*Source: Inside Earth by Prentice Hall Publishers, Science Explorer

What is stress?

• Stress is a force that acts on a rock to change its shape or volume.
• Tension - stretches rock so that it becomes thinner in the middle
• Compression - pushes rock together until it folds or breaks
• Shearing - pushes a mass of rock in two opposite directions

Faults

• A fault is a break in the rock of the crust.
• Most faults occur along plate boundaries, where the forces of plate motion push or pull the crust so much that the crust breaks.  There are three main types of faults.
• Normal Faults - tension in Earth's crust pulls rock apart
• Reverse Faults - compression pushes the rock of the crust together
• Strike-Slip Faults - the rock on either side of the fault slip past each other

• Seismic waves are vibrations in the earth, beginning at the focus of an earthquake.

• •A seismograph is an instrument used to record seismic waves.
• •Seismic waves cause the rocks in the crust to vibrate.

 

P-waves

• •The first waves to reach a seismograph are called primary waves, or P-waves.  These waves vibrate rocks back and forth.  P-waves compress and expand the ground like an accordion.

 

 

S-waves

• •Secondary waves, or S-waves, move more slowly than P-waves through rocks.  S-waves cause rocks to vibrate up and down and side to side.  S-waves do not move through liquids.

   Check out these web sites for moving animation of seismic waves.

 

http://www.classzone.com/books/earth_science/terc/content/visualizations/es1009/es1009page01.cfm?chapter_no=visualization

http://www.matter.org.uk/schools/Content/Seismology/pandswaves.html

 

 

L-waves or Surface waves

• •Long waves, or L-waves move along the surface of the earth. 
• •L-waves are the ones people feel during an earthquake. 
• •These waves often produce severe ground movements and cause damage. 
• Earthquakes can cause damage through shaking, liquefaction, aftershocks, and tsunamis.

 

What Causes Earthquakes?

• •There are two main causes of earthquakes.
• •Firstly, they can be linked to explosive volcanic eruptions; they are in fact very common in areas of volcanic activity where they either proceed or accompany eruptions.
• •Secondly, they can be triggered by Tectonic activity associated with plate faults. The majority of earthquakes world wide are of this type.

 

Seismologist

• A seismologist is a scientist who studies earthquakes.

  

Richter Scale

• •The Richter Scale is used to determine the magnitude (size) of an earthquake.

        

 

 

Three Major Earthquake Zones

• •Mid-Atlantic Ridge (a long range of mountains, volcanoes, and valleys deep underneath the ocean)
• •Mediterranean Sea
• •Pacific Ocean – The Pacific Ring of Fire
• Geologists can determine earthquake risk by locating where faults are active and where past earthquakes have occurred.