Transcript chapter 12 – earthquakes

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Chapter 12 Standard
3d. Students know why and earthquakes
occur and the scales used to measure their
intensity and magnitude.
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CHAPTER 12 –
EARTHQUAKES
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I. 11.1 FAULTS
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A. Stress and faults
1. The Earth’s lithosphere is constantly
moving. During this move, rocks in the
crust are squeezed, stretched, and twisted
resulting in force being exerted onto the
rocks. The force on each unit of area is
called stress.
2. Stress occur in the Earth’s crust at
divergent, convergent and transform
boundaries.
3. Stress can cause rocks to break. There
are two categories of rock breakage:
a. Fracture is a break where there is no
movement of the surrounding rock.
b. Fault is a break where there is movement of
the surrounding rock.
B. There are different types of faults.
1. Normal fault
a. Type of fault in which the hanging wall
moves downward relative to the footwall
2. Reverse fault
a. Type of fault in which the hanging wall
moves upward relative to the footwall
3. Thrust fault
a. Type of reverse fault in which the fault plane
is at a low angle or nearly horizontal.
4. Strike-Slip fault
a. Rock on either side of the fault plane slides,
or slips, horizontally
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footwall
hanging wall
hanging wall
hanging wall
footwall
Types of Faults
footwall
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II. 12.1 HOW AND WHERE
EARTHQUAKES HAPPEN
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A. What are earthquakes?
1. Earthquakes are movements of the
ground that are caused by a sudden
release of energy when rocks along a fault
move.
2. The rocks may move suddenly and without
warning.
B. Why do earthquakes happen?
1. Geologists think earthquakes are a result of
elastic rebound.
2. Elastic rebound is the sudden return of
elastically deformed rock to its undeformed
shape.
a. When plates move, rocks on either side of a fault
bend or deform. Rocks build up stress when they
bend. This stores energy in the rock.
b. When enough stress builds, rocks release their
accumulated energy and rebound or spring back to
their original shape.
c. This “rebound” results in an earthquake.
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Elastic Rebound
C. Anatomy of an earthquake
1. The focus is the location within the Earth
where the first motion of the earthquake
occurs.
2. The epicenter is the point on the Earth’s
surface directly above the focus.
3. Vibrations start from the focus and radiate
outward and eventually towards the surface.
4. Earthquakes with the most damage usually
have shallow foci because there is still a lot of
energy by the time the vibrations reach the
surface.
Anatomy of an Earthquake
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(energy)
D. Rocks release energy in the
form of seismic waves.
1. Seismic waves are the form of vibrations
released from rocks during an earthquake.
2. The waves travel outward in all directions.
3. There are two main types of waves.
E. The two main types of seismic waves are body waves and
surface waves.
1.
Body waves are waves that travel through the body of a
medium. There are two types:
a. P waves (primary or compression waves)
i. fastest of the body waves
ii. the first wave to be detected
iii. causes rocks to move back-and-forth that is parallel to
the movement of the waves
iv. can travel through solids, liquids and gases
v. moves faster through rigid material
b. S waves (secondary or shear waves)
i. slowest of the body waves
ii. arrives after P waves
iii. causes rocks to move in a side-to-side
direction that is perpendicular to the waves
iv. can travel only through solids
2. Surface waves are waves that travel along
the surface rather than through the middle
and are the slowest-moving waves. There
are two types:
a. Love waves
i.
cause rock to move side-to-side and perpendicular
to the waves
b. Rayleigh waves
i.
cause ground to move with a rolling motion
Types of Waves
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picture.
F. Seismic waves help scientists learn
about the Earth’s interior.
1. The speed and direction of P waves and S
waves are affected by the material they
travel through.
2. By studying the speed and direction of
seismic waves, scientists have been able to
locate the layers of the Earth.
3. Seismic waves also show shadow zones
which are location on Earth’s surface where
no body waves can be detected since the
Earth’s interior are not uniform in rigidity.
Shadow Zones
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G. Earthquakes and plate tectonics
1. Earthquakes are the result of stresses in Earth’s lithosphere.
There are three main tectonic plate boundaries where stress
on the rock is greatest.
a. convergent oceanic plates
i. Two plates are moving towards each other with one
plate subducting or sinking under the other plate
ii. As the top plate scrapes across the bottom plate,
earthquakes occur.
b. divergent oceanic plates
i. Two plates are moving away from each other.
ii. The spreading motion causes earthquakes.
c. continental plates
i. Two continental plates are moving away, towards or in
opposite directions to each other.
ii. Any of those movements causes earthquakes.
