What is an Earthquake? - Definition & Explanation

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  • 0:00 The Definition Of An…
  • 0:45 Rocks Under Stress
  • 2:10 What Makes The Shakes?
  • 3:38 Lesson Summary
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Lesson Transcript
Instructor: Charles Spencer

Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

Defining the word 'earthquake' is pretty easy. Understanding why they happen is a bit more complicated. This lesson will give you the information you need to understand how and why earthquakes happen, along with a few associated terms.

The Definition of an Earthquake

There are many scientific terms that are difficult to understand, but once in a while scientists actually use words that make sense! The word 'earthquake' literally means ground-shaking. Geologists also use the term in a broader sense when they talk about what happens below ground that produces the energy that causes the shaking.

An area near Union Square in San Francisco after the 1906 earthquake.
Earthquake damage 1906 San Francisco

As you can see in the above photograph of the aftermath of an earthquake in 1906, building collapse is one of the obvious results of ground shaking during an earthquake. You can see that the buildings in the foreground fell down, but the steel structures at left-center and right, which were buildings under construction at the time, remained standing - a clue that what a building is made of can determine whether it survives an earthquake.

Rocks Under Stress

You may think that rocks areā€¦ well, solid as a rock. They are, but they also can be squeezed, stretched and broken. The rocks in the earth's crust are constantly being stressed by forces exerted on them by natural processes, such as the movement of the earth's lithospheric plates or the pressure built up by magma beneath a volcano. Rocks can either change shape or break in response.

Try to reproduce those conditions yourself using a different kind of solid: a twig or thin strip of wood. If you slowly apply force downward on each end, the stick will likely change its shape by bending. But if you try to bend it too fast (in other words, apply force too quickly) the stick will more likely snap. You are applying force to the stick just as rocks are subjected to forces within the earth.

Although rocks aren't as flexible as a stick, they can change shape when force is applied slowly. (Another word geologists use to describe rocks changing shape is deformation). On the other hand, they will break if too much force is applied or if force is applied too rapidly. Breaks in rocks are called fractures.

Deep underground rocks are squeezed tightly together along fractures. If stress (the applied force) builds up to a point that it overcomes the friction between the rocks on either side of the fracture, the rocks will abruptly slide past each other. Once that shifting has occurred, the fracture is thereafter known as a fault.

What Makes the Shakes?

Think about how a rubber ball deforms when you squeeze it or how a rubber band deforms when you stretch it. What happens when you let it go? Sure enough, it snaps back to its original shape.

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