Download IM_chapter8 Earthquakes and Interior

Instructor’s Manual
GEOL
Chapter 8
Earthquakes and Earth’s Interior
Chapter 8
Earthquakes and Earth’s Interior
Table of Contents
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Chapter Outline
Learning Outcomes
Chapter Summary
Lecture Suggestions
Enrichment Topics
Common Misconceptions
Consider This
Key Terms
Internet Sites, Videos, Software, and Demonstration Aids
Chapter Outline
Introduction
LO1 Elastic Rebound Theory
LO2 Seismology
LO3 Where Do Earthquakes Occur, and How Often?
LO4 Seismic Waves
LO5 Locating an Earthquake
LO6 Measuring the Strength of an Earthquake
LO7 What are the Destructive Effects of Earthquakes?
LO8 Earthquake Prediction
LO9 Earthquake Control
LO10 What Is Earth’s Interior Like?
LO11 The Core
LO12 Earth’s Mantle
LO13 Earth’s Internal Heat
LO14 Earth’s Crust
Learning Outcomes
After reading this unit, the students should be able to do the following:
LO1 Explain Elastic Rebound Theory
LO2 Describe seismology
LO3 Identify where earthquakes occur, and how often
LO4 Identify different seismic waves
LO5 Discuss how earthquakes are located
LO6 Explain how the strength of an earthquake is measured
LO7 Describe the destructive effects of earthquakes
LO8 Discuss earthquake prediction methods
LO9 Discuss earthquake control methods
LO10 Describe Earth’s interior
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LO11
LO12
LO13
LO14
Chapter 8
Earthquakes and Earth’s Interior
Examine Earth’s core
Examine Earth’s mantle
Describe Earth’s internal heat
Examine Earth’s crust
Chapter Summary
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Earthquakes are vibrations caused by the sudden release of energy, usually along a fault.
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The elastic rebound theory is an explanation for how energy is released during
earthquakes. As rocks on opposite sides of a fault are subjected to force, they accumulate
energy and slowly deform until their internal strength is exceeded. At that time, a sudden
movement occurs along the fault, releasing the accumulated energy, and the rocks snap
back to their original undeformed shape.
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Seismology is the study of earthquakes. Earthquakes are recorded on seismographs, and
the record of an earthquake is a seismogram.
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An earthquake’s focus is the location where rupture within Earth’s lithosphere occurs and
energy is released. The epicenter is the point on Earth’s surface directly above the focus.
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Approximately 80% of all earthquakes occur in the circum-Pacific belt, 15% within the
Mediterranean–Asiatic belt, and the remaining 5% mostly in the interior of the plates and
along oceanic spreading ridges.
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Chapter 8
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The two types of body waves are P-waves (primary waves) and S-waves (secondary
waves). P-waves are the fastest seismic waves and travel through all materials, whereas
S-waves are somewhat slower and can travel only through solids. P-waves are
compressional (expanding and compressing the material through which they travel),
whereas S-waves are shear (moving material perpendicular to the direction of travel).
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Rayleigh (R-waves) and Love waves (L-waves) move along or just below Earth’s
surface.
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An earthquake’s epicenter is determined using a time–distance graph of the P- and Swaves to calculate how far away a seismic station is from an earthquake. The greater the
difference in arrival times between the two waves, the farther away the seismic station is
from the earthquake. Three seismographs are needed to locate the epicenter.
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Intensity is a subjective, or qualitative, measure of the kind of damage done by an
earthquake. It is expressed in values from I to XII in the Modified Mercalli Intensity
Scale.
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The Richter Magnitude Scale measures an earthquake’s magnitude, which is the total
amount of energy released by an earthquake at its source. It is an open-ended scale with
values beginning at 1. Each increase in magnitude number represents about a 30-fold
increase in energy released.
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The seismic-moment magnitude scale more accurately measures the total energy released
by very large earthquakes.
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The destructive effects of earthquakes include ground shaking, fire, tsunami, landslides,
and disruption of vital services.
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Seismic risk maps help geologists in determining the likelihood and potential severity of
future earthquakes based on the intensity of past earthquakes.
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Earthquake precursors are changes preceding an earthquake and include seismic gaps,
changes in surface elevations, and fluctuations of water levels in wells.
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A variety of earthquake research programs are underway in the United States, Japan,
Russia, and China. Studies indicate that most people would probably not heed a shortterm earthquake warning.
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Because of the tremendous energy involved, it seems unlikely that humans will ever be
able to prevent earthquakes. However, it might be possible to release small amounts of
the energy stored in rocks and thus avoid a large earthquake and the extensive damage
that typically results.
