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Earthquakes and Volcanoes
Earthquakes
What are earthquakes?
Imagine bending a stick until it breaks. When the stick
snaps, it vibrates, releasing energy. Earthquakes release
energy in a similar way. Earthquakes are the vibrations in the
ground that result from movement along breaks in Earth’s lithosphere.
These breaks are called faults.
Why do rocks move along a fault? The forces that move
tectonic plates also push and pull rocks along a fault. If these
forces become large enough, the blocks of rock on either side
of the fault can move past each other. The rocks might move
vertically—up or down—or horizontally—sideways.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
The size of an earthquake depends on the amount of
force applied to the fault. The greater the force applied to a
fault, the greater the chance of a large and destructive
earthquake occurring.
Earthquakes can cause billions of dollars in damage.
Injuries and fatalities often occur during earthquakes.
Earthquakes are common in the state of California. In 1994,
the Northridge earthquake along the San Andreas Fault in
California caused $20 billion in damage.
Where do earthquakes occur?
Few earthquakes occur in the middle of a continent.
Most earthquakes occur in the oceans and along the edges
of continents where tectonic plates meet.
Earthquakes and Plate Boundaries
Stress builds up along plate boundaries. Earthquakes
result from the buildup and release of this stress along the
active plate boundaries. The deepest and strongest
earthquakes occur along convergent plate boundaries. At a
convergent plate boundary, plates collide. The denser
oceanic plate subducts, or drops down, into the mantle.
These earthquakes release great amounts of energy.
Shallow earthquakes commonly occur where plates
separate along a divergent plate boundary or along a
transform plate boundary. Earthquakes occur at varying
depths where continents collide. Continental collisions
form large, deformed mountain ranges.
Rock Deformation
Force, or pressure, applied along plate boundaries can
cause a body of rock to bend and change shape. This is
called rock deformation. Over time, the rocks can break
and move.
Faults
Types of Faults
Fault Name
Location
Movement
Strike-slip
transform plate
boundaries
Two blocks of rock slide
horizontally past each
other in opposite
directions.
Normal
divergent plate
boundaries
Forces pull two blocks
of rock apart. One block
drops down relative to the
other.
Reverse
convergent plate
boundaries
Forces push two blocks
of rock together. One
block moves up relative to
the other.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
A fault is a break in Earth’s lithosphere where one block of rock
moves toward, away from, or past another. When rocks move in
any direction along a fault, an earthquake occurs. The table
below describes three types of faults. The forces applied to
a fault determine the direction the rocks move.
Earthquake Focus and Epicenter
When rocks move along a fault, they release energy.
Energy that travels as vibrations on and in Earth is called seismic
waves. Seismic waves originate where rocks first move along the fault,
at a location inside Earth called the focus. Earthquakes can occur
anywhere between Earth’s surface and depths greater than
600 km. In a news report, you might hear a reporter identify
the earthquake’s epicenter. The epicenter is the location on Earth’s
surface directly above an earthquake’s focus.
Seismic Waves
During an earthquake, there is a rapid release of energy
along a fault. This release of energy produces seismic waves.
The waves travel outward in all directions through rock,
much like ripples in water. As the waves travel, they transfer
energy through the ground and produce the motion
associated with an earthquake. The energy released is
strongest near the epicenter. As seismic waves move away
from the epicenter, their energy and intensity decrease. The
greater the distance from an earthquake’s epicenter, the less
the ground moves.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Types of Seismic Waves
During an earthquake, particles in the ground can move
back and forth or up and down. Particles can also move in
an elliptical motion parallel to the direction the seismic
wave travels. Scientists use wave motion, wave speed, and
the type of material the wave travels through to classify
seismic waves. The three types of seismic waves are primary
waves, secondary waves, and surface waves.
Primary waves, also called P-waves, cause particles in the ground
to move in a push-pull motion, similar to a coiled spring. P-waves
move faster than any other seismic waves. They are the first
waves detected and recorded after an earthquake.
Secondary waves, also called S-waves, cause particles to move up
and down at right angles relative to the direction that the wave travels.
These waves move like a coiled spring when it is shaken
side-to-side and up and down at the same time.
Surface waves cause particles in the ground to move up and
down in a rolling motion, similar to ocean waves. Surface
waves travel only on Earth’s surface. P-waves and S-waves
can travel through Earth’s interior. However, scientists have
discovered that S-waves cannot travel through liquid.
Properties of Seismic Waves
Seismic Waves
Description
Primary waves
(P-waves)
• Cause rock particles to vibrate in same
direction as waves travel
• Fastest seismic waves
• First waves that seismometers detect
• Travel through solids and liquids
Secondary waves
(S-waves)
• Cause rock particles to vibrate
perpendicular to direction that waves
travel
• Slower than P-waves; faster than surface
waves
• Second waves that seismometers detect
• Travel only through solids
Surface waves
• Cause rock particles to move in a rolling
or elliptical motion in the same direction
that waves travel
• Slowest seismic waves
• Cause the most damage at Earth’s surface
Mapping Earth’s Interior
Inner and Outer Core Seismologists discovered that S-waves
cannot travel through the outer core. This discovery proved
that Earth’s outer core is liquid, unlike its solid inner core.
