Friday, September 22, 2017

Mexico City Was Built on an Ancient Lake Bed. That Makes Earthquakes Much Worse.

Area of ancient
lake bed
Shock waves from
a simulated earthquake
The New York Times | Source: Víctor Cruz-Atienza, National University of Mexico
The earthquake that on Tuesday killed at least 135 people in Mexico City and toppled dozens of buildings there was all the more destructive because of the city’s unusual position atop an ancient lake bed.
The animation above, based on a model by Víctor Cruz-Atienza, a professor of geophysics at the National University of Mexico, shows how the shock waves of a hypothetical earthquake near Mexico City would spread. Darker red areas indicate the strongest ground movement.
The shaking in this simulation is strongest in the low parts of the Valley of Mexico, which cradles the city, and it weakens when it meets the surrounding hills. That’s no coincidence. The darker red areas showing the strongest shock waves trace the shape of an ancient lake.
The Spanish built modern Mexico City over the ruins of the Aztec capital of Tenochtitlan, which they conquered in 1521. The Aztec city was on an island in Lake Texcoco, but the Spanish drained the surrounding lake over centuries and expanded Mexico City onto the new land.
Today, much of the city stands on layers of sand and clay — up to 100 yards deep — that used to be under the lake. These soft, water-laden sediments make the city uniquely vulnerable to earthquakes and other problems.
2 MI.
The city stands on soft
ancient lake sediments.
The lake bed
contains some hills...
...and is surrounded
by mountains.
The New York Times
During an earthquake, the looser sediments near the surface cause shockwaves to slow down from about one and a half miles per second to about 150 feet per second as they enter the valley. The slower waves grow in amplitude, similar to a tsunami approaching a coastline, and cause more violent shaking.
Worse still, the denser, deeper material below the looser sediments causes waves to linger in the valley, making the amplified shaking last longer.
The map below, based on seismological readings taken at the National University of Mexico, shows how violently the ground shook in Mexico City during Tuesday’s earthquake. Like the simulation map, the redder the area, the more violently the ground shook.
Lake bed
Strength of shaking in Mexico City
The New York Times | Source: The Institute of Engineering at the National University of Mexico
These actual readings confirm what the simulation shows: Tuesday’s earthquake grew worse in the city as its waves moved through the ancient lake bed below.
Mexico City is already a hot spot for earthquakes because vast chunks of the earth’s crust, called tectonic plates, are slowly smashing into one another nearby. Mexico rests on the North American Plate, and the Cocos Plate slides underneath it along the country’s southwestern coast.
Mexico City
The collision as one plate plunges below the other, a movement called subduction, releases huge amounts of energy, making earthquakes a common occurrence in Mexico. Unlike Tuesday’s, many of these earthquakes are small.
The unique geology of Mexico City’s basin can amplify earthquake waves to be a hundred times stronger than they would be otherwise, a phenomenon that Dr. Cruz-Atienza said is not matched anywhere else in the world.
Partly because of this amplification, earthquakes that happen relatively far away from Mexico City can still cause significant damage. A devastating earthquake in 1985 that killed as many as 10,000 people originated over 200 miles away, near the Pacific coast of Mexico.
The epicenter of Tuesday's quake was closer, around 50 miles away, but the map below shows that it shook Mexico City more violently than other areas that were a similar distance from the epicenter.
Mexico City
The New York Times | Source: United States Geological Survey
The 1985 earthquake prompted improvements to building codes that are thought to have lessened the damage on Tuesday. But earthquakes will always pose a unique threat to Mexico City, because of the geologic implications of the ancient lake bed that lies beneath it.

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