Chapter 2 Earthquakes (page 43)
I.
Earthquakes occur along faults (page 45)
A. Rocks move along faults
(page 45)
1. A fault is a fracture or break in the Earth’s lithosphere
where blocks of rock move past each other.
2. Rocks along faults may slide slowly and constantly or
stick and lock together.
3. Rocks bend under stress which is the force when one
object pushes or pulls against another object.
4. A sudden release of stress in the lithosphere causes an
earthquake which is the shaking of ground caused by
sudden movements of large blocks of rock along faults.
5. Most faults are found along tectonic plate boundaries.
6. Earthquake strength depends on two things
a. How much stress is built up
b. The distance rocks move along a fault.
7. 80% of earthquakes occur in the Pacific Ocean
a. “Ring of Fire”
b. San Andreas Fault – best known active fault in the
U.S.A.
8. All earthquakes occur in the lithosphere because rocks
are brittle here.
B. Faults are classified by How Rocks Move (page 47)
1. Scientists classify a fault according to how rocks on one
side move with respect to rocks on the other side.
2. There are 3 main types of faults.
a. Normal Faults – a block of rock above the fault plane
slides down, common near divergent boundaries.
FAULT PLANE
Block of rock sliding downward
PULL
PULL
b. Reverse Faults – a block of rock above the fault
plane moves up relative to the other block,
common in collision zones (convergent
boundaries).
FAULT PLANE
PUSH
PUSH
c. Strike-Slip Faults- blocks of rock moving
sideways on either side of the fault plane where
rocks scrape past each other (Transform
Boundaries) San Andreas Fault.
II. Earthquakes release energy (page 51)
A. Energy from Earthquakes travels through Earth.
1. Energy released by an earthquake can be compared
to ripples moving out from a rock thrown into a
pond.
2. Energy released by an earthquake also moves in all
directions- up down and side to side.
3. The energy released from an earthquake travels as
seismic waves.
4. All earthquakes start beneath the earth’s surface this
point under the surface where the rocks begin to
move is called the earthquake’s focus.
5. The term epicenter is used for the point on the
surface directly above the focus.
6. Typically a shallow focus will cause more damage
than a deep focus earthquake.
B. Waves and Energy
1. Waves are part of our daily lives, for example our
voices as well as music travels in sound waves.
2. On October 17, 1989 an earthquake stopped the
World Series Baseball Game at Candlestick Park in
San Francisco California. The stadium shook for
about 15 seconds but the waves continued to travel
and after 20 minutes they reached the other side of
the world.
C. Primary Waves (page 54)
1. The fastest waves
2. Also called “P” waves
3. Travel through earth’s crust
4. First to reach any location
5. Travel about 5 kilometers per second which is
about 3 miles per second!
6. Can travel through solid, liquid or gas
7. They “push” and “pull” material as they move
through it.
D. Secondary Waves
1. Second wave to arrive at any location.
2. Travel at about half the speed of P waves.
3. Also called “S” Waves
4. They shake material side to side and up and
down as they pass through.
5. Can move through solid rock but not liquid or
gas.
E. Surface Waves
1. Move along the earth’s surface not the interior.
2. Make the ground roll up and down and move
side to side.
3. Cause the most damage
4. Travel the slowest of all the waves.
F. Seismic Waves can be measured.
1. A seismograph is a tool that constantly
records ground movements.
2. There are seismographs that measure side to
side movement and other that measure up and
down movements.
3. A seismogram is the recording produced by
the seismograph.
4. Locating Earthquakes (page 57)
a. To locate an earthquake epicenter scientists must
have seismograms from THREE different seismic
stations.
b. The difference between arrival times of the waves
is calculated.
c. The greater the distance the greater the further the
quake is from a station
d. Three Circles are drawn around each station and
the point where the circles meet is the epicenter.
e. Seismograph data also helps scientists locate the
focus of an earthquake along with the earthquakes
magnitude (strength).
III. Earthquake damage can be reduced. (page 60)
A. Earthquakes can cause severe damage and loss of life.
1. Every year on average an earthquake with a
magnitude of 8 or higher strikes the earth.
2. Earthquake Magnitude (the strength of an
earthquake) is measured using scales.
a. The first earthquake scale was developed in
California during the 1930’s by Charles Richter
and Beno Gutenberg and is called the Richter
Scale.
b. The Richter Scale measures the magnitude of an
earthquake based on how fast the ground moves
at a seismic station.
c. The Moment Magnitude Scale is a new scale
more accurate scale that is preferred by
scientists.
d. On both the Richter Scale and the Moment
Magnitude Scale the value of 10 is usually at the
top.
e. On both scales one whole number is 32 times
stronger than the next.
3. Damage from Earthquakes
a. Structures (buildings) that are weakened by
earthquakes can collapse from aftershocks which
are smaller earthquakes that follow the more
powerful earthquake.
b. Fires can break out after an earthquake and cause
damage as in the 1906 San Francisco quake when
90% of the damage was caused by fires.
c. Earthquakes also cause major damage by affecting
the soil and other loose materials.
d. Landslides can occur as a result of earthquakes.
e. Earthquakes can cause soil liquification which is
when the shaking of the ground causes the soil to
act like water.
4. Damage from Tsunamis
a. Tsunami waves can cause waves as tall as a 20
story building.
b. Tsunamis can be triggered by earthquakes,
volcanoes or landslides.
c. Tsunamis are often called tidal waves but are not
caused by the same forces that produce tides.
d. Tsunamis can move very quickly at speed of
700km/hr (430 mi/hr)
e. In 1946 an Earthquake in Alaska caused a
Tsunami that swept across the Pacific Ocean and
reached Hawaii in less than 5 hours.
f. In 2004 a Tsunami in the Indian Ocean off the
coast of Sumatra killed over 280,000 people.
g. Because many earthquakes occur around the
Pacific Ocean Hawaii and other areas are likely to
be hit by Tsunamis.
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