*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


  • 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.

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

  • 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.

  • 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.
  • 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.

The links in the area will let you leave the Fayetteville-Manlius School District site. The linked sites are not under control of the District and the District is not responsible for the contents of any linked site or any link contained in a linked site, or any changes or updates to such sites. The District is providing these links to you only as a convenience, and the inclusion of any link does not imply endorsement nor responsibility by the District of the content of the site.