Unconsolidated soil partly to blame for damage in Mexico City during earthquake

UTSA geologist explains source of 7.1 earthquake, damage

SAN ANTONIO – The 7.1-magnitude earthquake that shook Mexico City on Tuesday did not start there.

But thanks in part to the ground Mexico's capital city is built on, the earthquake didn't need to begin there.

The unconsolidated soil that sits under at least parts of the city doesn't do much to muffle the seismic waves from an earthquake, said Dr. Jaime Hincapie, a lecturer with the UTSA Department of Geological Sciences.

Related: Why Mexico City is so vulnerable to earthquakes

Hincapie used a plate of Jell-O as an analogy to show how the city's soil reacted when the seismic waves arrived from the epicenter, 76 miles away.

"If you hit it suddenly, the Jell-O is going to be shaking for some time. But if you have something stronger -- I don't know, maybe a brick -- if you shake it with the same strength at the same time you're not going to see the brick moving, shaking around. But the Jell-O will," Hincapie said.

Hincapie said the earthquake resulted from the Cocos and North American plates pushing against each other, building up stress and then finally slipping. The slip, 32 miles underground, sent out the seismic waves that has claimed at least 223 lives.

The waves can travel a long way, and can be detectable with sensitive equipment for days as they make their way around the globe, Hincapie said.

Local construction also plays a role in how much damage happens, and the destruction earthquakes cause is amplified by the surprise factor, Hincapie said.

"For a hurricane, sometimes we have at least 24, 36 hours, to gather our things and evacuate," Hincapie said. "Well, an earthquake hits you without notice."

Related: SA families anxiously wait to hear from loved ones after Mexico earthquake

That's not to say Mexico City residents should have been completely surprised by the earthquake.

"Those are the two same plates that caused that earthquake in 1985," Hincapie said. "So we knew the likelihood of an earthquake happening again in Mexico City was pretty high."

The question was just when it would happen.

It's the same question the city will face in the years to come.

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