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Chapter 8
Earthquakes
The New Madrid Earthquakes
Eyewitnesses to the 1811-1812 earthquakes in
New Madrid, Missouri, reported seeing bright
flashes of light and a dull glow in the sky over a
wide area.
Reeking sulfurous odors also
accompanied the quakes. Many survivors were
convinced that the quakes were a heavenly sign
meant to frighten the local citizens back to
church. As a result, church attendance in the
area skyrocketed between 1811 and 1812.
IS IT A FACT!
The strongest earthquake recorded to
date occurred in Chile in 1960. It
measured 9.5 on the Richter scale.
This is equivalent to detonating more
than 1 billion tons of TNT!
What Are Earthquakes?
There is a branch of Earth Science
devoted to earthquakes called
seismology. Seismology is the study
of earthquakes.
Earthquakes are
complex, and they present many
questions for seismologists, the
scientists that study earthquakes.
Where Do Earthquakes
Occur?
Most earthquakes take place near the
edges of tectonic plates. Tectonic plates
are giant masses of solid rock that make
up the outermost part of the earth.
A fault is a break in the Earth’s crust along
which blocks of the crust slide relative to
one another. Earthquakes occur along
faults due to this sliding. This is why
earthquakes are so common near tectonic
plate boundaries.
The largest and most active
earthquake zone lies along
the
plate
boundaries
surrounding
the
Pacific
Ocean.
What Causes Earthquakes?
As tectonic plates push, pull, or
scrape against each other, stress
builds up along the faults near the
plates’ edges. In response to this
stress, rock in the plates deforms.
Deformation
Deformation is the change in the
shape of the rock in response to
stress. Rock along a fault deforms in
mainly two ways – in a plastic
manner, like a piece of molded clay,
or in an elastic manner, like a rubber
band.
Deformation contd.
Plastic deformation does not lead to
earthquakes.
Elastic deformation, however,
does lead to earthquakes. While rock can
stretch farther than steel without breaking, it will
break at some point.
Think of elastically
deformed rock as a stretched rubber band. You
can stretch a rubber band only so far before it
breaks.
When the rubber band breaks, it
releases energy, and the broken pieces return to
their un-stretched shape.
Elastic Rebound
Like the return of the broken rubber band
pieces to their un-stretched shape, elastic
rebound is the sudden return of elastically
deformed rock to its original shape. Elastic
rebound occurs when more stress is applied to
rock than the rock can withstand. During
elastic rebound, rock releases energy that
causes an earthquake.
MISCONCEPTION ALERT!!
Earthquakes are not a rare phenomenon.
In fact, more than 3 million earthquakes
happen each year, about one every 10
seconds! Most earthquakes are too weak
to be felt by humans. The Ring of Fire, a
volcanic zone which lies along the plate
boundaries surrounding the Pacific Ocean,
is also the world’s largest and most active
earthquake zone.
Are All Earthquakes the
Same?
Earthquakes differ in strength and in
the depth at which they begin. These
differences depend on the type of
tectonic plate motion that produces
the earthquake. Examine the chart
and the diagram below to learn how
earthquakes differ.
WEIRD SCEINCE
Many people assume that major
earthquakes in the United States occur
only on the West Coast. However, major
quakes have occurred in South Carolina
and Missouri---far from any active plate
boundaries. The four major tremors of the
1811-1812 earthquakes in New Madrid,
Missouri, were so intense that, according
to reports, they altered the flow of the
Mississippi River and rang church bells in
Boston!
How Do Earthquakes
Travel?
Remember rock releases energy when it springs
back after being deformed. This energy travels
in the form of seismic waves. Seismic waves
are waves of energy that travel through the
Earth. Seismic waves that travel through the
Earth’s interior is called body waves. There are
two types of body waves: P waves and S waves.
Seismic waves that travel along the Earth’s
surface is called surface waves. Different types
of seismic waves travel at different speeds and
move the materials that they travel through
differently.
Earthquake Waves
Seismographs reveal that two main
groups of seismic waves are
generated by the slippage of a rock
mass.
Surface Waves- travel along the
surface
Body Waves- travel through the
Earth’s interior
“L” - Waves
Surface waves consist of two types of
wave motion. One motion produces a
complex up-and-down motion similar to
ocean swells, while the other surface wave
whips the ground from side-to-side without
any vertical motion. Surface waves have a
longer time interval between crests and
are referred to as L-Waves. These waves
cause the greatest destruction.
“S” - Waves
“S” Waves- (secondary) shake the
particles at right angles to the
direction of wave travel. This can be
illustrated by tying one end of a rope
to a post and shaking the other end.
S waves do not travel through liquids
and gasses, only through solids.
“P” - Waves
“P”
Waves(primary)
are
compressional waves and travel
through solids, liquids and
gasses. This wave action is like
that generated by human vocal
chords as they move air to create
sound.
Earthquakes
The epicenter is the location on the surface
of the Earth directly above the focus
(source) and can be measured by a
seismograph.
Seismograph- A device used to measure
the low frequency waves generated by an
earthquake. They are placed at over 100
locations throughout the world and provided
geologists with data about the Earth’s
composition,
by
understanding
the
characteristics of the low frequency waves
generated by an earthquake.
Earthquakes
Seismograph of an Earthquake
Locating Earthquakes
Seismographs
are
instruments
located at or near the surface of the
Earth that record seismic waves.
When
the
waves
reach
a
seismograph,
the
seismograph
creates
a
seismogram.
