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Earthquakes
Introduction
Faults & Earthquakes
Seismic Waves
Effects of Earthquakes
Measurement of Earthquakes
Distribution of Earthquakes
Earthquake Prediction
Summary
Diseased nature oftentimes breaks forth
In strange eruptions; oft the teeming earth
Is with a kind of colic pinch'd and vex'd
By the imprisoning of unruly wind
Within her womb; which, for enlargement striving,
Shakes the old beldame earth and topples down
Steeples and moss-grown towers.
William Shakespeare
Introduction
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Earthquakes represent the vibration of Earth because of
movements on faults.
The focus is the point on the fault surface where motion
begins.
The epicenter is the point on Earth's surface directly above
the focus.
The deadly Izmit earthquake struck northwest Turkey on
August 17, 1999, at 3 a.m. Over 14,000 residents of the
region were killed as poorly constructed apartment
complexes pancaked to the ground, each floor collapsing on
the one below (Fig. 1). The death toll from this single event
was greater than the average annual loss of life from all
earthquakes worldwide.
An earthquake occurs when Earth’s surface shakes because
of the release of seismic energy following the rapid
movement of large blocks of the crust along a fault. Faults are
breaks in the crust that may be hundreds of kilometers long
and extend downward 10 to 20 km (6-12 miles) into the
crust. The 1,200 km (750 miles) long San Andreas Fault that
separates the North American and Pacific Plates in
California is the most active fault system in the contiguous
U.S. The Izmit earthquake occurred on the North Anatolian
Fault, a fault that is of similar length and sense of movement
as the San Andreas Fault. Unraveling the movement history
of large faults that produce devastating but infrequent
earthquakes can help predict the potential threat from similar
faults elsewhere.
Figure 1. Collapsed structures
destroyed by the shallow 1999 Izmit,
Turkey (top), and 1994 Northridge,
California (bottom), earthquakes.
The point on the fault surface where movement begins, the
earthquake source, is termed the focus. Seismic waves
radiate outward from the focus. Earthquake foci (plural of
Images courtesy of USGS Expedition to
focus) occur at a range of depths. The majority of
Turkey and USGS Open-File Report 96earthquakes occur at shallow depths that range from the
263 (Northridge).
surface down to 70 km (44 miles). Less frequent
intermediate (70-300 km; 44-188 miles) and deep (300-700
km; 188-438 miles) earthquakes are generally associated with
subduction zones where plates descend into the mantle.
Damage is greatest from shallow earthquakes because the
seismic waves travel a shorter distance before reaching the
surface. The earthquake effects, the type of damage
associated with earthquakes, include changes in the natural
environment such as landslides but most attention is focused on
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the impact on constructed structures. Building codes are in
place in most earthquake-prone areas but they are of little use if
enforcement is lax, as was the case in Turkey. Following the
earthquake it was discovered that some contractors had cut
corners in the construction of multistory apartment complexes.
The poorly built structures were left as piles of rubble amongst
other apartments that remained standing.
Figure 2. A smashed car
buried under fallen bricks
resulting from the 2001
Nisqually earthquake near
Seattle, Washington. The
Northridge (see Fig. 1) and
Nisqually earthquakes were
of similar magnitudes but the
latter occurred further below
the surface, reducing the
scale of the damages
resulting from the shaking.
Image courtesy of FEMA News
Photos.
Figure 3. The focus is the
source of the earthquake and
the epicenter is the point on
the surface directly above the
focus.
In contrast, on February 28, 2001, the strong Nisqually
earthquake (Fig. 2) occurred below western Washington 56 km
(35 miles) south of Seattle. Buildings in Seattle and the
surrounding communities sustained relatively little structural
damage, no one was killed, and only a handful of people
received anything more than minor injuries. Seattle has
enforced a stringent building code over the last 30 years that
requires new structures to be able to withstand large
earthquakes. In addition, over the last decade, many older
buildings and bridges were retrofitted to ensure that they could
endure the big earthquake predicted for the region. Residents in
western Washington were doubly fortunate, not only did they
have well-built structures but the earthquake occurred much
further below the surface than the Izmit quake, further reducing
the resulting ground shaking.
Seismic waves are captured by a recorder known as a
seismograph. The relative arrival times of different types of
seismic waves is used to determine the distance of the
seismograph station from the origin of the earthquake. Three or
more records can be used to pinpoint the earthquake's
epicenter, the geographic location of the point on the earth’s
surface directly above the focus (Fig. 3). Earthquakes are
named for the epicenter location, for example, the Nisqually
earthquake occurred 53 km (33 miles) below the mouth of the
Nisqually River in western Washington. Loss of life in the
Turkish earthquake was greatest in the city of Izmit, located
close to the earthquake's epicenter. Earthquake distribution
is far from random. Earthquakes occur on faults that are
preferentially located along plate boundaries. The largest
earthquakes along convergent plate boundaries.
One method of earthquake measurement is to determine the
level of destruction following an earthquake. However, as the
Turkish earthquake so vividly illustrates, the degree of damage
is often related more to human activity than the earthquake
itself. Consequently, more quantitative measures have been
adopted that measure the magnitude and timing of the vibration
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of sensitive instruments (seismographs) and the distance from
the earthquake to calculate a value for the event. This
information is often combined with data on the geology
surrounding the fault to generate an even more accurate
measure.
Over two million people were killed during the previous
century by earthquakes and associated phenomena. The threat
of future earthquakes in heavily populated regions like
California (and Turkey) has spurred efforts in earthquake
prediction. Analysis of past earthquakes allows the
determination of the potential size and location of future
events, the problem is determining when such events will occur
with any degree of accuracy. The principal difficulty is that
large, dangerous earthquakes occur at intervals measured in
decades or centuries yet our record of past earthquake activity
stretches back only a few hundred years, making it inadequate
for rigorous predictions.
Earthquakes are expensive. Even the relatively minor damages
from the Nisqually earthquake cost $2 billion to repair and the
substantial damages resulting from the 1994 Northridge quake
have been estimated to cost $30 billion, making it one of the
costliest disasters in U.S. history. People living in areas with
the potential for damaging earthquakes may seek to purchase
earthquake insurance to provide some financial protection for
their property. Approximately 40% of residents in Northridge
had insurance and their insurance companies endured
significant losses in paying an estimated $15 billion in claims.
