Mexico City sits on geologic bowl of jelly
WASHINGTON — The soft soil that lines the ancient lake bed on which Mexico City is built amplified the shaking from Tuesday’s earthquake and increased its destructive force, seismologists say as they try to better understand the quake that has killed more than 200 people.
Scientists are looking at other quirks of the magnitude 7.1 earthquake, including the absence of aftershocks and if it is somehow related to a distant, even stronger, Mexican temblor that struck 12 days earlier.
Mexico City is built on deep, soft soil that was once the bottom of a lake. Instead of cushioning the city from earthquakes, it exaggerates their effects, said James Jackson, a professor of geophysics at the University of Cambridge in England.
The vibrations, or seismic waves, from the hard rocks below are amplified by the soil and sediments above, making the surface — and the structures built on the surface — shake longer and more intensely.
“It’s like being built on jelly on top of something that is wobbling,” Jackson said.
The soft sediment was the major cause of damage in Mexico City’s 1985 earthquake, according to Cornell University geophysicist Geoffrey Abers.
The same deep soft soil effect worsened the deadly 2015 Nepal earthquake because Katmandu is also built on a dry lake bed, Jackson said. While the geology is not quite the same, Los Angeles, Seattle and the San Francisco Bay area have soft soil that can amplify seismic waves, U.S. Geological Survey seismologist Oliver Boyd said.
Scientists have been unable to detect any aftershocks as of Wednesday afternoon, said USGS seismologist Paul Earle. Usually an area can expect an aftershock one magnitude lower, which would be in the 6.1 range, he said.