Chapter 6 - Earthquakes  What is an earthquake? An earthquake is the vibration or movement of the earth produced by the rapid release of energy in wave form usually along a fault line.  These fault lines are typically found at plate boundaries.  The center of an earthquake is called the focus, while the place directly above the focus at the surface is called the epicenter.

Earthquakes: It’s Not My Fault!  At fault lines there is constant motion as one plate slides or moves against another.  Since rocks exhibit elastic rebound, this energy can build up a the rocks compress.  Eventually, the energy is released causing earthquakes while the rocks snaps back much like an elastic band.  The San Andreas Fault is probably the most studied fault.

Fault movement  Three types: 1. Fault creep – low gradual displacement, hardly noticeable. 2. Stick-slip – alternating periods of sticking followed by slippage causing rapid release of energy. 3. Fault scarp – vertical causing cliff formation

Absolutely Shocking!  The adjustments of the bedrock as it settles after an earthquake often results in aftershocks. These are usually weaker than the original quake.  Small minor quakes can precede a major quake and they are called foreshocks.

Seismo Stuff  Seismology – the study of earthquake waves.  Seismologists – the people who do the studying.  Seismographs – instrument used to record quake waves.  Seismograms – the actual print out from a seismograph. See page 160

You Don’t Want to Catch These Waves Dude!  There are two main groups of seismic waves: 1. Surface waves – these travel along the outer surface of the lithosphere. 2. Body waves – These are divided into two groups:A. Primary or P-waves B. Secondary or S-waves

P-Waves  P-waves are push-pull waves that cause the volume of the rock to temporarily change. They act on the rock in the same direction the wave is traveling.  They travel the fastest and they travel through all materials.  They are recorded first by seismographs.

S-Waves  S-waves travel slower than p-waves and they act on the rock perpendicular to the direction the wave is traveling.  They temporarily change the shape of the materials they travel through. However, fluids like liquids and gases do not respond to this type of change. Therefore, s-waves can not travel through them.