The Northridge event caused insurers to drastically rethink
earthquake coverage. Deductibles rose to 10 to 15% and
policies now exclude loss of building contents and reductions
in other costs. These less generous insurance policies would
have trimmed claims from the Northridge earthquake to about
$4 billion.
Think about it . . .
How frequently do earthquakes occur near where you live?
Where do earthquakes occur in your state? Go to the USGS
National Earthquake Information Center’s website
(http://wwwneic.cr.usgs.gov/neis/states/states.html) and
answer these basic questions.
4
Estimated annual
cost of earthquake
insurance for a
$100,000 home in
California: $280
Faults & Earthquakes
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•
•
•
Earthquakes represent the vibration of Earth because of
movements on faults.
Faults can be identified by the offset of rock layers on
either side of the fault surface.
Normal and reverse faults are types of dip-slip faults.
Left-slip and right-slip faults are types of strike-slip faults.
An earthquake occurs when Earth shakes because of the release
of seismic energy following the rapid movement of large
blocks of the crust along a fault. A fault is a fracture in the
crust. During the Izmit earthquake, the crust broke along the
North Anatolian Fault in northern Turkey (Fig. 4). When
fault movement occurs it may be slow and gradual and
generate only small earthquakes, or it may be rapid and
catastrophic causing widespread destruction. Ground shaking
associated with most earthquakes is over in a matter of seconds
but it involves such large regions of Earth’s crust that
tremendous amounts of energy are released. The ground shook
for 45 seconds in the Izmit earthquake and affected a region of
approximately 100,000 square kilometers.
Faults may be hundreds of kilometers in length but only part of
longer faults typically break during an earthquake (Fig. 4).
Fault segments that have not experienced a recent earthquake
are termed seismic gaps and are considered potential sites for
future events. The Izmit earthquake occurred in a 150 km (94
mile) long gap at the western end of the North Anatolian Fault.
Figure 4.
Earthquake
sequence along
the North
Anatolian fault,
Turkey, 19391999. A series of
large earthquakes
have occurred on
the fault system;
each resulting from
only one segment
of the fault
breaking at a time.
Image courtesy of
USGS.
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Two adjacent fault segments to the east and west broke during
large earthquakes in 1963 and 1967. Eleven earthquakes of
magnitude 6.7 or greater occurred along segments of the fault
over the previous 60 years. Even though scientists can identify
potential seismic gaps, the faults may not cooperate to generate
an earthquake. Earthquake specialists predicted an earthquake
would strike the region around the small California town of
Parkfield before the end of 1993 but the quake still hasn't
shown up, despite the fact that there are millions of dollars
worth of instruments in the ground waiting for the big day to
arrive.
Even the largest earthquakes require relatively small fault
movements because such large volumes of rock are involved.
Offsets on faults for the largest of earthquakes are less than 10
meters, and typically less than 5 meters per quake. The
accumulated movement from hundreds of thousands of
earthquakes over millions of years results in the formation of
mountains in association with plate boundaries.
Fault Classification
Faults are distinguished as dip-slip or strike-slip faults. Two
types of dip-slip faults are identified on the basis of the
relative motion of rocks across an inclined fault surface (Fig.5).
The block of rocks above a fault is termed the hanging wall;
the footwall lies below the fault. Miners working in shafts that
crossed faults were able to hang their lanterns from the hanging
wall while their feet remained below the fault in the footwall.
Inclined faults can be identified by the offset of rock layers on
either side of the fault surface. The hanging wall moves down
relative to the footwall in a normal fault. In contrast, the
hanging wall moves up relative to the footwall in a reverse
fault. Movement on a dip-slip fault often results in a break or
Figure 5. Top: The
hanging wall (hw) lies
above an inclined
fault; the footwall (fw)
lies below the fault.
Bottom: Normal fault.
Figure 6. Fault scarp
formed during the Hebgen
Lake earthquake,
Montana, 1959. Person in
foreground is
approximately 2 meters
tall. Surface at bottom of
slope was at same
elevation as upper surface
prior to movement on the
normal fault.
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offset at the land surface. This break in slope is known as a
fault scarp (Fig. 6).
Figure 7. An example
of a left-slip strike-slip
fault.
Two types of strike-slip faults, left-slip and right-slip faults,
are identified on the basis of the motion on vertical fault
surfaces (Fig. 7). An observer, standing on one side of the
fault, sees objects on the other side of the fault move to the
right for a right-slip fault or to the left for a left-slip fault. The
1,200 km long San Andreas Fault, California, is a famous
example of a right-slip fault. Areas of frequent earthquake
activity are laced with faults. Maps of California show that
several faults make up the San Andreas Fault system. The
North Anatolian Fault that broke during the Izmit earthquake is
also a right-slip fault (Fig. 8) and is of similar length as the San
Andreas fault.
Figure 8. Offset in a fence
as a result of the Izmit
earthquake. Note the
relatively small movement
on the fault, even though
the earthquake was large.
Can you classify the fault?
Image courtesy of USGS
Expedition to Turkey.
Faults and Plate Boundaries
Earthquake distribution is far from random. Earthquakes occur
on active faults and active faults are preferentially located
along plate boundaries (Fig. 9). Although, both dip-slip and
strike-slip faults are associated with all types of plate boundary,
each type of boundary is characterized by a specific fault style.
Strike-slip faults are common at transform plate boundaries
Figure 9. Plate tectonic
setting for the Izmit
earthquake. The small
Anatolian Plate is moving
westward as it is wedged
between the converging
Arabian and Eurasian
plates. A subduction zone
in the eastern
Mediterranean Sea marks
the boundary with the
African Plate to the south.
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where two plates move in opposite directions. Reverse faults
occur most frequently at convergent boundaries where plates
collide; and normal faults are most common at divergent
boundaries such as oceanic ridges, where plates break apart.
Think about it . . .
Finish the partially completed concept map for faults and
earthquakes found at the end of the chapter. Print the page
and fill in the blanks with appropriate terms.
Seismic Waves
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Seismic waves can be divided into surface waves that
travel on Earth's surface and body waves that travel
through Earth.
Body waves are further divided into S waves and P waves.
Seismic waves are recorded on a seismogram at a
seismograph station.
The distance of an earthquake epicenter from a
seismograph station is determined by the difference in the
arrival times of P and S waves at a seismograph station.
Earthquake magnitude is calculated using the amplitude
(height) of the S wave recorded on a seismogram.
