Earthquakes

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Earthquakes

• "There is science, logic, reason there is
  thought verified by experience. And then there is
  California."
• --Edward Abbey, novelist
• From Writers Almanac, Garrison Keillor

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Fig. 2-18, p.24
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Fault

• A surface along which a rock body has broken and
  been displaced.
• Note Most faults are inactive (we have tons in
  PA), but the active ones get the press.
• Note been displaced, i.e. offset, is key to
  the definition of fault.
• We have fractures in rock with no offset. These
  are called joints.

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3 kinds of Faults

• Normal Faults The hanging wall has moved down
  relative to the footwall
• Reverse Faults The hanging wall has moved up
  relative to the footwall
• Transform faults (also called strike-slip faults,
  or wrench faults) No hanging wall or footwall,
  blocks across the fault move side to side
  relative to each other.

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Anderson Theory of Faulting

• Orientation of maximum principal stress and
  minimum principal stress determine the
  orientation and type of faulting normal,
  reverse (thrust), and transform.

Anderson, E.M. 1951. The dynamics of faulting and
dyke formation with applications to Britain.
Oliver and Boyd, Edinburg.
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Cross Section View Normal Fault
Fig. 3-15, p.43
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Figure 4.4
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Fig. 2-21, p.25
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Cross Section View
Range
Range
Range
Basin
Basin
Horst
Graben
Horst
Horst
Graben
Normal Faults
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Fig. 2-20, p.25
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Cross Section View Thrust Fault
Fig. 3-15, p.43
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Fig. 2-28, p.28
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Map View Transform Fault
Fig. 3-15, p.43
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Fig. 2-18, p.24
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Relate to the Plates

• Divergent Margin Normal Faults
• Convergent Margin Thrust Faults
• Transform Margin Transform Faults

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Seismic

• Seismic (seismicity) Pertaining to earthquakes
  or to waves produced by an earthquake.
• Seismic wave a mechanical wave or vibration
  produced within the Earth by an earthquake.
• Seismograph an instrument that records seismic
  waves

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What happens?

• Something breaks!!
• (releasing stored strain energy, i.e. mechanical
  energy)

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Fig. 3-16, p.44
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Elastic Rebound Theory

• Theory that earthquake energy is due to
  mechanical energy produced as strain rebounds.

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Figure 4.4
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Fig. 3-7, p.39
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Earthquake waves are mechanical waves. They
transmit energy from a source, an Earthquake
focus, or a nuclear explosion, through the Earth.
There are 3 kinds P, S, and Surface waves. P
and S waves are body waves traveling through the
earth. Surface waves travel along the Earths
surface. 1.) P-waves Primary waves arrive
first. They are compressional waves. 2.)
S-waves Secondary waves arrive second. They
are shear waves. 3.) Surface waves Travel
along surface. 2 types (Rayleigh and Love)
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Fig. 4.06
P-wave compressional
S-wave shear
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Rayleigh Wave (rolling motion)
Love Wave (shearing motion)
Fig. 3-12, p.41
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Fig. 3-9, p.40
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Fig. 3-9a, p.40
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Fig. 3-9b, p.40
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Earthquakes are measured with magnitude and
intensity Intensity Measure of effects on
humans and on surface features (damage). Usually
measured with Mercalli Intensity
Scale. Magnitude Measure of the amount of
ground motion, recorded as the amplitude of waves
recorded on a seismograph. Usually measured with
the Richter Scale. Sometimes these are compared,
but not reliably. Damage can vary according to
the geology because of liquefaction as well as
amplification due to wave interference. Of
course damage can vary according to the design of
buildings and other structures.
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Table 4.03
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Fig. 4.08
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• Intensity Measure of effects on humans and on
  surface features (damage). Usually measured with
  Mercalli Intensity Scale.
• How well an earthquake is felt is very
  subjective. Lately, instrument intensities are
  measured with accelerometers to produce shake
  maps the measured accelerations have been
  correlated to the Mercalli scale.

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Fig. 4.15
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Fig. 3-21, p.46
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Richters (1935) Richter Magnitude Scale Richter
used logarithm (factors of 10) differences in
amplitude (A) size to account for the geometric
increase in energy delivered, proportional to A2
Measured amplitude A in millimeters 1) At any
distance from the epicenter, an amplitude
increase of 10 x corresponds to a magnitude
difference of 1 (see fig 3-23) 2) The scale
starts at a background amplitude A0 1
micrometer .001 mm --Reference location is
taken as 100 km from the epicenter. M log(A)
log(A0) correction for difference from 100
km Example A 1 cm amplitude at 100 km (no
distance correction) is log 10 / (0.001)
4 (see next slide, note 10 mm at 100 km gt M
4) 3) Energy released by an Earthquake is
proportional to the amplitude squared. A 1 pt
increase in Richter magnitude has 10 times the
amplitude size. The corresponding energy
increase is proportional to 10 x 10 100 times.
The energy delivered is about 30 times more
(proportionality constant lt 1). A 1 pt increase
in Richter magnitude yields 30 times the energy!!!
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Fig. 3-19, p.45
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Fig. 4.08
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Problem For earthquakes above Richter 6.5, more
energy goes into ground shaking of longer
duration than Richters reference period 0.1 to
3 seconds. Better modern scale is the Moment
Magnitude Scale M A D u Where A is fault area
that broke (taken from surface rupture length and
an assumed depth) D is displacement u is shear
strength of rock
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Fig. 3-26, p.50
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Fig. 3-27, p.50
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Fig. 3-28, p.50
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Fig. 4.28
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Seismic Gap

