Download Earthquakes - for Jack L. Pierce

3
Earthquakes
and
Earth’s Interior
Questions:
• How many have felt an earthquake?
• What is an earthquake (EQ)?
Earthquake Statistics
• 30,000 EQ’s strong enough to feel annually
• 75 significant EQ’s each year
• 1960 Southern Chile, M 9.5
• 1964 Alaska, M 9.2
• 2004, Sumatra-Andaman, M 9.1
5
(M 9.5), So. Chile-1960
(M 9.2), Alaska-1964
(M 9.1), Sumatra-Andaman-2004
6
How are earthquake vibrations similar to a rock
entering water?
Earthquake – vibrations within the earth that
produce rapid release of energy
in the form of seismic waves
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Is there evidence for earthquakes?
San Francisco, 1906
8
July 21, 1952 Bakersfield EQ
19th st and Chester
19th street, Bakersfield
23 mi south of Bakersfield
Earthquake Terminology
Focus
actual rupture takes
place
Epicenter
rupture point directly
above the focus
Seismic Waves
vibrations released from
an actual rupture
10
I
a good earthquake.
Discuss with a friend:
1. How would you define an earthquake?
2. About how many significant EQ’s
take place per year?
3. Define the following:
epicenter
seismic waves
focus
I will get an A on my exams and quizzes.
11
What is taking place?
12
How do EQ’s occur?
The Elastic Rebound Theory:
•Stresses begin to build and strain the crust.
•Continued stress stores energy and begins to
reach the “elastic” limits of the rock.
•The rock SNAPS, releasing stored energy
in the form of seismic waves.
•Rock begins to reposition back into place,
producing aftershocks.
13
“Good Vibrations” – released seismic waves
Three major seismic waves are released:
•The P-wave
body waves
•The S-wave
•The L-wave (surface wave)
What measures the incoming seismic waves?
•The Seismograph
The Seismograph
The inertia of the suspended
mass keeps the seismograph
motionless while the seismic
waves vibrate the recording
drum anchored to the
bedrock – produces a seismogram.
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The P-wave
The P-wave (the primary wave)
•compressional wave
•compresses and expands rock material
•fastest of the 3 seismic waves
• typically 6 km/s in the upper crust
•travels through both solids and liquids
15
The S-wave
The S-wave (secondary wave)
•the shearing wave
•moves particles up and down (right angles to
each other)
•travels 2/3 the speed of the P-wave
• typically moves about 3.5 km/s in the
upper crust
•travels through solids ONLY
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The L-wave and Rayleigh wave (Surface Waves)
The L-wave (Love Wave)
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•exhibits horizontal motion (like an S-wave)
•travels the slowest
•travels along the surface – producing horizontal
motion
The Rayleigh wave
•travels similar to an ocean wave
•the slowest wave
•travels along the surface – producing a
rolling motion
SAF
Delano - Bako
Surface Waves
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19
Seismogram
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I
a good earthquake.
Discuss with a friend:
1. Explain the elastic rebound theory.
2. Describe the motions of P, S, and surface
waves.
3.Give three characteristics of each wave.
I will get an A on my exams and quizzes.
21
Did you feel the earthquake? – Measuring an EQ
What’s the difference between an 8 magnitude and
a 6.5 magnitude?
Three ways to measure EQ’s:
Mercalli Intensity Scale
Richter Scale
Moment Magnitude Scale
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Modified Mercalli Intensity Scale
• measures the intensity of the EQ – how much
damage occurs
•closer to the epicenter-- more damage
Less intense
•farther from the epicenter -- less damage
• people report the damage
• destruction is assigned a Roman Numeral (I – XII)
•I – least intensity
•XII – greatest intensity
More intense
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Richter Scale (Charles Richter – 1935)
• measures the intensity of an EQ using a seismogram
• maximum intensity = largest amplitude recorded
• EQ intensity is based on the logarithmic scale.
• each step (M1…M2…M3..)
largestincreases
amplitudethe amplitude X10
• M5 has an amplitude 10 times larger than an M4
• M5 has an amplitude 100 times larger than an M3
• What about the energy released?
•Each increase in Richter amplitude (by 10) increases energy 32
times so,
•An M6 releases 32 more times energy than an M5.
1,024 more times energy than an M4.
•An M6 releases _____
•An M6 releases 32,768
_____ more times energy than an M3.
•An M6 releases 1,048,576
________ more times energy than an M2.
What observations can you make about the increasing energy
release from one magnitude to the next?
Richter Magnitudes
Per/year
< 2.0
generally not felt, but recorded
600,000
2.0 -2.9
potentially perceptible
300,000
3.0 -3.9
felt by some
49,000
4.0-4.9
felt by most
6,200
5.0 – 5.9
damaging shocks
800
6.0 – 6.9
destructive in populous regions
266
7.0 – 7.9
major EQ – inflicts serious damage 18
8.0
great EQ – destroys communities
near epicenter
1.4
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M 6.6
M 4.2
M 5.6
M 7.6
M 8.0
M 7.8
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The Moment Magnitude Scale (MMS or Mw)
•based on “work” done (Mw)
•measures the energy released
•based on the moment of the earthquake
Moment
Rigidity of
the earth
Average amt. of slip
X
=
and
(rock properties) size of the area that slipped
• developed in the 1970’s – replaces the
Richter Scale
• uses magnitude values defined by the
Richter Scale
• used by the USGS folks
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Richter scale vs. Moment magnitude using
different starting assumptions:
Richter Scale:
• assumes EQ focus
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is a point
• best suited for EQ energy release from a
small area
• EQ intensity is based on amplitude
measurements.
Moment Magnitude Scale:
• assumes energy is released over a large area
• measures the actual energy released and
considers the various rock types that waves
move through
I
a good earthquake.