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Earth has an outer layer of oceanic and continental crust below which lies a rocky mantle
and an iron-rich core with a solid inner part and a liquid outer part.
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Chapter 8
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Studies of P- and S-waves, laboratory experiments, comparisons with meteorites, and
studies of inclusions in volcanic rocks provide evidence for Earth’s internal structure and
composition.
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Density and elasticity of Earth materials determine the velocity of seismic waves.
Seismic waves are refracted when their directions of travel change. Wave reflection
occurs at boundaries across which the properties of rocks change.
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Geologists use the behavior of P- and S-waves and the presence of the P- and S-wave
shadow zones to estimate the density and composition of Earth’s interior, as well as to
estimate the size and depth of the core and mantle.
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Earth’s inner core is probably made up of iron and nickel, whereas the outer core is
mostly iron with 10–20% other substances.
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Peridotite, an igneous rock composed mostly of ferromagnesian silicates, is the most
likely rock making up Earth’s mantle.
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Oceanic crust is composed of basalt and gabbro, whereas continental crust has an overall
composition similar to granite. The Moho is the boundary between the crust and the
mantle.
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The geothermal gradient of 25°C/km cannot continue to great depths; within the mantle
and core, it is probably about l°C/km. The temperature at Earth’s center is estimated to be
6500°C.
Lecture Suggestions
1. The difference between P- and S-wave motions can be effectively illustrated using a
Slinky.
2. Check out the earthquake database of the National Earthquake Information Center,
maintained by the U.S. Geological Survey, in Golden, Colorado (see information under
Internet Sites, Videos, Software, and Demonstration Aids below). Collect data on
numbers and magnitudes of earthquakes that have occurred in the past week or two. The
class will be interested to see how many quakes occur every day.
3. Demonstrate density differences among iron (meteorite, if available), peridotite, basalt,
and granite to suggest how concentric layers in Earth’s interior may have developed.
4. Bring to class samples of the interior of Earth: granite and basalt for the two types of
crust, peridotite for the mantle, and, if possible, an iron–nickel meteorite for the core.
Point out, of course, that these are not entirely accurate representations.
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Enrichment Topics
Topic 1. What a Unit Means. This chart, from the USGS, is useful for students who want to
check on what a unit oar partial unit change in magnitude means for the displacement and
energy of an earthquake. http://neic.usgs.gov/neis/eqlists/eqstats.html
Magnitude
Change
1.0
0.5
0.3
0.1
Ground Motion Change
(Displacement)
10.0 times
3.2 times
2.0 times
1.3 times
Energy Change
About 32 times
About 5.5 times
About 3 times
About 1.4 times
Topic 2. Intraplate Earthquakes. The great magnitude New Madrid earthquakes of 1811–
1812, 7.5 to 8, occurred in the middle of the North American Plate far from any plate
boundaries. The faulting in this region appears to be the result of compressional forces that
occur as the plate absorbs strain from westward motion and convergence with the Pacific
Plate. However, this fault zone is developed within, and made possible by, tectonic events
which occurred during the rifting of Pangaea, and possibly the events which attended the
breakup of a supercontinent some 600 million years ago. Rifting of the earlier supercontinent
produced a zone of down-faulted crust, which underlies a structure known as the Illinois
basin and which remained active for some 300 million years. A second rifting event—that of
Pangaea—produced a zone of down-faulted crust known as the Reelfoot, Delaware, and
Oklahoma Rifts to the immediate south and west—in the vicinity of New Madrid. It is this
rift zone that became the axis of the Mississippi River drainage system. Scientists now
estimate that there is a 7% to 10% chance of a repeat quake and a 25 to 40% chance of a
magnitude 6 quake in the next 50 years.
Topic 3. More about the Boxing Day Tsunami. On December 26, 2004, the most massive
earthquake in 40 years, measuring 9.0 on the Richter Scale, struck 60 miles off the
northwestern coast of Sumatra. The quake released the stresses that build up as the India plate
subducts into the Sunda Trench beneath the overriding Burma plate. The earthquake was the
result of thrust faulting, and it had a shallow focus. About 1200 km of the plate boundary
slipped, and the average displacement on the fault was about 15 meters. The seafloor above
the fault was likely uplifted several meters, and it was this vertical motion that displaced the
trillions of tons of water above it and generated the tsunami. Several aftershocks, measuring
about 5.0 on the Richter Scale, followed. The tsunami sped across the sea surface, barely
noticeable in open water, but grew as it went up the shoreline. Within one hour, the wave
slammed into Sumatra, and within two, it reached India. The tsunami reached Africa about
eight hours later. The tsunami struck in separate waves, the third being the most massive. In
all, eleven countries around the Indian Ocean suffered damage. Wave heights were greatest
near the earthquake epicenter and decreased with distance, ranging from over 33 feet (10 m)
tall at Sumatra down to 13 feet (4 m) at Sri Lanka, Thailand, and Somalia. The death toll
from the tsunami is around 230,000, making it the most deadly earthquake in recorded
history. About 1.2 million people were left homeless.
