Transcript Earthquakes - Needham.K12.ma.us

Earthquake
• The shaking that results from
the movement of rock beneath
Earth’s surface
There are about _____
earthquakes per day, worldwide.
• 8,000
Where do Earthquakes Occur?
• Earthquakes occur at tectonic plate
boundaries, and along fracture zones
(fault lines).
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Red lines = plate boundaries
Orange lines = fracture zones
The movement between plates and
along faults is not smooth. The plates
move in jerks, giving rise to
earthquakes. The locations of
earthquakes throughout the world
mark the major tectonic boundaries.
The red dots
indicate areas
where
earthquakes
have
occurred.
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The movement of Earth’s plates
creates powerful forces that ___
or ___ the rock in the crust.
• squeeze
• pull
Stress
• Stress: A force (push or pull) that
acts on rock to change its shape or
volume
Compression
• Stress that squeezes rock until it
folds or breaks
Tension
• Tension: Stress that stretches rock so
that it becomes thinner in the middle
Shearing
• Shearing: Stress that pushes a mass of
a rock in opposite, horizontal directions
Deformation
• Deformation: A change in the
volume or shape of Earth’s crust
(which causes it to bend, stretch,
break, tilt, fold or slide)
• Most changes in the crust occur so
slowly that they cannot be observed
directly
Review: How do stress forces
affect rock?
• The three kinds of forces that affect rock are:
• Shearing
– The rocks break and slip apart
• Tension
– The rock stretches and becomes thin in
the middle
• Compression
– The rock squeezes until it folds or breaks
• These stresses work over millions of years to
change the shape and volume of rock
Faults
• A break in the Earth’s crust, no more
than 10 miles deep, where slabs of
rock slip past each other
• Faults occur when enough stress
builds up in rock to make it move.
Why do faults form and
where do they occur?
• Faults usually occur along plate
boundaries or at fracture zones,
where the forces of plate motion
compress, pull, or shear the crust so
much that the crust breaks (deforms)
• Rocks on both sides of the fault can
move up or down, or sideways
Strike-Slip Faults
• A type of fault where rocks on either
side move past each other sideways
with little up or down motion.
• Shearing causes these types of faults
Normal Faults
(sometimes called Thrust Faults)
• A type of fault where the hanging
wall slides downward
• Tension forces cause normal faults
Hanging Wall & Footwall
• Hanging wall: The block of rock that forms
the upper half of a fault
• Footwall: The block of rock that forms the
lower half of a fault
Reverse Faults
• A type of fault where the hanging wall
slides up
• Compression forces cause reverse faults
Review: What are the three
types of fault? What force of
deformation produces each type?
• Strike-slip faults
• Produced by shearing
• Normal faults
• Produced by tension
• Reverse faults
• Produced by compression
What is friction?
• A force that opposes the motion of
one surface as it moves across
another surface
Friction exists because…
• surfaces are not perfectly smooth.
Describe what occurs when
the friction along a fault line
is low.
• The rocks on both sides of the fault
slide by each other without much
sticking
Describe what occurs when
the friction along a fault line
is moderate.
• The sides of the fault jam together
• From time to time they jerk free
• Small earthquakes occur
Describe what occurs when
the friction along a fault line
is high.
• Both sides of the fault lock together and
do not move
• The stress increases until it is strong
enough to overcome the force of friction
• Larger and/or more frequent earthquakes
will occur
The San Andreas
fault in California is
a transform
boundary that
contains ___ stress.
• high
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Fault-Block Mountain
• A mountain that forms where a
normal (thrust) fault uplifts a block
of rock
How does the process of a
fault-block mountain begin?
• Where two plates move away from each other,
tension forces create many normal faults
• When two of these normal faults form parallel to
each other, a block of rock is left lying between
them
• As the hanging wall of each normal fault slips
downward, the block in between moves upward
• When a block of rock lying between two normal
faults slides downward, a valley forms
Focus
• The point beneath Earth’s surface
where rock breaks under stress and
causes an earthquake
Epicenter
• The point on Earth’s surface directly
above an earthquake’s focus
Seismic Waves
Seismic Wave
• A vibration that travels through
Earth, carrying the energy released
during an earthquake
At what point do seismic waves
first reach the surface?
• The epicenter
What determines how much
the ground shakes during an
earthquake?
• How close a location is to the
epicenter
• The types of rock and soil surrounding
the epicenter determines how much
the ground shakes
There are three types of
seismic waves:
• Primary Waves (P waves)
• Secondary Waves (S waves)
• Surface Waves
• An earthquake releases two types of
waves: P waves and S waves
• When the waves reach Earth’s surface at
the epicenter, surface waves develop.
Primary Waves (P Waves)
• A type of seismic wave that compresses
and expands the ground
• The first wave to arrive at epicenter
Secondary Waves (S Waves)
• A type of seismic wave that moves the
ground up and down or side to side
• Second wave to arrive at epicenter
How are P waves different
from S Waves?