Seismic waves represent the energy released from the
earthquake focus. There are two types of seismic waves:
•
•
Surface waves travel on Earth’s surface and cause much
of the destruction associated with earthquakes.
Undulations of the land surface during an earthquake are a
representation of surface waves (Fig. 10). Surface waves
may result in vertical motions (Rayleigh waves), much
like waves traveling through water, or sideways motions
(Love waves) with no vertical component of movement.
Figure 10. Rayleigh (top) and
Love waves (bottom) are surface
Body waves travel through Earth’s interior. These are
waves with contrasting motion
further subdivided into P (primary) waves and S (secondary directions generated during an
earthquake.
or shear) waves based upon their vibration direction and
velocity. Variations in seismic wave velocity are used to
infer the properties of Earth’s interior.
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P waves vibrate parallel to their travel direction in the same
way a vibration passes along a slinky toy (Fig. 11). P waves
travel at speeds of 4 to 6 km per second (2.5-4 miles per
second) in the uppermost part of the crust. S waves vibrate
perpendicular to their travel direction, like the wave that passes
along a rope when it is given a sharp jerk (Fig. 11). S wave
velocity is 3 to 4 km per second (2-2.5 miles per second) in the
shallow crust.
Figure 11. Analogs of P
wave (left) and S wave
motion (right). P waves
are similar to the
passage of a vibration
through a slinky. The
vibration occurs in the
same direction as the
wave travels. S wave
motion is analogous to a
vibration moving along a
rope. The vibration
occurs perpendicular to
the direction in which the
wave travels.
The velocity of seismic waves is lower in loose, unconsolidated
materials (sand, partially melted rock) and higher in solid
materials (rock).
Both P and S waves are generated at an earthquake focus as a
result of movement on a fault. P waves will arrive at a
recording station (seismograph station) first because of their
greater velocity. Surface waves are the last to arrive because P
and S waves travel a more direct route through the earth (Fig.
12).
Figure 12. Contrasting
travel paths for surface
waves and body waves
following an earthquake.
The record of an earthquake at a seismograph station is a
seismogram (Fig. 13). The principal elements of a seismogram
that interest seismologists (scientists who study earthquakes)
are the relative size of the recorded waves and the difference in
time that the first P and S waves were recorded. The amplitude
of the recorded wave is proportional to the magnitude of
shaking associated with the earthquake but shaking may vary
with the character of the material underlying the seismograph
station. Loose, unconsolidated materials (e.g., mud, sediment)
may exaggerate the shaking whereas solid bedrock may result
in smaller vibrations.
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Figure 13. An idealized
seismogram illustrating the
sequential arrival of seismic
waves. Determination of
the distance from an
epicenter require
calculating the difference in
arrival time of P and S
waves (~14 seconds).
Earthquake magnitude is
related to the amplitude of
the recorded S wave.
The difference in arrival time between P and S waves on a
seismogram can be used to determine the distance of the station
from the earthquake source and the amplitude (height) of the S
wave recorded at the station can be used to determine
earthquake magnitude (see Measurement of Earthquakes).
The time interval between the recorded arrival of P and S
waves increases the further the seismograph station is located
from the epicenter. Seismologists match the time difference
with standard curves (Fig. 14) to determine distance from the
earthquake.
Figure 14. Graph of
distance from the epicenter
and time for seismic waves
to reach seismograph
station. The time interval
between the arrival of P
and S waves increases with
increasing distance from
the epicenter.
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Data at a single seismograph station are insufficient to pinpoint the earthquake epicenter because a seismogram yields
only the distance from the earthquake source. The epicenter
could be located anywhere along a circular arc of the calculated
distance from the seismograph station. Seismologists must use
data from multiple recording stations to learn the location of
the event. The common intersection point for several circles
plotted relative to different stations represents the point on the
surface above the earthquake source (Fig. 15).
Figure 15. An earthquake
originating in the Pacific
Northwest would be
recorded at seismograph
stations in Denver,
Quebec, and Lima (Peru).
The difference in arrival
times between P and S
waves would be least at
Denver and greatest at
Lima. Circles plotted at
each station reflect the
distance from the
epicenter but the direction
can only be determined by
identifying the intersection
point for three or more
circles.
Think about it . . .
Try the Virtual Earthquake exercise that guides users through the
determination of the location of an earthquake epicenter and
earthquake magnitude using records of seismic waves recorded
at three seismograph stations. Print the "Virtual Seismologist"
certificate upon completion of the exercise.
http:// vcourseware4.calstatela.edu/VirtualEarthquake/VQuakeIntro.html
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Effects of Earthquakes
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A major earthquake under a heavily populated area in the
U.S. could result in thousands of deaths.
Several effects of earthquakes could result in extensive
damages.
Ground shaking can collapse buildings.
Uplift may raise or lower large areas of Earth's surface.
Liquefaction in water-saturated sediment can result in the
collapse and subsidence of the ground surface.
Landslides are a potential hazard on steep slopes in seismic
zones.
Tsunamis, giant sea waves, are dangerous to coastal
communities, especially around the Pacific Ocean.
A magnitude 6.7 earthquake struck the Northridge suburb of
Los Angeles on January 17, 1994. The earthquake resulted in
the deaths of 57 people and injured over 9,000 more. There are
about 150 earthquakes of this magnitude worldwide each year
but this was the first time a quake of this size occurred in a
heavily developed area of the U.S. An earthquake of similar
size killed over 50,000 people in Iran in 1993.
The Elysian Park fault was recently discovered below
downtown Los Angeles and may produce substantial future
earthquakes. Movement on the 55 km (35 miles) long fault
could result in up to 5,000 deaths, leave 750,000 homeless, and
cause $100 billion in damages in Los Angeles. A comparable
earthquake in Kobe, Japan (exactly one year after the
Northridge quake), killed over 6,000 people.
The images in the following figures show damage from the
largest recorded U.S. earthquake (Alaska, 1964) and illustrate
the effects of earthquakes. All images taken from USGS
National Earthquake Information Center (NEIC).
Sudden changes on or near the earth’s surface result from
earthquakes and may include:
Ground Shaking: Rapid horizontal movements associated
with earthquakes may shift homes off their foundations and
cause tall buildings to collapse or "pancake" as floors collapse
down onto one another. Shaking is exaggerated in areas where
the underlying sediment is weak or saturated with water (Figs.
