The New Yorker:
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.
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