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Prentice Hall
EARTH SCIENCE
Tarbuck

Lutgens
Chapter
8
Earthquakes and
Earth’s Interior
8.1 What Is an Earthquake?
 An
earthquake is the vibration of
Earth produced by the rapid
release of energy
 Focus and Epicenter
• Focus is the point within Earth
where the earthquake starts.
• Epicenter is the location on the
surface directly above the focus.
8.1 What Is an Earthquake?

Faults
• Faults are fractures in
Earth where movement
has occurred.
Focus, Epicenter, and Fault
Slippage Along a Fault
San Andreas Fault
• Aerial view of the San
Andreas fault slicing
through the Carrizo Plain
in the Temblor Range
east of the city of San
Luis Obispo. (Photograph
by Robert E. Wallace,
USGS.)
8.1 What Is an Earthquake?
 Elastic Rebound Hypothesis
• Forces within Earth slowly
deform crustal rocks on both
sides of the fault
• Forces cause rocks to bend
and store energy (like in a
rubber band)
8.1 What Is an Earthquake?
• Eventually, resistance is
overcome and rocks slip at the
weakest point (focus).
•Slippage allows deformed rock
to snap back into place.
Elastic Rebound Hypothesis
8.1 What Is an Earthquake?
 Aftershocks
and Foreshocks
• An aftershock is a small
earthquake that follows the
main earthquake.
• A foreshock is a small
earthquake that often precedes
a major earthquake.
8.2 Measuring Earthquakes
Earthquakes are measured
by two things:
Magnitude and Intensity
8.2 Measuring Earthquakes
 Magnitude=how
much the
ground moves from an
earthquake
• Measured by a seismograph
(instrument that records earthquake
waves)
• One rating for the entire earthquake
Seismograph
8.2 Measuring Earthquakes
•
•
Used to use Richter scale, now we
use Moment Magnitude Scale
Moment Magnitude Scale includes
seismographic data, how much the
fault moved, area of surface break,
strength of broken rock
Earthquake Magnitudes
8.2 Measuring Earthquakes
 Intensity=earthquake’s
effects at different locations
(how much damage)
• Measured on Modified Mercalli Scale
• Different ratings for different areas
8.2 Measuring Earthquakes
Two main types of waves
produced by earthquakes
 Body
Waves – P & S waves
 Surface
waves
8.2 Measuring Earthquakes
P-waves
-
Primary, move fastest, push-pull
waves that push (compress) and
pull (expand) in the direction that
the waves travel
P-waves
8.2 Measuring Earthquakes
S-waves
- Secondary, move slower, don’t
go through liquids, move up and
down
S waves
8.2 Measuring Earthquakes
 Surface
waves=slowest waves,
can cause the greatest damage
•
can move up-and-down and sideto-side, rolling motion, like ocean
waves
Seismic Waves
8.2 Measuring Earthquakes
A
seismogram shows all three
types of waves
• The first way to arrive are the Pwaves, followed by the S-waves
and last are the Surface waves
Seismogram
8.3 Destruction from Earthquakes
Seismic Vibrations
 The
damage to buildings
and other structures from
earthquake waves depends
on several factors.
8.3 Destruction from Earthquakes
These factors include the
intensity and duration of the
vibrations, the nature of the
material on which the structure
is built, and the design of the
structure.
Earthquake Damage
Earthquake Risk in the United States
8.3 Destruction from Earthquakes
Liquefaction
 When
soil is saturated with
water during an
earthquake, it turns into
liquid that can’t support
buildings
Some Notable Earthquakes
8.3 Destruction from Earthquakes
Tsunamis
• Tsunami is the Japanese word for
“seismic sea wave.”
• Can occur when the vibration of a
quake sets an underwater landslide
into motion or when a slab of ocean
floor is displaced along a fault.
Movement of a Tsunami
8.3 Destruction from Earthquakes
Other Dangers
 Landslides
and Fire
8.3 Destruction from Earthquakes
Predicting Earthquakes
 Short-Range
•
Not successful.
 Long-Range
•
•
Predictions
Forecasts
Not accurate-make 30-100 year
predictions.
Study seismic gaps- an area along a
fault where there has not been any
earthquake activity for a long period of
time.