(PT) Earthquakes Lesson

Earthquakes

What are earthquakes and what causes them to occur? How often are earthquakes happening? Are they rare or do they happen more often than people realize?

Quick Review

The Earth's crust and the upper layer of its outer mantle are called the Lithosphere.

This Lithosphere is made up of tectonic plates – plates that float on the bottom layer of Earth's outer mantle, which is called the Asthenosphere.

There are a dozen or so major plates and a number of minor ones. These plates move constantly.

Watch this video as an introduction to Earthquakes.

Where Earthquakes Occur and Why

Many earthquakes occur along the edges of these plates, which scientists call plate boundaries. Other earthquakes occur along fault lines, which can be either near the edges of plates or in the middle of them.

Earthquakes can happen anywhere. They happen most often along plate boundaries or established fault lines.

The Earth has three primary earthquake zones:

• The Circum-Pacific Seismic Belt
• The Alpide
• The Mid-Atlantic Ridge

An earthquake is the shaking and trembling that results from the movement of rock beneath earth's surface. When Earth's plates move forces are created that squeeze or pull the rock in the crust.

Stress

STRESS: A force that acts on rock to change its shape or volume.

Because stress is a force, it adds energy to the rock. The energy is stored in the rock until the rock either breaks or changes shape.

There are three types of stress that occur in the crust.

• Shearing - stress that pushes a mass of rock in two opposite directions.
• Tension - stress that pulls on the crust, stretching rock so that it becomes thinner in the middle.
• Compression - stress that squeezes rock until it folds or breaks. One plate pushing against another can compress rock like a giant trash compactor.

DEFORMATION: Any change in the volume or shape of Earth's crust,

Q: How does deformation change Earth's surface?

A:  It causes it bend, stretch, break, tilt, fold, and slide. 

Earthquakes Along Plate Boundaries

Many earthquakes occur along the edges of oceanic and continental plates.

These plates are constantly bumping into each other, pulling away from each other or trying to slide past one another.

When two plates run into each other or try to slide past each other, an earthquake occurs.

That's because when the plates rub up against each other, they don't just slide smoothly, they stick a little. They keep pushing against each other but they can't move – they're stuck.

After a while, pressure builds up and the rocks break.

Earthquakes Along Fault Lines

Earthquakes can also occur along fault lines far from the edges of plates.

A fault is a crack in the earth where sections of a plate (or two plates) are moving in different directions.

There are three major kinds of faults.

Normal Faults

In a normal fault, one block of rock sitting at an angle is sliding downward and away from another block of rock, found lower. Tension forces in Earth's crust cause normal faults. The higher plate is called the hanging wall. The half of the fault that lies below is called the footwall.

These faults usually occur in areas where one plate is splitting apart or where two plates are pulling away from each other.

An example of normal faults occur along the rio Grande rift valley in New Mexico where two pieces of Earth's crust are diverging.

Strike-Slip Faults

Strike-slip faults occur between two plates that are sliding past each other.

California's San Andreas Fault is a Strike-slip Fault. It marks the plate boundary between the Pacific Oceanic Plate and the North American Continental Plate. It is over 650 miles long. A strike-slip fault that forms the boundary between two plates is called a transform boundary.

Many powerful earthquakes have taken place there.

Reverse Faults

Reverse faults occur when one plate is pushing into another. Compression forces produce reverse faults. Reverse faults are similar to Normal faults but the blocks of rock move in the opposite directions. The hanging wall of a reverse fault slides up and over the footwall. Reverse faults produced part of the Appalachian Mountains in the eastern United States.

They also occur where one plate is folding up because another plate is pushing against it.

Put it all together

Strike - slip faults are produced by shearing.

Normal faults are produced by tension.

Reverse faults are produced by compression.

FRICTION: The force that opposes the motion of one surface as it moves across another surface.

While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released. The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth's surface, they shake the ground and anything on it, like our houses and us!

Changing Earth's Surface

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