Discuss with a friend:
1. Explain the differences between the
Mercalli Intensity scale, Richter
scale and Moment scale.
2. What’s wrong with the Mercalli Scale
when compared to the Richter scale?
I will get an A on my exams and quizzes
29
How do EQ’s provide scientists a look into the
earth’s interior?
Seismic waves propagate through the earth’s
interior encountering different rock types.
• Seismic waves behave differently depending on the materials
they pass through.
• Seismic discontinuity:
• A boundary inside the earth where the velocities and
directions of seismic waves change abruptly.
refracted:
waves are bent as they pass from one material
to another
reflected:
waves are bounced back
absorbed:
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waves are blocked or impeded
The Mohorovicic discontinuity
(the moho) and asthenosphere
Rocks are ductile with little
strength (upper mantle) – low
velocity zone
The mantle:
Mesosphere: 350km to the
core mantle boundary is
highly compressed rock
(more dense) increasing
seismic wave velocities.
Seismic waves are refracted,
or reflected changing the wave
velocity within the material
they pass through.
The earth’s mantle has several
seismic boundaries
Seismic discontinuities
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How do geologists understand the “layering” of our
earth??
An earthquake takes place.
P and S waves measured
at seismic stations
The absence of both P and S
waves – NO WAVES
The Shadow Zone
40o
P-waves ONLY
33
I
a good earthquake.
Discuss with a friend:
1. What is a seismic discontinuity?
2. Explain the differences between refraction,
reflection and absorbsion of seismic
waves.
3. What seismic evidence suggests a lowvelocity zone within the upper mantle?
4. What is the shadow zone?
I will get an A on my exams and quizzes
34
What is a tsunami?
an enormous ocean wave created from an earthquake
or an undersea volcanic eruption
2.0
1.0
3.0
4.0
5.0
7.0
6.0
Sea floor shifts,
displacing water and
creating tsunami.
I
A good earthquake
1. Explain why the shadow zone exists
2. Why do P-waves occur on the other side
of the earth and S-waves do not?
3. How is the earth’s interior layering
understood --- if a person has not been
there??????
Discuss with a friend
I will get an A on my exams and quizzes
37
Can earthquakes be predicted???????????
At this time – EQ’s cannot successfully be
predicted.
In order to predict an EQ – science folks must
predict a small range of uncertainty
as to the location and timing.
Predictions must be able to produce few failures
and no false alarms.
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Can earthquakes be predicted along the SAF??
Seismologists warn that
an EQ larger than 6.5 will
occur along the San
Andreas Fault.
How do seismologists
predict this warning?
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Bakersfield
How geologists predict EQ’s
Long Range EQ-Prediction – Seismic gap method
• Prediction of an EQ is based on probability – What is the probability
of an EQ?
•Statistical estimation when EQ’s take place over a given time
span (the frequency of EQ’s)
•Assumption that EQ’s are cyclic -- repetitive
Seismic gap method - SAF
Based on studies of the San Andreas Fault,
the probability of an EQ is:
•Less than 10% in Northern CA
•Parkfield (central CA) = 90%
•Southern CA = 30%
Use of the seismic gap
method along the SAF
Cyclic EQ’s in Parkfield,CA
Parkfield
90%Bakersfield
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1857 1 EQ about
1881
every 22 years
1901
1922 Cyclic EQ M6 or greater
1934 rupturing in the same
1966 area of the fault zone
The next predicted EQ
before 1993
DID NOT HAPPEN!
90% chance of a major EQ
EC – Did the EQ on 9/28 reduce the
probability of 90%?
www.quake.usgs.gov/recenteqs/
September 28, 2004
M6 at Parkfield
42
I
a good earthquake
Discuss with a friend:
1. Can EQ’s be predicted? – Why or Why not?
2. Draw a picture of CA and show where
the San Andreas fault is located.
3. Explain how the seismic-gap method
works when predicting earthquakes.
I will get an A on my exams and quizzes
43
Structural Geology
Faults
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Structural Geology – Earth Deformation
Fault
Folds
How is rock deformed?
(stress and strain relationships)
Stress
• force (pressure) acting on the rock surface
Strain
• a change in the shape (deformation) to the
response of stress
Compression
Tensional
undisturbed
cube
Shear
Faults
Joint – fracture within a rock body
Fault – movement along a fracture or joint
B
A
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fault
Joint
A
B
Displacement
I
A good earthquake
1. Describe the differences between stress
and strain
2. Describe the stress types compression
tensional, and shear
3. Draw a fault block and label:
1. hanging/footwalls
2. fault plane
Discuss with a friend
I will get an A on my exams and quizzes
50
Normal Fault
• HW down relative to FW
HW
FW
• Tensional stress
• Extension of crust
FW
HW
HW
FW
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Reverse Fault
• HW up relative to FW
• Compression stress
• Shorting of crust
Vertical type faults
FW
HW
FW
HW
Normal faulting
(extension of crust)
FW
Reverse faulting
(compression of crust)
HW
HW
FW
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Strike-slip faulting: two blocks sliding past one another
Map View
Turned to Right
Turned to Left
What is the type of strike-slip fault?
Right-Lateral – strike slip
Wallace Creek
Right-Lateral
Strike slip
San Andreas Fault – Wallace Creek,
Carrizo Plane
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What type of fault ?
Normal fault
57
I
a good earthquake
Discuss with a friend:
1. Draw a diagram showing the differences
between a normal and reverse fault.
2. Explain the differences between the
cross-sectional fault views of a normal,
reverse, and strike-slip fault
3. Describe the type of stresses that produce
both normal and reverse faults.
I will get an A on my exams and quizzes
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