By analyzing the speed of P-waves, seismologists also
discovered that the inner and outer cores are mostly iron
and nickel.
The Mantle Seismologists have used seismic waves to model
convection currents in the mantle. The speeds of seismic
waves depend on the temperature, pressure, and chemistry
of the rocks that the waves travel through. Seismic waves
tend to travel slower as they move through hot material,
such as in areas of the mantle below mid-ocean ridges.
Seismic waves travel faster in cooler areas of the mantle near
subduction zones.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Scientists who study earthquakes are called seismologists
(size MAH luh justs). They use properties of seismic waves, as
described in the table above, to map Earth’s interior. P-waves
and S-waves change speed and direction as they travel
through different materials. Seismologists measure the speed
and direction of waves as they move through Earth at
different depths. Using these measurements, seismologists
can determine the materials that make up Earth’s layers.
Locating an Earthquake’s Epicenter
An instrument called a seismometer (size MAH muh ter)
measures and records ground motion and can be used to determine the
distance seismic waves travel. A seismometer records ground motion
as a seismogram, a graphical illustration of seismic waves.
Seismologists use a method called triangulation to locate
an earthquake’s epicenter. This method uses the speeds and
travel times of seismic waves to determine the distance to
the earthquake’s epicenter from at least three seismometers
at different locations.
1. Find the arrival time difference. First, scientists
determine the number of seconds between the
arrival of the first P-wave and the first S-wave on the
seismogram. This time difference is called lag time.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
2. Find the distance to the epicenter. Next, seismologists
plot the lag time against distance on a graph. This
reveals the distance of the epicenter from the
seismograph’s location.
3. Plot the distance on a map. Seismologists determine the
distance of the epicenter to seismographs in at least
three different locations. The map below shows circles
around the locations of three seismometer stations.
The distance from each station to its circle measures
the distance from that station to the earthquake’s
epicenter. The epicenter must lie somewhere on
the circle around each station. Only one point lies
on all three circles. The point where the three circles
intersect is the epicenter.
Station 1
Epicenter
Station 3
Station 2
Determining Earthquake Magnitude
The Richter magnitude scale uses the amount of
ground motion at a given distance from an earthquake to
determine magnitude. Each increase of one unit on the
Richter scale represents 10 times the amount of ground
motion. For example, a magnitude 8 earthquake produces
10 times greater shaking than a magnitude 7 earthquake
does and 100 times greater shaking than a magnitude 6
earthquake does (10 × 10).
The moment magnitude scale measures the total amount
of energy released by an earthquake. For each increase of
one unit on the scale, an earthquake releases 31.5 times
more energy. For example, a magnitude 8 earthquake
releases more than 992 times the amount of energy than
that of a magnitude 6 earthquake (31.5 × 31.5).
Describing Earthquake Intensity
Another way to measure and describe an earthquake is
to examine the amount of damage that results from the
shaking. The Modified Mercalli scale measures the intensity
of an earthquake based on descriptions of its effects on
people and structures. The scale, shown below, ranges from I,
an earthquake that people do not feel, to XII, an earthquake
that destroys everything. The higher the number is, the
greater the effects.
Modified Mercalli Scale
I
Not felt except under unusual conditions.
II
Felt by few people; suspended objects swing.
III
Most noticeable indoors; strong vibrations.
IV
Felt by many people indoors but few outdoors; dishes rattle.
V
Felt by nearly everyone; dishes break.
VI
Felt by all; furniture shifts.
VII
Everyone runs outdoors; some chimneys break.
VIII
Chimneys, smokestacks, and walls fall.
IX
Great damage occurs; buildings shift off of foundations.
X
Most ordinary structures are destroyed; landslides occur.
XI
Few structures remain standing; bridges are destroyed.
XII
Damage is total.
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An area’s geology also influences earthquake damage. The
shaking produces more damage in areas covered by loose
sediment than it does in places built on solid bedrock.
Earthquake Risk
In the United States, the highest risk of earthquakes
occurs near tectonic plate boundaries of the western states.
The transform plate boundary in California and the
convergent plate boundaries in Oregon, Washington, and
Alaska have the highest earthquake risks. However, not all
earthquakes occur near plate boundaries. Some of the largest
earthquakes in the United States have occurred far from
plate boundaries.
High-energy, destructive earthquakes are not very
common. Only about ten earthquakes with a magnitude
greater than 7.0 occur worldwide each year. Earthquakes
with magnitudes greater than 9.0, such as the Indian Ocean
earthquake in 2004, are rare.
Seismologists evaluate risk in several ways because
earthquakes threaten people’s lives and property. They
study the probability that an earthquake will occur in an
area. Seismologists study past earthquake activity, the
geology around a fault, the population density, and the
building design in an area.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Engineers use these risk assessments to design buildings
that can withstand the shaking during an earthquake. City
and state government officials use risk assessments to help
plan and prepare for future earthquakes.