Soft Soil Made the Mexico Earthquake Worse

Fagjun | Published 2017-09-30 11:57

Seismologists say that soft soil made the recent Mexico earthquake shake the country's capital like it was “built on jelly”.


Rescue operations at the Enrique Rebsamen school [AP/Marco Ugarte]



It was just less than two weeks before this latest earthquake that a powerful magnitude 8.1 earthquake hit Mexico, killing almost a hundred and rendering thousands more homeless. On Tuesday, September 19, another strong earthquake hit the country, which was still reeling from the effects of the first quake. This second quake was a magnitude 7.1, and it occurred 32 years to the day after Mexico's deadliest earthquake ever. Now, the death toll is rising even as rescue workers and volunteers scramble to search for survivors. Mexican authorities report that there have been at least 230 fatalities across southern Mexico.


Mexico has had a long and tragic history of destructive earthquakes. This latest earthquake is the second in the 12 days. According to seismologists, this history is due in part to the geology of Mexico and that of its capital city especially.



The Anatomy of the Mexico Earthquake


The 1985 Mexico earthquake [Photo by the U.S. Geological Survey Department of the Interior/USGS/I.D. Celebi]



The movement of tectonic plates caused the two latest earthquakes in the country. Right under Mexico is a complicated juncture of tectonic plates. These tectonic plates have been slowly colliding with each other for more than a million years. When the tension from the slow collisions finally breaks, an earthquake occurs.


What makes the tremors cause worse damage is that the soil underneath Mexico City is quite soft. "It's like being built on jelly on top of something that is wobbling," geophysics professor James Jackson said of the city.


Mexico City stands on land that had once been a lake bed. The soft soil worsens the effects of the earthquake instead of shielding the city from them. Hard rocks under the sediment send seismic shocks up through the soil, making the structures built on the surface shake even more intensely. The soft soil was also found to have been the culprit behind the devastation left by the deadly 1985 earthquake.


The US Geological Survey has also found that about 19 other earthquakes with a magnitude of over 6.1 have occurred within 155 miles of the epicenter of the latest Mexico earthquake. Other places in the world also experience the same earthquake effects due to the presence of soft soil.



Soft Soil in Other Places


Rescue operations in Mexico City [Photo by Yuri Cortez/AFP/Getty Images]



Soft soil is also the reason why 2015 Nepal earthquake, which killed almost 9,000 people and injured almost 22,000, was so destructive. Kathmandu, like Mexico City, also stands on what was once a lake bed. Other places, such as Los Angeles, the San Francisco Bay area, and Seattle also have the same soft soil, though their geology differs.


According to geologists, though the two earthquakes were just 12 days apart, the latest one is not an aftershock of the one before it. The September 19 earthquake occurred about 650 kilometers away from the other. Usually, aftershocks occur within 100 kilometers of the actual earthquake. So far, however, Tuesday's earthquakes have not had aftershocks. Usually, quakes like the Mexico earthquake are the type to have fewer aftershocks.

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