16, 17).
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Figure 16. Part of a railroad
bridge over the Copper River
was shaken loose by the 1964
Anchorage earthquake. Image
courtesy of USGS.
Figure 17. This map shows in
color those parts of the
contiguous 48 states that have a
10% chance of experiencing an
earthquake strong enough to
cause appreciable damage in a
50-year period. In the yellow
areas, maximum ground
shaking would be strong enough
to damage unreinforced
masonry buildings, even those
built on bedrock. Darker colors
are at the same risk for more
intense shaking, and areas left
blank would have less intense
shaking. Image courtesy of USGS.
Fault Rupture and Uplift: Break of the ground surface by the
fault plane may form a step in the surface known as a fault
scarp (Fig. 6). Large sections of Earth’s surface may change
elevation as a result of uplift on an earthquake fault (Fig. 18).
Mountains east of Los Angeles were uplifted 0.3 meters (1
foot) by the 1994 Northridge earthquake.
Liquefaction: Liquefaction occurs when water-saturated
sediment is reorganized because of violent shaking. The
sediment collapses, expelling the water, and causing the ground
surface to subside.
Figure 18. Top: Uplifted sea
floor in Prince William Sound,
Alaska. The 400-meter-wide
white surface was raised
above sea level. Bottom:
Diagonal crack represents the
upper part of a landslide in an
Anchorage residential district
associated with the 1964
earthquake. Images courtesy of
USGS.
Landslides: Earthquakes are often associated with mountains
formed along convergent plate boundaries. The steep slopes
present in these environments are prone to landslides when
shaken (Fig. 18). Landslides are common following
earthquakes in California.
Tsunamis: Giant sea waves are generated by submarine
earthquakes, especially noted from the Pacific Ocean.
Tsunamis caused by earthquakes around the ocean’s perimeter
may travel thousands of miles to destroy coastal property in
Hawaii. Tsunami waves may reach heights of 15 meters (50
feet) near shore and travel at speeds up to 960 km/hr (600
mph). Many casualties associated with the 1964 Alaska
earthquake resulted from tsunamis.
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The Pacific Tsunami Warning System (PTWS) is a network of
stations that attempt to identify potentially damaging tsunamis
from earthquakes in or around the Pacific Ocean. The PTWS
issues warnings or watches that predict tsunami arrival times
for coastal areas.
Think about it . . .
1. Review the possible effects of earthquakes and examine
a description of the 1989 Loma Prieta earthquake and/or
the 1906 San Francisco earthquake and suggest what
could be done to diminish the potential for damages and
loss of life resulting from earthquakes.
2. Use the Venn diagram located at the end of the chapter to
compare and contrast the characteristics and effects of
the 1989 Loma Prieta and 1906 San Francisco
earthquakes.
Loma Prieta information available here:
http://www.es.ucsc.edu/~jsr/EART10/Trips/FT3/index.html
San Francisco earthquake information available here:
http://quake.wr.usgs.gov/info/1906/index.html
10 largest U.S.
Earthquakes
(with momentmagnitude values)
1. Prince William Sound,
Alaska 1964 (9.2)
Measurement of Earthquakes
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There are three methods used for measuring earthquakes.
The Modified Mercalli scale measures intensity and is often
used to rank the cultural effects of historical earthquakes.
Mercalli scale values vary with distance from epicenter,
building materials used, and population density.
The Richter scale is the most well known and measures
earthquake magnitude using the amplitude (height) of the S
wave recorded on a seismogram.
Each division in the Richter scale represents a 10-fold
increase in amplitude and an approximate 30-times increase
in energy released.
The moment-magnitude scale has recently found favor as a
method that more accurately measures energy release on
large faults.
There are three principal methods of measuring the effects of
earthquakes.
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2. Andreanof Islands,
Alaska 1957 (8.8)
3. Rat Islands, Alaska
1965 (8.7)
4. Shumagin Islands,
Alaska 1938 (8.3)
5. Lituya Bay, Alaska
1958 (8.3)
6. Yakutat Bay, Alaska
1899 (8.2)
7. Cape Yakataga, Alaska
1899 (8.2)
8. Andreanof Islands,
Alaska, 1986, (8.0)
9. New Madrid, Missouri,
1812 (7.9)
10. Fort Tejon, California,
1857 (7.9)
•
Modified Mercalli scale is used to measure damage and
human perception of an earthquake.
•
Richter scale is the most familiar and measures the size of
the seismic waves recorded at a seismogram.
•
Moment-magnitude scale has replaced the Richter scale in
popularity among geophysicists because it gives a more
accurate interpretation of the amount of energy released by
an earthquake.
Five most
destructive historical
earthquakes
(number of deaths)
1556
Shansi, China
(830,000)
1737
Tangshan, China
(255,000)
1138
Aleppo, Syria
(230,000)
1927
Xining, China
(200,000)
856
Damghan, Iran
(200,000)
Modified Mercalli Scale
The Mercalli scale measures earthquake intensity: the level of
destruction of the earthquake (higher values) and the effect of
the event on people (lower values). The scale ranks intensity
from I to XII (1-12) using Roman numerals. The table below
summarizes the characteristics of the Mercalli scale.
Index
Effects of Earthquake on People and Structures
I
II
III
Not felt by people.
Felt by people at rest on upper floors of buildings.
May be felt by people indoors. Vibrations similar to the
passing of a truck.
Felt indoors by many, outdoors by few. Dishes,
windows, doors disturbed; walls make cracking sound.
Sensation like heavy truck striking building.
Felt by nearly everyone; many awakened. Some dishes,
windows broken. Unstable objects overturned.
Felt by all; many frightened. Some heavy furniture
moved; a few instances of fallen plaster. Damage slight.
Slight to moderate damage in ordinary structures;
considerable damage in poorly built or badly designed
structures; some chimneys broken.
Slight damage in buildings designed to withstand
earthquakes; heavy damage in poorly constructed
structures. Chimneys, columns, walls may fall.
Considerable damage in specially designed structures.
Damage great in substantial buildings, partial collapse.
Buildings shifted off foundations.
Some well-built wooden structures destroyed; most
masonry and frame structures destroyed with
foundations.
Few, if any (masonry) structures remain standing.
Bridges destroyed. Rails bent greatly.
Total damage, objects thrown into air.