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Topic 4. The Origin of Deep Earthquakes. The focus of most earthquakes is less than 50
km depth because at greater depths rock deforms plastically and does not generate
earthquakes. Yet the focus of some quakes is deeper. These quakes occur beneath subduction
zones where the down-going slab is relatively old and cold; since it is colder than the
surrounding mantle, it behaves as a brittle solid. Geologists cannot agree on the mechanism
for these quakes. The favored possibility is that, as temperatures and pressures in the slab
increase, minerals in the slab transform from one phase to another, and the quakes are due to
the transformation. For example, olivine, which is a major mineral in the down-going oceanic
crust, transforms into its denser polymorphs (same composition, different structure).
However, in some slabs, this transformation takes place long after the slab has reached the
temperature and pressure conditions in which the polymorphs are stable. In this case, the
transformation occurs as a shear instability on transform faults, which causes earthquakes.
The evidence for the phase transformation model is that deep earthquakes first appear at
about 325 km depth, where the phase change is expected to begin, and end at about 700 km
depth, where another phase changes starts. Also, in this model, detached slab fragments can
host earthquakes, a phenomenon that is seen. Other possible explanations include that deep
earthquakes occur where faulting is by rapid creep. Alternatively, deep earthquakes occur by
brittle fracture, which is due to the release of water from mineral structures. None of these
models is supported by all the data. Science, October 29, 1999:
http://geology.about.com/od/earthquakes/a/aa_deeEQs.htm
Topic 5. Imaging Earth’s Interior with Tomography. There are many beautiful images
available on the Internet that show the results of seismic tomography studies. To find some,
Google seismic tomography, and choose images. Here is one in which scientists have drawn
in features of a subduction zone: http://www.whoi.edu/cms/images/lstokey/2005/1/v42n2detrick2en_5301.jpg. And here is another, showing blobs in the mantle:
http://www.geo.cornell.edu/geology/classes/Geo101/graphics/s12fsl.jpg.
Common Misconceptions
Misconception: When “the big one” hits, Southern California will break away from the rest of
the continent and fall into the ocean.
Fact: Although the State of California straddles the boundary between two plates of Earth’s
crust, and movement along this boundary (the infamous San Andreas Fault zone) is
responsible for many large earthquakes, nonetheless, this boundary is a transform fault. Thus,
the plates on either side of the transform fault are sliding past, and tectonic forces are really
fighting against separation of the plates. Los Angeles will someday be a suburb of California
though.
Misconception: A one unit increase on the Richter Magnitude Scale means a 10-fold increase
in size.
Fact: It is true that each whole-number increase in magnitude represents a 10-fold increase in
wave amplitude, but each magnitude increase of one unit corresponds to a roughly 30-fold
increase in the amount of energy released.
Consider This
1. Does the observation that lithospheric plates drag the underlying mantle along as they
move support either a slab-pull or ridge-push mechanism of plate movement?
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2. What tests could you design to determine whether mantle convection takes place only in
the upper portion or if it involves the entire mantle?
3. Suggest a reason why, as seismic tomography has indicated, there are large pockets of hot
material beneath the interiors of continents rather than along the spreading ridges, and
why there are zones of cold rock that encircle and extend inward beneath the continents
on their Pacific margins.
Key Terms
discontinuity
earthquake
elastic rebound theory
epicenter
focus
geothermal gradient
intensity
Love wave (L-wave)
magnitude
Modified Mercalli Intensity Scale
Mohorovicic discontinuity (Moho)
P-wave
P-wave shadow zone
Rayleigh wave (R-wave)
Richter Magnitude Scale
seismograph
seismology
S-wave
S-wave shadow zone
tsunami
Internet Sites, Videos, Software, and Demonstration Aids
Internet Sites
1. As the World Churns. NASA Jet Propulsion Laboratory, California Institute of
Technology, News & Features: http://www.jpl.nasa.gov/news/features.cfm?feature=2420
2. National Earthquake Information Center: http://earthquake.usgs.gov/regional/neic/
To determine the size and location of all destructive earthquakes worldwide and to
disseminate the information to interested parties.
