Mexico City Was Built on an Ancient Lake Bed. That Makes Earthquakes Much Worse.<br />By DEREK WATKINS and<br />Area of ancientlake bed<br />Shock waves froma simulated earthquake<br />The earthquake that on Tuesday killed at least 145 people in Mexico City<br />and toppled dozens of buildings there was all the more destructive because of the city’s unusual position atop an ancient lake bed.<br />Strength of shaking in Mexico City<br />These actual readings confirm what the simulation shows: Tuesday’s earthquake<br />grew worse in the city as its waves moved through the ancient lake bed below.<br />During an earthquake, the looser sediments near the surface cause shock waves to slow down from about one<br />and a half miles per second to about 150 feet per second as they enter the valley.<br />The animation above, based on a model by Víctor Cruz-Atienza, a professor of geophysics at the National University<br />of Mexico, shows how the shock waves of a hypothetical earthquake near Mexico City would spread.<br />The map below, based on seismological readings taken at the National University of Mexico,<br />shows how violently the ground shook in Mexico City during Tuesday’s earthquake.
