An informative article from Saturday, September 23rd (and excellent graphics after the jump) from the New York Times detailing why the tragedy is even worse than it would have been had it happened elsewhere.
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.
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.