H. Earthquakes and fault zones
1. At some plate boundaries, there are fault
zones which are regions of closely spaced
faults.
2. Some earthquakes are not caused by
movements along plate boundaries but
can be caused by an ancient fault zone
deep within the Earth’s crust.
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Summary (two paragraphs) – Page 4
Stress is ____________________________. Fault is
_____________________________. The different
types of faults are (choose any 2) ___________ and
________. Earthquakes are ___________________.
Scientists believe earthquakes happen because of
___________________. The focus is the
__________________whereas the epicenter is the
_____________________.
The two types of seismic waves are ___________
and ____________. There are two types of body
waves which are _________ and _________. There
are two types of surface waves which are _________
and ___________. Seismic waves help scientists
learn
about the Earth’s ___________.
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III. 12.2 STUDYING
EARTHQUAKES
A. Earthquakes are recorded.
1. Seismology is the study of earthquakes
and seismic waves.
2. Seismograph is an instrument that records
and detects vibrations in the ground.
a. One device records the vertical motion of the
ground.
b. A second device records the horizontal eastwest motion.
c. A third device records the horizontal northsouth motion.
3. Seismogram is the recording or tracing of
the earthquake motion.
a. P waves are recorded first since they are the
fastest.
b. S waves are recorded second.
c. Rayleigh and Love waves are recorded last.
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picture.
B. Earthquakes (epicenters) are
located using two techniques.
1. Scientists look at the arrival times of both
the P waves and S waves.
a. The longer the lag time between the two, the
further the epicenter.
2. Scientists use computers to perform
complex triangulations.
a. Information is used from several seismograph
stations to pinpoint the location of the
epicenter.
C. Earthquakes are measured by magnitude
and intensity.
1. Magnitude is the measure of the strength of an
earthquake. The larger the magnitude, the greater
the energy of the earthquake. Magnitude has two
scales:
a. The Richter scale measures the ground motion
from an earthquake to find the earthquake’s
strength.
b. Moment magnitude is a measurement of
earthquake strength based on three factors:
i. size of the area of the fault that moves
ii. the average distance that the fault blocks move
iii. the rigidity of the rocks in the fault zone.
2. Intensity is the
measure of the
effects of an
earthquake.
Intensity has one
scale:
a. Mercalli scale
expresses intensity
and provides a
description of the
effects of each
earthquake intensity.
IV. 12.3 EARTHQUAKES AND
SOCIETY
A. Earthquakes cause much damage.
1. Earthquakes with epicenters on the ocean
floor may cause a giant ocean wave called
a tsunami.
a. can cause much damage when the waves
crashes into land
2. Buildings whose walls are too weak to
withstand the swaying motions of an
earthquake may collapse completely.
B. There are earthquake safety measures.
1. Before an earthquake
a. Be prepared by keeping the appropriate
supplies on hand.
2. During an earthquake
a. Stay calm while moving to a safer position.
3. After an earthquake
a. Be cautious and check for hazards.
C. Can earthquakes be predicted?
1.
Scientists study past earthquakes to predict where future
earthquakes are most likely to occur. However, there is
currently no reliable way to predict exactly when or
where an earthquake will occur.
2. Scientists have identified zones of low earthquake
activity, or seismic gaps.
a. A seismic gap is an area along a fault where
relatively few earthquakes have occurred recently but
where strong earthquakes occurred in the past.
3. Some earthquakes are preceded by foreshocks but this
is not a successful method to predict earthquakes.
4. Scientists continue to study seismic activity so they may
one day make accurate forecasts and save more lives.
Areas with earthquake hazard levels
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Summary (one paragraph) – Page 6
Seismology is _________________________.
A seismogram is _______________________.
P waves are recorded ______ since they are
the __________. ___ waves are the second.
Earthquakes are measured by ___________
and ____________. Magnitude is __________
___________________ while intensity is
_____________________.
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Notes Points
Name __________________________________ Period _______
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Chapter 12 Notes
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Copy Notes: pictures + color + label
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J. Nguyen – Physical Science
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