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.