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The Causes of Earthquakes

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  • 0:07 What Causes an Earthquake?
  • 1:27 Faults and Elastic Rebound
  • 3:10 Energy Shakes the Ground
  • 4:41 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

In this video lesson you will learn what causes earthquakes and where they occur. You will also understand the theory of elastic rebound, and how this relates to movement of the earth's crust.

What Causes an Earthquake?

Earthquakes are one of the most dangerous natural disasters on Earth. This is because they strike with little or no warning, and can cause catastrophic damage. All that shaking comes from deep underground, but as you know, the surface shakes a lot too, which is where all the damage occurs. Buildings fall down, roads and bridges collapse, and land and mud come sliding down from hillsides.

But what causes all that shaking in the first place? Earthquakes happen deep underground along tectonic plate boundaries. Tectonic plates are what make up the earth's crust, its outermost layer. These plates fit together like puzzle pieces but they don't stay in one place. They're always moving because the part of the earth underneath them is like a fluid. And because the plates are sitting on top of this fluid like ice on top of a pond, they are not locked in place and are sort of floating about.

However, each plate is lined up pretty well with the other plates around it. So as they move, they create tension and pressure as they slide past and bump into each other, sometimes even sticking together. And though the plate boundary is stuck, the plate itself keeps moving and pulling the rest of the plate with it. Eventually, the pulling becomes too much and the plates suddenly break free from each other, causing an earthquake.

Faults and Elastic Rebound

The place where the bumping and sliding occurs along the plate boundaries is called a fault. Plate boundaries can have many faults, and most of the world's earthquakes occur along plate boundaries for this reason. The 'Ring of Fire' is an area where most of the world's earthquakes occur because it lines up with many of the plate boundaries. The San Andreas Fault in California is one of the most famous because it runs much of the length of the state and is very active. California's many earthquakes are a result of this dynamic plate boundary.

Imagine it like this: you're playing a game of tug-of-war with your friend, and you're both pulling pretty hard on the rope from each end. Suddenly, your friend lets go of the other side of the rope. All of the tension quickly leaves the rope and you go tumbling down to the ground. The release of energy during an earthquake is very much the same. The plates get stuck, building up tension. Suddenly, the tension releases and both plates break free. The tension that was built up gets sent through the ground, which is what causes all the shaking in the first place.

Scientists now know that the movement of the faults is what causes the ground to shake, but it was previously thought that the opposite was true: that the ground shaking caused faults to slip. The theory of elastic rebound explains that faults slip during an earthquake and cause ground shaking. This theory explains what you've just learned - that the plates keep moving even though the part stuck along the fault does not, which causes a sudden slip along the fault when it finally breaks free. The theory also explains what happens to the land around the fault once it does slip. The ground slowly gets deformed as it sticks, and then 'rebounds' back into shape once it breaks free.

Energy Shakes the Ground

You fall on the ground when your friend lets go of the rope because all of that built-up tension has to escape somewhere. The tension built up from a slipping fault also needs an outlet once the tension is released, but this gets sent through the earth as waves of energy known as seismic waves. There are two different types of seismic waves: body waves and surface waves. Body waves are seismic waves that travel through the ground under Earth's surface and surface waves are seismic waves that travel through Earth's surface. Makes sense, right? Body waves occur in Earth's 'body', while surface waves occur on the surface.

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