The New Yorker
The Strange Tectonic Coincidence of Mexico’s September Earthquakes ...
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.
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