IV
V
VI
VII
VIII
IX
X
XI
XII
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Figure 19. Locations
of U.S. earthquakes
causing damage
1750-1996, Mercalli
intensity VI to XII.
Large red squares
represent locations of
largest earthquakes
(intensity XII). Note
squares in southeast
Missouri from New
Madrid (1811-1812)
earthquakes. Source
NEIC.
The Mercalli scale is relatively easy to use but it is not widely
applicable to modern earthquakes because its interpretation is
dependent upon:
•
Variations in population density: earthquake intensity
would be underestimated in sparsely populated areas.
•
Building materials and methods: earthquakes of similar size
could give different values depending upon building codes.
•
Distance from the epicenter: values decrease with
increasing distance from the epicenter. Each earthquake has
several different intensity values making it difficult to
compare individual events.
Some of the largest historical U.S. earthquakes occurred in
the eastern half of the country (Fig. 19). Three major
earthquakes were centered in southeastern Missouri (New
Madrid) over a three-month period from December 1811 to
February 1812.
The Mercalli scale is useful in ranking historical earthquakes
that occurred before the widespread use of seismographs
(after World War II). Notice that the map above contains
many historical earthquakes in the eastern half of the U.S.,
some equally severe as those in California. Isoseismal maps
can be created that show areas of equal earthquake intensity
(Fig. 20). A comparison of isoseismal maps for earthquakes of
similar size from the eastern (New Madrid, Missouri) and
western (San Francisco, California) U.S. illustrates that the
eastern event was felt over a much larger area.
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Figure 20. Isoseismal map of
1964 Alaska earthquake
showing areas with equivalent
damages following the largest
recorded U.S. earthquake.
Richter Scale
The Richter scale measures earthquake magnitude, the
amplitude of seismic waves recorded on a seismograph
following an earthquake. (See the Virtual Earthquake exercise
on page 11).
Charles Richter developed the scale in the 1930s to measure
shallow earthquakes in California. These early measurements
of magnitude (ML- local magnitude) simply relied on using two
factors (the difference in P- and S-wave arrival times and Swave amplitude). The measured earthquakes were less than
600 km (375 miles) from the seismograph stations and
occurred at similar depths in the crust.
Mb = log10(A/T) + Q
Formula to determine
magnitude from body
waves (Mb) where A is the
amplitude of ground motion
(microns); T is time taken
for motion (seconds); and Q
is a correction for distance
from the epicenter and the
focal depth (kilometers).
More complex formulae to determine magnitude from seismic
body waves or surface waves were developed as the number of
seismograph stations increased and it was recognized that
earthquakes occurred at a range of depths.
The Richter scale is logarithmic, each division represents a 10fold increase in the ground motion associated with the
earthquake, and ~30-times increase in energy released. For
example, a magnitude 7 earthquake has ten times as much
ground motion (and releases over 30-times the energy) as a
magnitude 6, 100 times as much motion (900 times the energy)
as a magnitude 5, 1,000 times the motion of a magnitude 4, etc.
Magnitude
1
2
3
4
5
6
7
8
Ground Motion
1
10
100
1,000
10,000
100,000
1,000,000
10,000,000
Energy
1
30
900
27,000
810,000
24,300,000
729,000,000
21,870,000,000
Unlike the Mercalli scale, the Richter scale does not have a
maximum value; it is open-ended. The largest earthquakes
measured with the Richter scale have magnitudes between 8
and 9. It is probable that rocks in Earth’s crust are unable to
withstand stresses necessary to generate earthquakes of
magnitude 9 or more.
17
The terminology used to describe earthquakes is dependent
upon their magnitude.
Description
Great
Major
Strong
Moderate
Magnitude
8+
7-7.9
6-6.9
5-5.9
Equivalent
Intensity
XI-XII
IX-X
VII-VIII
VI-VII
Number per
Year
1
18
120
800
Moment-Magnitude Scale
Mw = 2/3 log10(Mo)-10.7
The moment-magnitude (Mw) scale measures the energy
released by the earthquake more accurately than the Richter
scale. The amount of energy released is related to rock
properties such as the rock rigidity, area of the fault surface and
amount of movement on the fault. It provides the most accurate
means of comparison of large earthquakes.
Formula to determine
magnitude where Mo = mSd,
where m is shear strength of
the faulted rock, S is the area
of the fault, and d is fault
displacement.
Think about it . . .
Answer the conceptest question below.
Three sites (L1, L2, L3) record earthquake intensity and
earthquake magnitude for the same earthquake. L1 is located
closest to the earthquake focus and L3 is farthest away.
The intensity values are greatest at _____ and the earthquake
magnitude (calculated using seismograms) _______________.
a) L1; is the same at each site.
b) L3; is the same for each site.
c) L1; decreases with distance from the focus.
d) L3; decreases with distance from the focus.
Distribution of Earthquakes
•
•
•
•
18
Earthquakes are most frequent along plate boundaries.
The largest earthquakes are associated with convergent
plate boundaries.
Oceanic ridges are characterized by shallow earthquakes.
Deep earthquakes (to depths of ~700 km) occur within
subduction zones along convergent plate boundaries.
•
•
•
The most devastating earthquakes are typically shallow
earthquakes (0-33 km depth).
Alaska has the largest U.S. earthquakes but California has
the most damages because of a larger population.
Some large historical earthquakes have been identified in
the eastern U.S., in particular a swarm of three major
quakes which occurred at New Madrid, Missouri, 18111812.
Where Do Earthquakes Occur?
Earthquakes occur at many sites around the world but
seismicity is concentrated in specific locations. A map of the
Pacific Ocean basin showing the location of large earthquakes
over a 20-year period (1975-1995) is presented below.
Compare the map with a map of plate boundaries for the same
area (Fig. 21).
Earthquake distributions have several characteristics:
• There is a strong correlation between earthquake foci and
plate boundaries (Fig. 21).
• Swarms of earthquakes resulting from collisions of
Figure 21. Top: Distribution
of earthquake focal depths
around the Pacific Ocean.
Orange and yellow dots
represent shallow focal
depths (0-70 km); green and
blue focal depths are 71 to
300 km; purple and red dots
represent focal depths of 301
km or greater. Bottom: Plate
boundaries (yellow lines)
within and around the Pacific
Ocean. Notice the
correlation between plate
boundaries and the
distribution of earthquake
foci. Images courtesy of USGS
NEIC.
