Transcript Earthquakes - Rosierulescience

Earthquakes
Chapter 8 & Then Some
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Where do Earthquakes
occur?
• Most earthquakes
take place near
the edges of
tectonic plate.
• Earthquakes
occur along
faults, where
blocks of crust
move.
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Earthquake Zones
• Earthquakes can occur near the Earth’s
surface or far below the surface.
• Most earthquakes occur at plate
boundaries (in earthquake zones),
but some happen at faults located in the
middle of tectonic plates.
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Earthquake Locations Around the World
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What Causes Earthquakes?
• Caused by movement along faults
(breaks in the Earth’s surface)
• Rock deforms (changes) due to
stress
• Two types of deformation:
– Plastic deformation: bending of rock; no
earthquakes created
– Elastic deformation: bending of rock
until it breaks; causes earthquakes
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What is Elastic Rebound?
• occurs when rock is suddenly returned to its
undeformed (original) shape
– Example: rubberband snapping back after being
stretched
• The rock will bend until enough force causes the
rocks on either side of a fault to move and
release energy.
• Think of it as a rubber band being stretched to
its limit and released—it snaps back.
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Elastic Rebound
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Earthquakes in
Alabama?!
Surely, you’re kidding? How?
Why?
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The New Madrid Fault
• If there is no plate boundary in the middle
of the United States, why do these
earthquakes take place?
• Geologists are beginning to understand the answer.
The New Madrid Fault Zone is part of an ancient
plate boundary.
• In this area, the North American Plate tried to form a
divergent plate boundary about 500 million years
ago. The splitting stopped before new plates could
form.
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The New Madrid Fault
• The faults in the New Madrid Zone are remnants of
this old event. Earthquakes occur because the North
American Plate is still "settling down".
• The faults in the New Madrid Zone do not reach the
Earth’s surface.
• They are buried beneath thousands of feet of rock
and sediment deposited by the Mississippi River.
• Geologists have located them by looking at the
patterns of earthquakes in the zone.
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New Madrid Fault
Several of the largest
earthquakes ever
recorded in the United
States occurred in the
Midwest, far from any
plate boundary. These
earthquakes took place
in an area called the
New Madrid Fault Zone,
named after the town of
New Madrid, Missouri.
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New Madrid Fault
• Over a three-month period in the winter
of 1811 to 1812, the New Madrid Fault
Zone was struck by three huge
earthquakes estimated to be greater than
magnitude 8.0 .
• The New Madrid Fault is what causes
Alabama to experience earthquakes.
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Alabama’s Earthquake Zone
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Alabama Earthquakes Since 1886
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Earthquakes in Alabama
• The last earthquake to occur in Alabama
was on July 27, 2007.
– It measured a 2.6 on the Richter scale.
(A little stronger than the one crazed LSU fans caused in
October 9,1988, when they defeated Auburn 7-6 for the
SEC title. 80,000 people jumping up & down created
surface waves, triggering an earthquake. Imagine that
many people jumping up & down on a trampoline.)
• This was not a very strong earthquake, but
it was recorded by a seismograph, an
instrument used to measure earthquakes.
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Earthquakes
Boundaries & Faults
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3 Types of Faults Associated
with Earthquakes
PLATE
MOTION
Transform
FAULT TYPE
Convergent
Strike –Slip
Fault
Reverse Fault
Divergent
Normal Fault
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Strike-Slip Fault occurs at a
Transform Boundary
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Reverse Fault occurs at a
Convergent Boundary
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Normal Faults occur at
Divergent Boundaries
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Earthquakes and Faults Organizer: Draw the boundary
on the plate motion side & the fault moving on the fault
type side (label Hanging Wall & Foot Wall)
PLATE MOTION
FAULT TYPE
Transform Boundary: Plates Strike –Slip Fault: Fault blocks
move past each other
move past each other
Convergent: Plates push
Reverse Fault: Fault blocks
move toward each other
toward each other
Divergent Boundary: Plates Normal Fault: Fault blocks move
move apart
apart
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Earthquake Energy:
Just How Strong Was
It Anyway?
Earthquake Energy=Seismic Energy
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How do Earthquake Waves
Travel?
• Energy released from moving plates &
faults travels through the Earth as
waves.
• Waves travel at different speeds & in
different ways, depending on what kind
of material they are moving through.
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Earthquake Energy
This energy is called seismic energy
and is measured in seismic waves:
– Two types of seismic waves:
• body waves
• surface waves
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Body Waves: waves that
travel through the Earth’s
interior
• There are two types of body waves:
– P-waves
– S-waves
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P and S wave comparison
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P-waves: primary waves or
pressure waves
• travel through solid, liquids and gases
• fastest waves (travel 5-8 km/h), arrive first
& are first ones to be detected
• P-waves cause rock to squeeze and stretch.
