Earthquakes

*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

 

What is an earthquake?
  • An earthquake is a tremendous movement of the earth’s crust.
  • Earthquakes cause rocks to move along fault lines.
 
 
Focus vs. Epicenter
  • The focus is the place inside the earth where an earthquake begins.
  • The epicenter is the place on the surface of the earth directly above the focus, where an earthquake is first felt.

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

Seismograph
  • 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.
 
 
 
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.
 








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