• A segment of an active fault that has not had an
  earthquake recently, relative to its historic
  periodicity.

Seismic Gap Theory
Theory that a seismic gap is most likely where
the next earthquake will occur along a fault.
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Fig. 4.28
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Fig. 4.23
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Fig. 4.25
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Fig. 4.22
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Factors that increase damage to
structures Amplification An increase in
seismic energy delivered to an area due to
lagging or reflection of waves, with
corresponding constructive wave
interference. Example Loma Prieta 1989
earthquake (Bay Bridge area) Example Mexico
City 1985 earthquake Liquefaction A process in
which water-saturated sands, jostled by an
earthquake are loosened, liquidizing the
sediment, then re-compacted into a closer packing
arrangement, expelling the water. While sediment
is fluidized, buildings sink. Example Loma
Prieta 1989 earthquake (Marina District)
Example Mexico City 1985 earthquake (lake
basin)
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Amplification

• Amplification (a.k.a. Amplified Ground Shaking)
  An increase in seismic energy delivered to an
  area due to lagging or reflection of waves, with
  corresponding constructive wave interference.
• Wave Inteference

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Wave Interference

• Destructive

• Constructive

Note with equal amplitude (A) waves, resulting A
is doubled
Note with equal amplitude (A) waves, resulting A
is zeroed
Energy delivered E proportional to A2 !!!
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Amplification

• Amplification (a.k.a. Amplified Ground Shaking)
  An increase in seismic energy delivered to an
  area due to lagging or reflection of waves, with
  corresponding constructive wave interference.
• Example Loma Prieta 1989 earthquake (Bay
  Bridge area)

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Amplification1989 Loma Prieta (San Francisco)
Earthquake.Lagging (sloshing) sets up waves
out of phase
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Fig. 3-1, p.36
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Amplification

• Amplification (a.k.a. Amplified Ground Shaking)
  An increase in seismic energy delivered to an
  area due to lagging or reflection of waves, with
  corresponding constructive wave interference.
• Example 1985 Mexico City Earthquake.

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Benioff Zone

• Earthquake foci can be shallow (0-100 km), medium
  (100 to 300 km), or deep (300-700km). A pattern
  like this

• is called a Benioff Zone. Note A Benioff Zone
  is a Subduction Zone!!!!

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• The Mexico City, 1985 Earthquake was from a deep
  focus Earthquake epicentered near Acapulco,
  Mexico. Acapulco was spared Mexico City was
  not. Why?

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AmplificationMexico City, 1985Reflection (and
lagging) sets up waves out of phase
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Wave Interference

• Destructive

• Constructive

Note with equal amplitude (A) waves, resulting A
is doubled
Note with equal amplitude (A) waves, resulting A
is zeroed
Energy delivered E proportional to A2 !!!
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Damage (amplification liquefaction)Mexico
City, 1985
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Liquefaction

• Liquefaction A process in which water-saturated
  sands, jostled by an earthquake are loosened,
  liquidizing the sediment, then re-compacted into
  a closer packing arrangement, expelling the
  water. While sediment is fluidized, buildings
  sink.
• Example 1985 Mexico City Earthquake.

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LiquefactionMexico City, 1985
This combined 6-floor residential and commercial
building sank more than one meter into the
partially liquefied soil. Photo credit
Reinsurance Company, Munich, Germany
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Liquefaction
Niigata, Japan, 1964
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Liquefaction

• Liquefaction A process in which water-saturated
  sands, jostled by an earthquake are loosened,
  liquidizing the sediment, then re-compacted into
  a closer packing arrangement, expelling the
  water. While sediment is fluidized, buildings
  sink.
• Example Loma Prieta, 1989
• (Marina District, San Francisco)

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Fig. 3-3, p.37
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Fig. 4.12
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Fig. 4.18
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Other Seismic Hazards

• Blind Thrusts (Los Angeles)

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Fig. 4.28
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Fig. 4-31, p.81
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Fig. 4-30, p.81
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Fig. 4-36, p.83