Throughout this Lesson, we've been telling you what the Earth is made of and about some of its dynamic processes.
 
As you remember, 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 (at about the same speed as your fingernails grow). Some plates are under the ocean, some are under land and some are under both land and ocean.
 
This might be a good time to review Instruction 3 in this Lesson, which dealt extensively with plates and their boundaries.
 
To see two excellent diagrams of the Earth's tectonic plates, click:
http://www.iris.washington.edu/edu/onepagers/no2.pdf 
 
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 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.
 
When these rocks break, the sudden release of energy causes seismic waves -- and it's the seismic waves that make the earth shake.
 
 
The Focus and the Epicenter
 
The underground spot where the rock breaks is called the focus of the earthquake.
 
It is usually found at depths less than 70 km.
 
Shallow focus earthquakes start above 70 km, while deep focus earthquakes begin below 300 km.
 
The spot directly above the focus (on top of the ground or ocean) is called the epicenter of the earthquake.
 
 
Seismic Waves

As we said, it's seismic waves that make the earth shake.
 
There are two different types of seismic waves: body waves and surface waves.
 
 
Body waves can travel through the Earth's inner layers, while surface waves can only travel along the Earth's surface. Both kinds of waves radiate seismic energy.
 
 
Body Waves
 
There are two kinds of body waves: P waves and S waves.
 
A P wave (primary wave or compressional wave) can move through solids, liquids or gas. It pushes and pulls at the same time.
 
Sometimes animals can hear P waves, but humans just feel their thump and rattle. In fact, the first indication of an earthquake is often a P wave's sharp thud.
 
A P wave is the fastest kind of seismic wave, traveling at from 1 to 5 miles per second.
 
An S wave (secondary wave or shear wave) is the second wave you feel in an earthquake.
 
An S wave is slower than a P wave. It can move through rock but not through liquid. It travels through the Earth's upper and lower mantles -- but stops at the liquid layer in its core.

An S wave moves from side to side or up and down.
 
To make your own P waves and S waves, click:
http://www.geo.mtu.edu/UPSeis/making.html 
 
 
Surface Waves
 
Surface waves move along the Earth's surface like ripples move on water.
 
There are two kinds of surface waves: Love waves and Rayleigh waves. They are named for the mathematicians who modeled them -- Love and Rayleigh.
 
A Love wave is the fastest kind of surface wave. It moves the ground from side to side.
 
A Raleigh wave rolls along the ground like a wave rolls across the ocean. Because it rolls, it moves the earth both up and down and from side to side (in the same direction the wave is moving).
 
Rayleigh waves cause most of the shaking you feel during an earthquake.
 
 
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 is sliding downward and away from another block of rock.
 
These faults usually occur in areas where one plate is splitting apart or where two plates are pulling away from each other.
 
 
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.
 
Many powerful earthquakes have taken place there.
 
 
Reverse Faults
 
Reverse faults occur when one plate is pushing into another.
 
They also occur where one plate is folding up because another plate is pushing against it.
 
 
Where Earthquakes Happen
 
Earthquakes can happen anywhere.
 
But they happen most often along plate boundaries or established fault lines.
 
The Earth has three primary earthquake zones:
 

The Circum-Pacific Seismic Belt
 
This belt, along the rim of the Pacific Ocean, is where 80% of the world's biggest earthquakes occur.
 
It extends from Chile northward along the South American coast through Central America, Mexico, the West Coast of the United States and Southern Alaska through the Aleutian Islands to Japan, the Philippines, New Guinea, the South Pacific and New Zealand.
 
This belt was responsible for 70,000 deaths in Peru in 1970 and 65 deaths in California in 1971 (along with billions of dollars of damage).
 
 
The Alpide
 
The second most important earthquake belt is called The Alpide.
 
It extends from Java to Sumatra through the Himalayas, the Mediterranean and out into the Atlantic.
 
This belt accounts for about 17% of the world's large earthquakes, including some of the most destructive -- like the Iranian quake that took 11,000 lives in 1968.
 
 
The Mid-Atlantic Ridge
 
The third prominent earthquake belt, the mid-Atlantic Ridge, follows the submerged mountain range we told you about in an earlier Instruction.
 
This is where earthquakes of the smallest magnitude occur.
 
 
Measuring Earthquakes
 
We'll tell you about how earthquakes are measured -- and about some of history's most significant earthquakes -- in our next Instruction.

for Students, Parents and Teachers

Now let's do Practice Exercise 3-5 (top). Choose printer friendly or online exercises. Printer friendly version requires the Adobe Acrobat Reader 5. Click HERE to obtain a free copy.