Imagine a slinky!
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P-Waves
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S-waves: secondary waves
• second fastest waves; 2nd to arrive;
shear waves
• slower than P-waves; travel 3-5 km/h
• S-waves move rock from side to side.
• S-waves can’t travel through parts of
the Earth that are completely liquid.
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S-Waves
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Surface Waves
• move along the surface of the Earth
• can move up, down & around or side to
side
• Surface waves move the slowest &
cause the most destruction.
– Think about it? Where do most people live:
on the Earth’s crust or in it? Shouldn’t
these waves be the most destructive since
they occur where there is more stuff to
destroy?
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Surface Waves
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Seismic Wave Arrival
• First- P-waves
• Second- S-waves
• Third- Surface Waves
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The area of the Earth that does not receive seismic
energy is called the Shadow Zone.
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Shadow Zone
The shadow zone results from S waves being
stopped entirely by the liquid core and P waves
being bent (refracted) by the liquid core.
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Mohorovicic discontinuity: The Moho
Zone
• The Moho is the boundary between the crust and the mantle in
the earth. This is a depth where seismic waves change velocity,
or speed. They usually increase at this location—the Moho zone.
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Quiz
•
•
•
•
•
•
1. Most Earthquakes occur along ________________.
2. The first seismic waves to arrive are______________.
3. The second seismic waves to arrive are _____________.
4. The last seismic waves to arrive are_______________.
5. Which seismic waves travel the fastest?___________
6. Which type of seismic wave can move through a solid,
liquid or a gas?________________
• 7. Which seismic wave cannot travel through material that
is completely liquid?______________
• 8. Which seismic waves are the slowest and the most
destructive?_______________
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Measuring Earthquakes
Seismographs & Seismograms
Wait aren’t those the same?
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Measuring Earthquakes
• Seismographs: instruments used to
sense earthquakes
– seismogram: tracing of
earthquake movement made by
seismograph
• Seismologists use seismograms
to locate earthquakes.
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Picture of a Seismograph
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Time and Location of Earthquakes
• Seismologists use seismograms to find the
epicenter of earthquakes:
– the point on top of the Earth above the true
center of an earthquake which is called the
focus—it’s inside the Earth. (Also called the
hypocenter.)
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Seismograms
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Measuring Earthquakes
• Earthquakes are defined by their
strength and depth, which is
measured at the place the
earthquake occurs.
• 2 Scales are used to rank the quakes:
– Richter Scale—measures magnitude
– Mercalli Scale—measures intensity
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Earthquake strength &
intensity
• Richter scale: measures magnitude (think
strength) of ground motion on a scale of
2.0-8.0 (numbers)
• Mercalli scale: measures degree to which an
earthquake is felt by people & amount of
damage caused, which is known as intensity
– Scale of I-XII (Roman Numerals)
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Magnitude
Effects
2.0
(2-2.9 very minor)
Only felt by seismograph
3.0
(3-3.9, minor)
Felt at epicenter
4.0
(4-4.9 light)
Felt by most people in area
5.0
(5-5.9, moderate)
Damage at epicenter
6.0
(6-6.9, strong)
Widespread damage
7.0
(7-7.9 major)
Great, widespread damage
8.0 or higher
(8 & up, great)
Catastrophic destruction
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Reading Richter Scale Maps
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Reading Mercalli Scale
Maps
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Seismograph
•
•
•
A seismograph is an instrument used by scientists to measure
earthquakes.
Seismologists who study earthquakes can determine when an
earthquake started by noting the arrival times of P-waves and S-waves.
A seismograph records vibrations in the Earth and determines the
strength and location of an earthquake.
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Chinese Seismograph
Ancient
Chinese
Seismograph.
The ball
would drop
from the
dragon to the
frog. It told the
people from
which
direction the
earthquake
came.
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Seismograms
0
1
2
3
Time in Minutes
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5
6
7
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1. How many minutes did it take for the
P-Waves to arrive?
2. How many minutes did it take for the
S-waves to arrive?
3. How long did the surface waves last?
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IV
Oahu
Maui V-VI
Hawaii
6.7
VII
VI
V
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Questions from the Hawaii intensity
map:
• 1. What was the magnitude of the Hawaii
earthquake?
• 2. Where was the intensity the greatest?
• 3.According to the Mercalli Scale on what
island did the most damage likely occur?
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Mercalli Intensity Scale
1. What is the intensity
at Monterey?
Smith
2. What is the intensity
at the epicenter?
3. What is the intensity
at San Jose?
4. What is the intensity
at Santa Cruz?
5. What is the intensity
at Smith?
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