GLY 150: Earthquakes and Volcanoes Spring 2005 04212005

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GLY 150 Earthquakes and VolcanoesSpring 2005
04/21/2005
Lecture 23
Re-landscaping the backyard ???
http//geology.utah.gov/utahgeo/hazards/index.htm
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AnnouncementsGLY 150 Earthquakes and Volcanoes

• The next journal assignment is due Today. If you
  still have questions regarding the grading the
  grading criteria please see me or the T.A.
  (recent small magnitude earthquake in NMSZ).
  Next Thursday will be your last journal
  assignment.
• Instructor office hours are Mon. 200-300 and
  Wed. 200-300
• A homework assignment will be posted by tomorrow.
  It will be due next Thursday.
• Both of your texts will now be helpful,
  especially with regard to figures shown in class
  and similar pictures in your geology text.

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Extra Credit OpportunityGLY 150 Earthquakes and
Volcanoes

• I have a list of events we have not studied in
  class. See Rachel or myself to find out the
  topics you will write about.
• For each talk, write a four page (single space,
  12 inch font, 1 inch margins) summary of the
  event, tectonic setting, its implications,
  reasons for damage and fatalities, related
  archeological evidence, etc. etc. and and receive
  4 bonus points added to your total exam score for
  the semester.
• Follow the directions closely or no credit will
  be given
• If you write two reports you will receive 8 bonus
  points
• For those of you who have written reports on one
  or both of the lectures, you may only accumulate
  8 bonus points total for the semester so if you
  have already accumulated your bonus points dont
  do this too. If you get less than 4 points on
  any of the assignments you can complete this
  project so as to account for points missed.
• The due date will very. You can choose to
  complete this assignment at any time for the
  remainder of the class. The assignment will be
  due 1 week after you are given a topic.

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Events this QuarterSpring 2005
Note Some of the eruptions may be ongoing so be
sure to check their current status
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WavesBasic Terms

• Wave a disturbance that propagates in time and
  space
• Crest highest point of the wave
• Trough lowest point of the wave
• Wavelength horizontal distance between
  consecutive peaks or troughs
• Period at a point, the length of time between
  the passage of equivalent heights on the wave
• Amplitude (a.k.a. wave height) vertical distance
  between the peak and the trough, i.e., the
  maximum size of the disturbance

Crest
Trough
http//oceanlink.island.net/oinfo/acoustics/soundw
ave.gif
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WavesPeriod and Frequency

• T Period
• V Wave Velocity
• l Wavelength
• f Frequency
• Wave Velocity how quickly the disturbance moves
• Period the time between successive peaks of the
  wave
• T l /V
• Frequency the number of oscillations per second.
  It is the inverse of the period
• F 1/T

Crest
Trough
V
http//oceanlink.island.net/oinfo/acoustics/soundw
ave.gif
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WavesFrequency

• Frequency the number of oscillations per second.
  It is the inverse of the period
• The frequency of seismic waves is a factor in
  determining the amount and type of damage due to
  earthquakes

Short Wavelength
Long Wavelength
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WavesAmplitude

• Amplitude (a.k.a. wave height) vertical distance
  between the peak and the trough, i.e., the
  maximum size of the disturbance

http//earthquake.usgs.gov/image_glossary/amplitud
e.html
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WavesWavelength, Period, and Velocity

• Wavelength horizontal distance between
  consecutive peaks or troughs
• Period at a point, the length of time between
  the passage of equivalent heights on the wave
• Wave Velocity how quickly the disturbance moves,
  i.e. its propagation speed

Wavelength
Velocity
Period
Velocity
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Seismic Waves

• Body Waves
• P-waves longitudinal waves
• S-waves transverse waves
• Surface Waves
• Love waves
• Rayleigh waves

P-wave
S-wave
Love Wave
Fig. 3.3 Bolt, 1999
Rayleigh Wave
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P-Waves
Body Wave

• Occur because the Earth resists compression
• Velocity depends on how strongly rocks resist
  compression
• Fastest elastic wave (typically 6 km/s for the
  Earths crust)
• Longitudinal particle motion (i.e., the motion of
  the wave is parallel to the direction in which
  the wave propagates)

Compression
Undisturbed
Undisturbed
Dilatation
Fig. 3.3 Bolt, 1999
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S-Waves
Body Wave

• Occur because the Earth resists shear
• Velocity depends on how strongly rocks resist
  shearing
• Slower than P-waves (typically 3.5 km/s for the
  Earths crust)
• VS 0.577 VP
• Transverse particle motion (i.e., the motion of
  the wave is perpendicular to the direction in
  which the wave propagates)
• CAN NOT propagate through liquids (e.g. the outer
  core)

Undisturbed
Fig. 3.3 Bolt, 1999
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Love Waves
Surface Wave

• Arise because the Earth has a free surface (like
  water waves)
• Travel more at about the same velocity as S-waves
  (VLove VS)
• Horizontally polarized transverse particle motion
• Cannot propagate through liquids (e.g. the outer
  core)
• Side-to-side motion particularly damaging
• Amplitude decreases with depth

Undisturbed
Fig. 3.3 Bolt, 1999
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Rayleigh Waves
Surface Wave

• Arise because the Earth has a free surface (like
  water waves)
• Travel more slowly than S-waves (VRayleigh 0.92
  VS)
• Retrograde-elliptical motion (counterclockwise)
• Like ocean waves
• Amplitude decays with