• P waves compress and expand the ground
• P waves can travel through solids, liquids
and gases
• S waves move the ground up and down or
side to side
• S waves can only travel through solids
Surface Waves
• A type of seismic wave that forms when
P waves and S waves reach Earth’s surface
• Surface waves are the most destructive of
the 3 types of waves, because they move
both sideways and up-and-down.
Why do you think surface waves
produce more severe ground
movements than P waves and S
waves?
• Surface waves travel through loose
soil, sand, gravel, mud, and small
rocks (not solid rock)
• These looser substances are more
likely to shift and slide
How does the energy of an
earthquake travel through
Earth?
• Seismic waves carry the energy of an
earthquake from the focus, through Earth’s
interior, to the epicenter, and across the
surface
Detecting Seismic Waves
Seismograph or Seismometer
• A device that records ground
movements caused by seismic waves as
they move through Earth
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The old-fashioned kind
iPhone seismometer-- yes,
there’s an app for that!
Describe how a mechanical
seismograph records ground
movement.
• A heavy weight attaches to a frame by
spring or wire
• A pen connected to the weight rests its
point on a rotating drum
• During an earthquake the seismic waves
cause the drum to shake while the pen
stays in place
• The pen records lines on the paper
around the drum
Measuring Earthquakes
Magnitude
• The measurement of an earthquake’s
strength based on seismic waves and
movement along faults
Intensity
• the strength of ground movement in
a given place.
Mercalli Scale
• A scale that rates earthquakes
according to their intensity and how
much damage they cause
• Developed in the early twentieth
century
The Mercalli scale has ______
steps and describes how an
earthquake affects ________,
________, and the ________.
•
•
•
•
12
People
Buildings
Land surface
How would you rate the damage
to the Foligno city hall on the
Mercalli scale?
• The
damage
would
probably
rate VII VIII
Richter Scale
• A scale that rates
the amplitude
(height) of seismic
waves measured by a
seismograph
• It is a logarithmic
scale, which means
each level has 10
times the magnitude
of the level below it.
Richter
Magnitude
-1.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
10.0
12.0
Comparable Energy Example
(approx.)
breaking a rock on a lab table
large blast at construction site
large quarry or mine blast
small nuclear weapon
average tornado (total energy)
Little Skull Mtn, NV quake, 1992
Double Spring Flat, NV quake, 1994
Northridge, CA quake, 1994
Haiti Earthquake, Jan.12, 2010
Landers, CA quake 1992
SF, CA quake 1906
Anchorage, AK quake 1964
Chilean quake, 1960
(San-Andreas type fault circling Earth)
(Fault through center of Earth)
What is a strength of the
Richter scale when measuring
earthquakes? What is a
weakness?
• Strength: Provides accurate
measurements for small, nearby
earthquakes
• Weakness: Does not measure larger
and more distant earthquakes well
How are the Mercalli scale
and the Richter scale similar?
How are they different?
• Both measure the strength of an earthquake
• The Mercalli scale measures the strength
in terms of the amount of damage caused
and the amount of shaking that people
experienced
• The Richter scale measures the size of an
earthquake’s seismic waves
Moment Magnitude Scale
• A scale that rates earthquakes by
estimating the total energy released
by an earthquake
Why is the moment magnitude
scale used today by geologists
to measure earthquakes?
• The moment magnitude scale determines
the total energy released by an earthquake
• This scale uses a electronic seismograph
that can measure earthquakes that are big
or small, and near or far
• Geologist examine movement along the fault
and the strength of broken rock
• These two measurements give a more
accurate measurement of an earthquake
On which scale would an
earthquake’s strength vary
from one place to another?
Explain.
• The Mercalli scale, because the
amount of shaking that people would
feel and the damage to objects would
be greater in a place closer to the
earthquake’s epicenter
Review: What are three scales for
measuring earthquakes? Explain what
each one measures.
• Mercalli Scale
• Measures earthquakes on how much damage they
cause
• Richter Scale
• Measures seismic waves using a seismograph
• Measures small and nearby earthquakes
• Moment Magnitude Scale
• Looks at the total energy released
• Measures large and distant earthquakes
• Helps scientists predict how much fault movement
there was
Locating the Epicenter
How do scientists calculate
how far a location is from the
epicenter of an earthquake?
• Scientists calculate the difference
between arrival times of the P
waves and S waves
• The further away an earthquake is,
the greater the time between the
arrival of the P waves and the S
waves
Figure 17: Use the map scale to
determine the distances from
Savannah and Houston to the
epicenter. Which one is closer?
• Houston
• 800 Km
• Savannah
• 900 km
Seismicity of the Caribbean plate
Haiti earthquake 1-12-2010
7.0
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