19
•
•
•
•
continental plates form a belt across central Asia, through
the Middle East and southern Europe.
Continental interiors that are far removed from plate
boundaries (e.g., Canada) have few earthquakes.
A belt of shallow earthquakes can be traced along the
global oceanic ridge system from the center of the Atlantic
ocean, through the Indian Ocean, around the southern
Pacific Ocean, and into the East Pacific.
Earthquakes are present under hot spots such as the
Hawaiian Islands in the central Pacific Ocean.
The largest earthquakes are associated with convergent
plate boundaries (Fig. 22).
Figure 22. Locations
and focal depths of
earthquakes of
magnitude 7 and
greater from 19751998. Colors
correspond to focal
depths (see
scale). Note that the
majority of large
earthquakes are
located around the
rim of the Pacific
Ocean, an area
characterized by
convergent plate
boundaries. Source:
USGS NEIC.
How Does Focal Depth Vary with Location?
Epicenter locations on the maps above are colored based upon
the depths of the earthquake foci. Several patterns are obvious
from the map:
• Deep earthquakes (focal depth >300 km) are present in
association with subduction zones along convergent plate
boundaries such as western South America (Nazca/South
American Plates), southern Alaska (Pacific/North
American Plates), and the southwest Pacific
(Pacific/Australian Plates).
• The focal depths increase below overriding plates at
convergent boundaries in the direction of inclination of
20
•
•
•
subduction zones. For example, the Nazca Plate descends
below South America and foci increase in depth toward the
interior of the continent.
The only area where deep earthquakes are not present along
the Pacific Rim is in the western U.S. where a transform
plate boundary exists.
The largest earthquakes are typically shallow earthquakes
where seismic energy is released closer to Earth's surface.
Divergent plate boundaries such as the oceanic ridge
systems in the north Atlantic Ocean and continental rift
valleys (East Africa) are characterized by earthquake focal
depths of less than 33 km.
Where Are the Most Seismically Active Areas in
North America?
The most seismically active states are along the western margin
of the continent (Fig. 23). In the U.S., Alaska and California, in
that order, experience the most earthquakes. Damage caused by
earthquake activity is greatest in California because of its
larger population. Most of the largest earthquakes in U.S.
history occurred on the southern coast of Alaska, along the
convergent boundary between the Pacific and North American
plates.
The effects of earthquakes in eastern North America are felt
further from their sources because the crust is less fractured
(more rigid) than in the west. Earthquakes of comparable size
Figure 23.
Seismicity in the
conterminous
U.S. reflected by
earthquakes
between 19771997.
21
in California affected a much smaller area (compare isoseismal
maps for the San Francisco and New Madrid earthquakes).
Some of the largest historical earthquakes occurred in the
eastern half of the continent. For example, three major
earthquakes were centered in southeastern Missouri (New
Madrid) over a three-month period from December 1811 to
February 1812.
States in the northern Great Plains of the U.S., such as North
Dakota, and adjacent provinces in central Canada (Manitoba,
Saskatchewan) have experienced the fewest significant
earthquakes.
Think about it . . .
Examine the world map at the end of the chapter and predict
which locations are most likely to have experienced recent
earthquake activity then go to online maps (URL below) of current
seismicity to check your predictions.
http://wwwneic.cr.usgs.gov/neis/general/seismicity/seismicity.html
Earthquake Prediction
•
•
•
•
•
•
Earthquakes represent the deadliest of natural hazards.
Earthquakes typically occur in areas of active faults,
especially along plate boundaries.
Earthquake magnitude increases with fault length.
Various instruments and satellite observations can be used
to measure the buildup of strain in rocks.
Scientists predict the long-term probability of earthquakes
for specific locations on the basis of information about
strain accumulations and recurrence interval.
Short-term prediction, days or weeks before an earthquake,
is still a long way off.
Earthquakes represent the most deadly natural hazard. Over
two million people have been killed this century alone by
earthquakes and associated phenomena. The threat of future
earthquakes in heavily populated regions like California has
spurred efforts to discover ways to predict future earthquake
22
Average annual
losses from floods:
$5.2 billion
Average annual
losses from
hurricanes:
$5.4 billion
Figure 24. The FEMA
report on the potential
damages from
earthquakes identified
West Coast states as
having the
combination of active
faults and large
population centers
that may result in the
greatest damages
from earthquakes.
The smallest risk
occurs in North
Dakota and
Minnesota where
earthquake damages
would account for
less than $10,000
annually.
activity. The basic questions in earthquake prediction are
When? Where? and How big?
A recent report by the Federal Emergency Management
Agency (FEMA) estimated that the average annual property
damage from U.S. earthquakes totaled $4.4 billion. California
alone accounted for 75% of this total. Several years may pass
with few large events and little associated damage but a single
big earthquake in a large city can have a price tag of as much
as $30 billion. Population density and active seismicity have
the greatest influence over estimates of potential damages.
When averaged over several decades, the potential cost of
earthquake damages for the populous eastern U.S. ranks
alongside that of the more seismically active Rocky Mountain
states where population density is much lower (Fig. 24). The
upper Midwest and Great Plains states have the least risk for
significant earthquake-related damages.
Where? How big?
Answers to the Where? and How big? questions are already
known in regions of frequent seismic activity. The answers to
these questions depend on an understanding of the earthquake
mechanism. We have already discussed the fact that
earthquakes occur on faults. Many active faults have already
been discovered but some questions remain about the potential
size of earthquakes on faults that have no associated historical
earthquakes. For such faults, scientists attempt to estimate
future earthquake magnitudes from fault size. Earthquake
magnitude is directly related to fault length - the longer the
fault the bigger the earthquake (Fig. 25). The 1906 San
Francisco earthquake (Mw 7.7) was caused by rupture of 400
km (250 miles) of the San Andreas Fault and shaking lasted for
23
nearly two minutes. In contrast the magnitude 6.7 Northridge
earthquake was caused by displacement on a 14 km (9 miles)
long fault segment and the duration of shaking was just 7
seconds.
Figure 25. Relationship
between earthquake
magnitude and fault
size for a series of
California earthquakes.
Earthquake magnitude
increases with fault
length.
When?