3. USGS Earthquake Hazards Program: http://earthquake.usgs.gov/
The latest earthquakes are reported here.
4. The Pacific Northwest Seismograph Network: Earthquake Prediction:
http://www.geophys.washington.edu/SEIS/PNSN/INFO_GENERAL/eq_prediction.html
All about earthquakes of the Pacific Northwest.
5. Northern California Earthquake Data Center (NCEDC): http://quake.geo.berkeley.edu/
An archive and distribution center for earthquake data for Northern and Central
California.
6. Understanding Seismic Tomography:
http://www.see.leeds.ac.uk/structure/dynamicearth/flash_gallery/layered_earth/seismic_t
omography.html
A simple description of seismic tomography and its importance.
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Videos
1. Tsunami: The Wave that Shook the World. NOVA, PBS, DVD (2005, 60 min.)
The causes and consequences of the Indian Ocean tsunami that struck in 2004.
2. Earthquake Tsunami: Wave of Destruction. Insight Media, DVD (2007, 30 min.)
The scientific causes of earthquakes and tsunamis with details about the 2004 Boxing
Day tsunami in the Indian Ocean.
3. Earthquakes in the Midwest. NOVA scienceNOW, DVD (2009)
Earthquakes that strike away from plate boundaries as evidenced by those in the
American Midwest.
4. Earthquake—The Science Behind the Shake. NOVA, PBS, DVD
Lives and dollars can be saved if earthquakes can be predicted.
5. Inside Planet Earth. Discovery Channel (2009, 84 min.)
An unusual and unique voyage into Earth’s interior
6. When the Earth Moves. Insight Media, DVD (27 min.)
Earth movement from earthquakes, mass wasting, floods, glaciers, and volcanoes and
how to mitigate the problems caused by these movements.
7. Earthquakes. Insight Media, DVD (3 min.)
The nature and consequences of earthquakes.
8. Earthquakes: Seismic Sleuths. Insight Media (2001, 51 min.)
Scientists who attempt to predict earthquakes by learning all they can about why and
where they occur.
9. Earth Revealed. Annenberg Media: http://www.learner.org/resources/series78.html
(1992, 30 min., free video):
 #3: Earth’s Interior. Using oil wells to understand what lies beneath the Earth’s
surface. Geophysical methods, such as seismic wave studies, are described.
 #9: Earthquakes. Footage of earthquakes and their aftermath, including how
earthquakes are studied.
 #25: Living with Earth, Destructive Natural Phenomena. Annenberg/CPB Collection.
How people cope with living in an earthquake zone, especially those who live along
the San Andreas Fault.
10. The Great San Francisco Earthquake: American Experience Series. PBS, DVD
(1987, 56 min.)
The 1906 earthquake on the San Andreas Fault near San Francisco and its impact then
and now.
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11. Killer Quake. Nova. PBS, DVD (1994)
The 1994 Northridge earthquake in Los Angeles is studied.
12. Hidden Fury: The Danger Posed by the New Madrid Earthquake Zone. New Madrid
Films (1993, 27 min.)
The 1811 New Madrid quake and the likelihood of another quake on that fault zone.
13. Aftershocks of the Loma Prieta Earthquake—Computer Animations. U.S. Geological
Survey. The Open Video Project: http://www.open-video.org/details.php?videoid=770
Slides
1. The National Geophysical Data Center (NGDC) has relevant slide sets,
http://www.sciencestuff.com/—an assortment of rock and mineral collections, including:
 Introductory Earth Science Rocks and Minerals
 Natural Crystal Collection
 Gem Minerals
 Scale of Hardness Collection
 Advanced Rock and Minerals Collection
2. National Geophysical Data Center (NGDC) Natural Hazard Slide Sets,
http://www.ngdc.noaa.gov/nndc/struts/results?eq_0=5&t=101634&s=0&d=1, including:
 Earthquake Damage—General
 Earthquake Damage in San Francisco, April 18, 1906
 Earthquake Damage in Mexico City, September 19, 1985
 Faults
 Loma Prieta Earthquake, October 18, 1989
 Many others
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3. National Information Service for Earthquake Engineering (NISEE) Slide Sets,
http://nisee.berkeley.edu/bertero/html/slides.html, including:
 Surface Faulting
 Ground Shaking
 Many others
4. Geo-Tech Imagery DVD, CD, and Slide Sets:
http://www.geo-tech-imagery.com/prod03.htm
 Loma Prieta
 Northridge
 Many others
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