The Science Behind Earthquakes: The how and why

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By: Charles Roop | WCTV Eyewitness News
September 20, 2017

TALLAHASSEE, Fla. (WCTV) -- Tuesday's earthquake near Mexico City was potent and deadly. But what exactly causes these quakes?

We first have to start with a basic setting of Earth's geology; the earth's crust is broken into plates - pretty much floating sheets of crust on the mantle.

But these plates move around.

These plates either run under each other, slide along each other or diverge from each other. But points of stress between the plates create faults. Some faults can be go for many miles, or go for short distances. Tension can build and release within these faults.

"And that rupture produces a shock wave, or a series of shock waves, known as earthquakes," Dr. James Tull, said geology professor and chair of the Department of Earth, Ocean and Atmospheric Science at Florida State University.

Those shock waves can vary in strength and magnitude. Tuesday's earthquake had a magnitude of 7.1. The depth of about 44 miles (70 km) was also a contribution in strength.

"That’s considered a relatively shallow earthquake," Tull said.

And the more shallow it is, the stronger the quake. Despite that the epicenter of the quake was nearly 100 miles from Mexico City, it still did significant damage. Tull said that Mexico has a "weird" system setup.

"It subducts (runs under another plate) at a shallow angle, almost flattens out and steepens up," he said. "Right under Mexico City, and that area of Mexico, the slab is almost flat."

Tull said that as the slab gets deeper, it begins to melt. This melting produces the volcanic chain, which is the volcanic arc near Mexico City. This volcanic arc goes beyond Mexico, and goes near the Pacific plate boundary. This area is also known as the Ring of Fire.

This ring is known for volcanoes and earthquakes.

There is another element that enhanced the damage in Mexico City: The city sits in an old lake bed. The lake existed when the Spanish arrived in current-day Mexico, according to Tull. The lake was drained, and is now dry. But the sediments that still sit there are rich in water.

"[The lake bed] makes it very susceptible to what we call ground motions - the actual seismic waves that move along the ground," Tull said.

Once the waves hit the lake bed, it practically turns it into jello. There is more ground motion with the sediment than bedrock. The local geology, according to Tull, can damage older masonry buildings. A lot of the modern buildings, Tull said, are able to withstand the tremors.

With two major earthquakes in Mexico in September, the Earth continues to show that it's constantly changing.

"The earth is a restless planet as we are finding out almost every day now," Tull said.

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