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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.

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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.When an earthquake occurs, the seismic waves radiate out in all directions from the focus, which is where the earthquake occurred underground. You can think of this as the focal point of the earthquake, where all the action starts. Body waves, which are the fastest seismic waves, begin to travel from deep underground. As the waves reach the surface and become surface waves, they’re not traveling as fast but they do inflict the most damage.

Directly above the focus on the surface is the epicenter, which you can think of as the center of the earthquake on the surface. Radiating out from the epicenter, surface waves move the ground up and down and side to side, which is what causes all the damage an earthquake produces.

Lesson Summary

Earthquakes are known for being deadly natural disasters. They occur without warning and can do some serious ground shaking. Tectonic plates are large areas of rock that make up the earth’s surface and they fit together like puzzle pieces.

They float on top of a fluid inner layer of the earth like ice on a pond and are always slowly moving around. As they move they bump against and slide past each other, sometimes getting stuck.An earthquake occurs when the two plates that are stuck suddenly break apart. The places along the boundaries where they bump and slip are called faults.

The plate edges get stuck here, but the plates themselves keep moving along like nothing has happened. It was once thought that the ground shaking caused faults to slip, but as explained with the theory of elastic rebound, we now know that the fault slipping is what causes the ground to shake during an earthquake.Just like when your friend lets go of the rope during a game of tug-of-war and the tension is quickly released, when the plates finally become ‘unstuck’ the tension is quickly released as waves of energy in the ground.This energy that gets sent through the ground during an earthquake is in the form of seismic waves. Body waves, which travel through the earth’s interior (or Earth’s ‘body’) are the fastest seismic waves.

They radiate outward in all directions from the focus, which is where the earthquake actually occurs underground. Directly above the focus on the surface is the epicenter, and from here, surface waves, which are slower than body waves but do more damage, radiate outward across the surface of the earth.

Learning Outcomes

After finishing this lesson, you might be able to:

  • Give details about the movement of the Earth’s tectonic plates
  • Identify the various faults around the planet and comprehend the theory of elastic rebound
  • Discuss the various ‘waves’ caused by an earthquake

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