Displacement on faults is related to crustal deformation
associated with plate tectonics and is concentrated in relatively
narrow zones along plate margins. Stresses build up in rocks
where plates interact. Faults exhibit movement when stresses
reach sufficient levels. Rocks adjacent to the fault may be
deformed prior to fault movement. Stresses cause deformation
of rocks (strain) and geologists can measure the accumulation
of strain in deforming rocks in an effort to predict the timing of
future earthquakes.
Strain can be measured in the vicinity of active faults using a
variety of instruments including creepmeters, strainmeters, and
satellite positioning systems. Creepmeters survey
displacement between two points on opposite sides of a fault.
As strain increases the distance between points increases.
Strainmeters measure the distortion of the originally circular
profile of cylindrical boreholes as a result of deformation.
Boreholes are distorted to an increasingly elliptical shape in
section as strain accumulates. Satellites of the Global
Positioning System (GPS) can be used to continually monitor
the location of receivers on the ground on either side of a fault.
Distances between stations distributed over an area of hundreds
of square kilometers can be determined to within a few
centimeters. Monitoring of stations over months or years
reveals changes in the relative positions of receivers related to
the buildup of strain along the fault.
24
Scientists can establish an average recurrence interval - the
time between earthquakes of similar magnitude - for individual
faults by determining the ages of offset layers of rocks and/or
sediment. Analysis of how much time has elapsed since the last
earthquake and the amount of energy that was released
(magnitude) help reveal which faults may be storing up
sufficient strain for earthquakes in the relatively near future.
Probability Theory
Researchers have used statistical methods to predict the
probability of future damaging earthquakes on particular
faults with sufficient record of seismicity. Faults with a high
probability of an earthquake exhibit a lot of stored strain and a
long time interval without fault movement.
Figure 26. The
probability of fault
movement varies along
the San Andreas Fault.
Segments along the
southern half of the
fault system are most
likely to break,
especially at Parkfield.
In 1990 a panel of experts convened by the National
Earthquake Prediction Evaluation Council estimated a 67%
probability for a major earthquake on one of four segments of
the San Andreas fault in the San Francisco Bay area between
1990 and 2020 (Fig. 26). Scientists predicted the near certainty
(95% probability) of an earthquake at Parkfield, California,
between 1986-1993. Parkfield, located on the San Andreas
Fault, averaged a magnitude 6 earthquake every 22 years since
1857. Geophysicists distributed an array of monitoring
instruments around Parkfield in the 1980s hoping to pick up a
signal that would aid in predicting future earthquakes.
25
However, the earthquake has still not occurred illustrating a
potential pitfall of prediction by probability.
Probability theory assumes a random occurrence of
earthquakes but recent analyses suggest that earthquakes
cluster together in groups of events. For example, one
magnitude 6 or larger earthquake occurred every four years on
average between 1836 and 1911 in and around San Francisco.
There were no more earthquakes of that magnitude in the 68
years that followed. However, since 1979 there have been four
more magnitude 6 events. Scientists are now concerned that the
release of strain on one fault may increase the potential for
movement on an adjacent fault in ways that cannot be
accounted for in traditional probability theory.
Even if it becomes possible to accurately predict earthquake
activity to within a specific year, it is unlikely that individual
events can be pinpointed to within a few months, let alone
weeks or days. Furthermore, it is unlikely that we would be
able to collect sufficient data to predict earthquakes in areas of
infrequent seismic activity. Given the difficulty in predicting
the timing of future earthquakes we would be well advised to
focus instead on engineering solutions that attempt to
earthquake-proof key structures.
Think about it . . .
Following graduation you get a job working for a county
planning task force in California. The task force must examine
the setting of several different cities and identify which is at
greatest risk for future earthquake damages from movement
on known faults. You are given the assignment to create an
evaluation rubric to rank the relative dangers for different
cities. Go to the evaluation rubric frame at the end of the
chapter to complete the exercise.
Summary
1. What is an earthquake?
Vibration of Earth due to a rapid release of energy. Energy is
released because of rapid movement on a fault.
26
2. What is a fault?
A fracture on which movement has occurred. Rapid movement
of 1 to 10 meters is typically necessary to generate a significant
earthquake. Faults are distinguished as dip-slip or strike-slip
faults.
3. What is the earthquake focus?
The focus is the point on the fault surface where movement
begins, the earthquake source. Seismic waves radiate outward
from the focus. Earthquake foci occur at a range of depths;
shallow (0-70 km), intermediate (70-300 km), and deep (300700 km). Shallow earthquakes are the most common.
4. What is the earthquake epicenter?
The epicenter is the geographic location of the point on Earth’s
surface directly above the focus. Earthquakes are named for the
epicenter location, for example the 1994 Northridge earthquake
occurred several kilometers below the city of Northridge in
metropolitan Los Angeles.
5. What are the differences between body waves and surface
waves?
Seismic waves represent the energy released from the
earthquake focus. There are two types of seismic waves.
Surface waves travel on Earth’s surface. Undulations of the
land surface during an earthquake are a representation of
surface waves. Body waves travel through Earth’s interior.
These are further subdivided into P (primary) waves and S
(secondary or shear) waves on the basis of their vibration
direction and velocity.
6. How do P and S waves differ?
P waves vibrate parallel to their travel direction in the same
way a vibration passes along a slinky toy. P waves travel at
speeds of 4 to 6 km per second. S waves vibrate perpendicular
to their travel direction, like the wave that passes along a rope
when it is given a sharp jerk at one end. S wave velocity is 3 to
4 km per second.
7. What is a seismogram?
The record of an earthquake at a seismograph station is a
seismogram. The difference in arrival time between P and S
waves on a seismogram can be used to determine the distance
of the station from the earthquake source. Furthermore, the
amplitude (height) of the S wave recorded at the station can be
used to determine earthquake magnitude.
27
8. What are the principal effects of an earthquake?
Ground Shaking: Rapid horizontal movements associated with
earthquakes. Shaking is exaggerated in areas where the
underlying sediment is weak or saturated with water. Fault
Uplift: Large sections of the earth’s surface (thousands of
square kilometers) may change elevation as a result of uplift on
an earthquake fault. Liquefaction occurs when water-saturated
sediment is collapses due to violent shaking. Landslides:
Earthquakes are often associated with mountains formed along
convergent plate boundaries. The steep slopes present in these
environments are prone to landslides when shaken. Tsunamis
are giant sea waves generated by submarine earthquakes,
especially noted from the Pacific Ocean.
