In 1325, the Aztecs, until then a nomadic people, chose the site of their capital, Tenochtitlan, based on a prophecy that the location would be marked by an eagle eating a snake while perched on a cactus. That the cactus in question happened to sit on an island in a mucky lake did not, apparently, deter them from seeing it as a divine revelation; they went ahead and built a great city with grand temples and market squares on a tiny patch of land in a swamp. That metropolis is now Mexico City.

The cruel coincidence of there being a large earthquake in Mexico City on September 19th, the exact anniversary of the devastating magnitude-8.1 quake that killed at least five thousand people in the city in 1985, seems similarly preordained. But a closer look at the details partly dispels its statistical improbability. Neither quake was actually centered on Mexico City. The epicenter of the 1985 event was two hundred and twenty miles to the west, off the coast of Michoacán, while the recent quake was generated about seventy miles to the southeast, in the state of Puebla. Seismic waves emanate in all directions from their origins, and regions closer to these epicenters were hit harder than Mexico City. But, with a population of close to twenty million, the capital simply has more people and buildings likely to be affected—and the old lake sediments on which Tenochtitlan was built have an unfortunate tendency to magnify seismic waves, and sometimes to liquefy altogether.

A more interesting coincidence is the fact that the two large earthquakes that struck Mexico this month—the 7.1-magnitude Puebla event, on September 19th, and the 8.1-magnitude Gulf of Tehuantepec quake, twelve days earlier—were both exceptions to some general geophysical rules. Since the start of the new millennium, millions of Earthlings have been involuntary students in a rigorous experiential course on plate tectonics. There have been particularly punishing lessons about subduction zones, where old, dense ocean crust, often stuck in place for centuries, slides back into Earth’s mantle in a matter of seconds. The 9.1-magnitude Sumatra-Andaman earthquake, in 2004, and the 9.0-magnitude Tohoku quake, in Japan, in 2011, both of which spawned enormous tsunamis, occurred at such boundaries. The spectre of a similar catastrophe along the Cascadia subduction zone, in the Pacific Northwest, keeps many residents of Portland, Seattle, and Vancouver awake at night.

The west coast of Central America is another region where subduction has shaped the landscape. The imposing volcanoes of central Mexico and Guatemala—including Popocatépetl, which looms on the horizon in Mexico City—owe their existence to the subduction of the Cocos Plate beneath the North American Plate at the brisk clip of three inches per year. The tragic 1985 earthquake that devastated Mexico City was in fact triggered by slip on a segment of the Cocos. It would therefore be reasonable to assume that the two large earthquakes that jarred Mexico this month were classic subduction-zone events. In fact, neither was, and both were anomalies in several respects.

To begin with, the quakes emanated not from the top of the Cocos Plate, where the subduction actually happens, but from significantly deeper, within that plate, at depths of about thirty and forty-five miles, respectively. And, in both cases, the plate slipped in an unusual way. So-called megathrust events, such as the Sumatra and Tohoku quakes, typically involve slip with “reverse” motion, like an Aztec temple builder pushing a rock slab up a ramp. The two September earthquakes, though, were caused by “normal” motion, akin to an errant slab sliding back down the ramp. Normal slip occurs when rocks are being stretched rather than squeezed—the opposite of what one would expect in a subduction setting. In other words, normal faulting is not the norm in subduction.

The unexpected stretching is most likely a result of the unusual geometry of the Cocos Plate as it dives downward into the mantle. Geophysical data suggest that the subducting plate is not a smoothly inclined plane but instead a warped surface with two points of flexure. Like a glow stick that is activated by bending, the rock at these points is stretched, and may finally snap, in earthquakes with normal-sense slip.

In “The Flamingo’s Smile: Reflections in Natural History,” Stephen Jay Gould writes, “The human mind delights in finding pattern—so much so that we often mistake coincidence for profound meaning. No other habit of thought lies so deeply within the soul of a small creature trying to make sense of a complex world not constructed for it.” While the two September earthquakes had broadly the same underlying mechanical cause, it is not yet clear whether their occurrence within two weeks of each other is tectonically meaningful. They were centered more than four hundred miles apart, on completely different fault surfaces, so the later event cannot be considered a classic aftershock. But seismologists are not ruling out the possibility that there could be a distal connection between the tremors. Detailed analysis of small seismic events in the days between the two quakes may make it possible to distinguish causality from coincidence.

It might have been prudent for the founders of Tenochtitlan to exercise similar skepticism before launching into the construction of the city in a marsh. According to Aztec cosmology, Earth had come through four great ages, or Suns, before the present era. Each had ended in cataclysm. The Fifth Sun, our world, would be the last, destined to be destroyed completely in a series of great earthquakes. By coincidence, the Aztecs may unwittingly have created a self-fulfilling prophecy in their seismically star-crossed siting of what would become one of the world’s largest cities.