A
seismogram is a tracing of
earthquake motion created by a
seismograph.
When Did It Happen?
Seismologists use seismograms to
calculate when an earthquake started. An
earthquake starts when rock slips
suddenly enough along a fault to create
seismic waves. Seismologists find an
earthquake’s start time by comparing
seismograms and noting the difference in
arrival times of P waves and S waves.
Where Did It Happen?
Seismologists also use seismograms
to find an earthquake’s epicenter. An
epicenter is the point on the Earth’s
surface
directly
above
the
earthquake’s starting point. A focus
is the point inside the Earth where an
earthquake begins.
QUIZ
1. How is an earthquake’s epicenter related
to its focus?
2. As seismic waves travel farther, what
happens to the difference in arrival times
of P and S waves?
1. The epicenter is the point on the Earth’s
surface directly above the focus, which is
where the earthquake originates.
2. It increases.
The Richter Scale
The Richter scale is commonly
used to measure earthquake
strength. It is named after
Charles Richter, an American
seismologist who developed
the scale in the 1930’s.
Richter Scale
The Richter Scale is used worldwide to
describe earthquake magnitude. Using
Richter’s scale, the magnitude is determined
by measuring the amplitude of the largest
wave recorded on the seismograph. Largemagnitude earthquakes will cause the
seismograph pen to be displaced farther than
small-magnitude earthquakes.
Earthquake Energy
There is a pattern in the Richter scale
relating an earthquakes magnitude and
the amount of energy release by the
earthquake. Each time the magnitude
increases by 1 unit, the amount of energy
released becomes 31.7 times larger. For
example, an earthquake with a magnitude
of 5.0 on the Richter scale will release
31.7 times as much energy as an
earthquake’s with a magnitude of 4.0 on
the Richter scale.
Earthquake Hazard
Earthquake hazard measures how
prone an area is to experiencing
earthquakes in the future. An area’s
earthquake-hazard
level
is
determined by past and present
seismic activity. The West Coast, for
example, has a very high earthquakehazard level because it has a lot if
seismic activity.
Earthquake Forecasting
Predicting when and where earthquakes
will occur and how strong will be a difficult
task. However, by closely monitoring
active faults and other areas of seismic
activity, seismologists have discovered
some patterns in earthquakes that allow
them to make some broad predictions.
The Gap Hypothesis
The gap hypothesis states that sections of
active faults that have had relatively few
earthquakes are likely to be the sites of stronger
earthquakes in the future. The areas along a
fault where relatively few earthquakes have
occurred are called seismic gaps.
The gaps hypothesis helped seismologist
forecast the approximate time, strength and
location of the 1989 Loma Prieta earthquake in
the San Francisco Bay area.
Are You Prepared for an
Earthquake?
If you live in an earthquake-prone area or ever
plan to visit one, there are many things you can
do to protect yourself and your property from
earthquakes. Plan ahead so you will know what
to do before, during, and after an earthquake.
Stick to your plan as closely as possible.
When The Shaking Starts… The best thing
to do if you are indoor is to crouch or lie face
down under a table or desk in the center of the
room.
WEIRD SCIENCE
Engineers have devised giant shock
absorbers for buildings. The shock
absorbers contain a ferrofluid solution
that becomes rigid in a magnetic field.
When an earthquake occurs, a
computer controls the electromagnets
in the shock absorbers to dampen the
vibrations!
QUIZ
1) What is gap hypothesis?
2) Why should you lie under a table or desk
during an earthquake?
3) What are aftershocks?
1)The gap hypothesis states that sections of
active faults that have had relatively few
earthquakes are likely to be the sites of stronger
earthquakes in the future.
2)The table or desk might prevent falling objects
from hitting you and causing injury.
3)They are weaker earthquakes that follow
stronger earthquakes.
After the Shaking Stops
Being in an earthquake is a startling
experience. Afterward, you should not be
surprised to find yourself and others
puzzled about what happened.
You
should try to calm down, get your
bearings, and remove yourself from
immediate danger, such as downed power
lines, broken glass, and fire hazards. Be
aware that there may be aftershocks.
Recall your earthquake plan, and follow it
through.
Earthquake Discoveries Near
and Far
The study of earthquakes has led to
many important discoveries about the
Earth’s interior. Seismologists learn
about the Earth’s interior by observing
how seismic waves travel through the
Earth. Likewise, seismic waves on
other cosmic bodies allow
seismologists to study the interiors of
those bodies.
Earthquakes
The Moho is a place within the Earth where the
speed of seismic waves increases sharply. It
marks the boundary between the Earth’s crust
and mantle.
The shadow zone is an area on the Earth’s
Surface where no direct seismic waves from a
particular earthquake can be detected. This
discovery suggested that the Earth has a liquid
core.
The solid inner core was discovered in 1936.
Before this discovery, seismologists thought that
the Earth’s entire core was liquid.
The Moon
In July 1969, humans set foot on the moon
for the first time. They brought with them
a seismograph. Not knowing if the moon
was seismically active, they left nothing to
chance – they purposely crashed their
landing vehicle back to the moon’s surface
after they left to create artificial seismic
waves. What happened after that left
seismologists astonished.
IS THAT A FACT!!
“Moonquakes”
fall
into
three
categories: deep quakes, which result
from the gravitational pull of the
Earth; shallow quakes, which may be
caused by the heating and cooling of
the moon’s surface; and quakes
caused by collision of objects with the
moon’s surface.
Ohio Earthquake