a few more frames.”
That’s why Suárez and his team turned to Aztec codices and the accounts of Spanish missionaries. Their work suggests that Mexico is more or less rumble-ready along the entire length of the volcanic belt, which means this sleeping seismic serpent presents a looming threat. Today, 52 million people—or 40 percent of Mexico’s population—live along this belt, largely unaware of the restless geological giants below their feet.
A link to the past
The Trans-Mexican Volcanic Belt’s angry mountains, from Popocatépetl to Parícutin, owe their existence to a process called subduction, in which the tiny Rivera and titanic Cocos tectonic plates dive beneath the North American plate. The resulting confrontation and water leakage from the downgoing plates creates a zone of intense melting deep within Earth, which then forms chains of magma reservoirs within the crust that spawn volcanoes.
Oddly, though, the volcanoes here don’t run along the length of the subduction zone but are oblique to it, suggesting that the subducting Cocos slab is somehow highly warped. Despite this rocky chaos, the belt also seems to have a curious dearth of so-called crustal earthquakes, which are relatively shallow temblors far from the deep-seated subduction zone. These types of earthquakes are the ones that more often chew up the ground people walk on.