What is an Earthquake? - Definition and Components

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  • 0:05 What Is an Earthquake?
  • 1:18 What Causes Earthquakes?
  • 2:26 The Components of an…
  • 3:48 Where Do Earthquakes Occur?
  • 5:04 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.

Earthquakes are well known for the damage and destruction they leave behind. But what is an earthquake? In this video lesson you will learn about what causes earthquakes and the various components involved in these powerful natural disasters.

What Is An Earthquake?

You know how when you have a really bad day and it seems like everything is going wrong? It's just one thing after another, and you get so frustrated that by the end of the day you are ready to explode from frustration. In fact, you may get so frustrated that you have a sudden outburst or release of all that built-up energy. And those unlucky enough to be around you when that happens should watch out!

Earth also builds up and releases energy in this way. When this occurs, we have an earthquake, which is the shaking of the ground when rock below Earth's surface breaks. Energy builds up underground and once it builds up enough it just can't hold back any longer, and we get the same explosive release of energy as a person might at the end of a long, bad day.

When the energy is released it radiates outward in all directions. As the energy travels toward Earth's surface from underground it shakes the ground, sometimes so much that it can cause damage above the ground. A seismograph is a machine that records ground movement from earthquakes. The information recorded tells us about the strength and speed of the energy traveling from the breaking point underground.

What Causes Earthquakes?

We now know what an earthquake is, but what exactly causes the rock underground to break? This is the result of stress along plate boundaries on Earth. The plates are dynamic, so they are always moving. Sometimes they move enough that they push into each other or pull apart. Compressional stress occurs when rocks are pushed together - they're pressed into one another. Tensional stress occurs when rocks are pulled apart - they're being stretched farther than they would be otherwise. Shear stress is when rocks slide past each other in opposite directions - it's like rubbing your hands together; they don't push or pull, but there's a lot of friction there!

When the stress gets to be too much, like all of the events building up on a bad day, the rock breaks and the ground begins to shake. If the rock splits into separate pieces, we get a fault, which is the line of fracture along the split rock. One of the most famous faults on Earth is the San Andreas Fault in California. This fault runs almost the entire length of the state of California, and is well known for causing frequent earthquakes in this area.

The Components of an Earthquake

Just like you may have some release of your tension at different times during your bad day, not all of the breaking of rock and energy release happens at once during an earthquake. There may be foreshocks and aftershocks, which are the energy released before and after the main quake.

The point underground where the actual breaking of the rock occurs is called the focus. It might help to remember this by thinking of it as the focal point of the earthquake. This is where the main event occurs underground. The point directly above the focus on the surface of Earth is called the epicenter. This is where the ground shaking is usually the strongest. From this point on the surface, the waves of energy from below ground begin to travel outward, so you can think of this as the central point of shaking above ground. Because the shaking is strongest here, this is also where the most damage usually occurs.

You may be surprised to learn that earthquakes don't actually kill people; it's all of the damage that occurs because of the shaking ground that causes people harm. Buildings crumble and fall, landslides and avalanches may be triggered, and roads and bridges can collapse. Falling objects are also quite likely to harm people during an earthquake, as items are shaken off of shelves, walls, and buildings.

Where Do Earthquakes Occur?

Since earthquakes are the result of plate boundary interactions, it's no surprise that most earthquakes occur in just a few specific areas on Earth. In fact, these areas are so famous for earthquakes and volcanoes, which also tend to occur where plates meet, it's been nicknamed 'The Ring of Fire.' There is a major plate boundary along California, which is where the San Andreas Fault is found. Along the west coast of South America we also find another major plate boundary, and quakes are quite common in this region as well. The Pacific Rim is another area with lots of shaking activity, so places like Indonesia are also very familiar with this dangerous natural disaster.

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