9. What methods can be used to measure an earthquake?
There are three methods used for measuring earthquakes. The
Modified Mercalli scale measures earthquake intensity
represented by damages associated with earthquakes. The
Richter scale is the most well known and measures earthquake
magnitude using the amplitude (height) of the S-wave recorded
on a seismogram. The moment-magnitude scale has recently
found favor as a method that more accurately measures energy
release on large faults.
10. How is the Modified Mercalli scale used?
The Mercalli scale measures earthquake intensity: the level of
destruction of the earthquake (higher values) and the effect of
the event on people (lower values). The scale ranks intensity
from I to XII (1-12) using Roman numerals. Values of I to VI
represent increasing awareness of people; VII to XII involve
increasing damages associated with the event. The Mercalli
scale is not widely used for modern earthquakes because it is
inaccurate in areas of low population density and in cities
which lack stringent building codes, and has a variety of values
with distance from the epicenter.
11. What regions of the U.S. have a history of earthquake
activity?
Earthquakes are common in states along present-day plate
boundaries (California, Alaska) and are least common in the
continental interior (North Dakota, Minnesota). However,
some ancient fault zones in Missouri (New Madrid) and South
Carolina (Charleston) have experienced major infrequent
earthquake events.
28
12. What is the difference between great, major, and strong
earthquakes?
Great, major, and strong earthquakes are differentiated by
Richter magnitude. Great earthquakes (magnitude 8+) are rare
(average 1 per year); an average of 18 major earthquakes occur
annually with a magnitude of 7 to 7.9; strong earthquakes are
more common (120 per year) with a magnitude of 6 to 6.9.
13. How is the Richter scale used?
The Richter scale measures earthquake magnitude, the
amplitude of S waves recorded on a seismograph following an
earthquake. The Richter scale is logarithmic, each division
represents a ten-fold increase in the ground motion associated
with the earthquake, and ~30-times increase in energy released.
For example, a magnitude 7 earthquake has 10-times as much
ground motion (and releases over 30-times the energy) as a
magnitude 6, 100 times as much motion (900 times the energy)
as a magnitude 5, 1,000 times the motion of a magnitude 4, etc.
14. What controls the distribution of earthquakes?
Earthquakes are concentrated in narrow seismic belts along
plate boundaries. The largest earthquakes are typically
associated with convergent boundaries.
15. Is there a difference in the distribution of deep and shallow
earthquakes?
Deep earthquakes (to depths of 800 km) occur only in
association with subduction zones along convergent plate
boundaries. Shallow earthquakes occur along all plate
boundaries.
16. Are all U.S. earthquakes confined to the active plate
boundary along the western U.S.?
Most U.S. earthquakes occur in Alaska and California but
several smaller quakes occur along old fault zones in the
continental interior. A swarm of major earthquakes of
magnitude 7 to 8 occurred near New Madrid, Missouri, in a
three-month span from December 1811 to February 1812.
17. What factors control the size of future earthquakes?
Earthquake magnitude is related to fault length. Longer faults
yield larger earthquakes that shake the ground for longer
periods. Future large earthquakes are anticipated where strain
has accumulated along faults that have not experienced recent
29
seismic activity. Scientists predict the long-term probability of
earthquakes for specific locations on the basis of information
about strain accumulation and recurrence interval.
18. How do scientists determine the time between earthquakes?
Scientists estimate the recurrence interval - time between
earthquakes of similar magnitude - for individual faults by
determining the ages of offset layers of rocks and/or sediment.
Analysis of how much time has elapsed since the last
earthquake and the amount of energy that was released
(magnitude) help reveal which faults may be storing up
sufficient strain for earthquakes in the relatively near future.
30
Concept Map: Faults and Earthquakes
Finish the partially completed concept map for faults and earthquakes below. Print the
page and fill in the blanks with appropriate terms. Try to complete the map after reading
the section on faults in the this chapter. If you need some help, use some of the terms in
the list below to complete the concept map. There are more terms than spaces available.
strike-slip
1-10 meters
fault scarp
dip-slip
New Madrid
horizontally
hot spots
California
plate boundaries
volcanoes
1-10 kilometers
stream valleys
San Andreas fault
1,000 meters
faults
Alaska
1,000 kilometers
segments
31
Venn Diagram: Loma Prieta vs. San Francisco Earthquakes
Use the Venn diagram, below, to compare and contrast the similarities and
differences between the 1989 Loma Prieta and 1906 San Francisco earthquakes.
Both events occurred in the same region. Print this page and write features unique to
either group in the larger areas of the left and right circles; note features that they
share in the overlap area in the center of the image.
Loma Prieta
32
San Francisco
Earthquake Locations
Examine the map below and answer the following questions.
1. Which location is likely to have experienced the largest number of recent
earthquakes?
a) A
b) B
c) C
d) D
e) E
2. Which location is likely to have experienced the deepest recent earthquake?
a) A
b) B
c) C
d) D
e) E
Go to St. Louis University’s (SLU) site to examine the distribution of earthquakes
over the last 14 days or view maps of current seismicity of the world from the USGS
National Earthquake Information Center (NEIC) to check your predictions.
SLU: http://www.eas.slu.edu/Earthquake_Center/quakemaps.html
NEIC: http://wwwneic.cr.usgs.gov/neis/current/world.html
33
Earthquake Risk Evaluation Rubric
Following graduation you get a job working for a county planning task force in
California. The task force must examine the setting of several different cities and identify
which is at greatest risk for future earthquake damages from movements on known faults.
You are given the assignment to create an evaluation rubric to assess factors that will
influence the risk of potential damage from a future earthquake. The city that scores the
highest using the rubric will receive additional county funds to protect key structures
from earthquake damage. One factor is included as an example in the table below,
identify four more. Consider the relationship between faults and earthquakes, the
geologic properties of the location, and cultural factors when developing your rubric.
Factors
Proximity to
fault
Low Risk
(1 point)
Moderate Risk
(2 points)
High Risk
(3 points)
Far
(more than 100 km)
Intermediate
(20-100 km)
Close
(less than 20 km)
Reviewing your evaluation rubric you realize that some factors are more significant than
others. Your team decides to double the score of the most important factor. Which do
they choose? Why?
34
A map of the county showing the locations and characteristics
of four cities is provided below. Use your rubric to decide
which site will receive funding to retrofit key buildings and
other structures.
35