Undisturbed
Fig. 3.3 Bolt, 1999
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Seismic WavesPropagation Pattern

• Surface waves (Love, Rayleigh) generated when the
  body waves (P-wave, S-wave) reach the Earths
  surface

http//www.essc.psu.edu/ammon/HTML/Classes/IntroQ
uakes/Notes/waves_and_interior.html
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Surface WavesAnother Perspective
Surface Waves
Love Wave

• Amplitude decays with depth

Rayleigh Wave
Both from http//www.essc.psu.edu/ammon/HTML/Clas
ses/IntroQuakes/Notes/waves_and_interior.html
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Propagating Waves General Terms

• Wavefront the propagating front of a wave (think
  of ripples in water)
• Ray a path oriented perpendicular to the
  wavefront
• Shows the direction that the wave is going

http//www.eas.purdue.edu/braile/edumod/slinky/sl
inky.htm
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Propagating Waves Expanding Wavefronts

• Body waves expand as spherical wavefronts within
  the Earth (3D)
• Surface waves expand as circular wavefronts on
  the Earths surface (2D)

a.k.a. focus
http//earthquake.usgs.gov/faq/meas.html
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Wave Propagation EffectsRefraction - I
http//160.94.61.144/courses/2301/min15_optics1.ht
ml
Rays

• The bending of waves that results because of
  velocity changes in the material through which
  the wave is propagating
• The velocity of a wave changes when the
  properties of the material through which it is
  traveling change

Wavefronts
http//www.scitoys.com/scitoys/scitoys/light/perma
nent_rainbows/permanent_rainbows.html
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Wave Propagation EffectsRefraction - III

• The real Earth is a layered body and material
  properties change with depth
• Generally, seismic velocities increase with depth
• Seismic waves in the Earth travel along a
  distinctive a curved path

http//www.essc.psu.edu/ammon/HTML/Classes/IntroQ
uakes/Notes/waves_and_interior.html
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Wave Propagation EffectsReflection

• Waves bounce off interfaces between materials
  with different properties
• e.g., crust-mantle boundary
• e.g., core-mantle boundary
• Numerous other boundaries found within the Earth

Rock Type 1
Rock Type 2
http//www.surfacesensor.com/Principles/refraction
.htm
http//160.94.61.144/courses/2301/min15_optics1.ht
ml
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Wave Propagation EffectsReflection and Refraction

• There are numerous material interfaces in the
  Earths crust (upper 15-20 km). As a result,
  earthquake waves can travel an infinite number of
  paths
• Direct waves
• Refracted waves
• Reflected waves

Fig. 1.10a Bolt, 1999
Seismic waves reflect and refract from various
layers within the earth
Direct Wave
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Seismic WavesSeismic Wave Density Variations
With Depth

• Below the asthenosphere P S-wave velocities and
  densities increase with depth

• At core mantle boundary
• S-waves disappear because they cannot propagate
  through the liquid outer core
• P-wave velocities and density drop but then
  continue to increase
• S-waves reappear in the inner core
• Note that wave velocity increases with density
  (proportional to pressure)

Upper Mantle
P-wave
Density
P-wave
S-wave
Density
S-wave
Lower Mantle
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Wave Propagation In The Earth

• Seismic waves from earthquakes propagate through
  the entire Earth (layered body material
  properties change with depth)
• There are numerous material interfaces throughout
  the Earth. The result is that earthquake waves
  can travel an infinite number of paths
• Direct waves, refracted waves, and reflected
  waves
• Note S-waves do not propagate through the outer
  core, its liquid

http//www.eas.purdue.edu/braile/edumod/slinky/sl
inky.htm, similar to Fig. 4.3, Bolt, 1999
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Seismic WavesWhat Controls the Level of Shaking
and Damage?

• Magnitude of Earthquake
• The more energy released during the earthquake
  the greater the shaking
• Distance From Focus
• Shaking decays with distance from the focus
  (spherical spreading, attenuation)
• Directivity
• If the fault ruptures towards you the shaking
  will be higher than if the fault ruptures away
  from you
• Local Amplification of Seismic Waves
• Topography
• Sedimentary Basins
• Unconsolidated soils (sand, mud, fill, etc.)
  amplify shaking

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Earthquake RuptureArea of Rupture

• The area of the fault that ruptures is
  proportional to the size of the event
• Earthquakes only occur within the brittle upper
  zone of the crust (a.k.a. seismogenic layer)
• upper 15-20 km of the curst
• The largest earthquakes rupture the entire
  thickness of the seismogenic crust

http//www.essc.psu.edu/ammon/HTML/Classes/IntroQ
uakes/Notes/earthquake_size.html
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Distance From FocusDecay In Amplitude With
Distance Spherical Spreading
2D or 3D

• As a seismic wave spreads out, the same amount of
  energy is distributed over a larger/longer
  wavefront
• As a result, the amplitude of the wave gets
  smaller with distance (i.e., energy must be
  conserved)
• Effects P and S waves more rapidly than Love and
  Rayleigh waves
• P S waves spread out in a 3D volume (they are
  body waves)
• Love and Rayleigh waves spread out across the
  Earths 2D surface (they are surface waves)

http//www.npmoc.navy.mil/KBay/soundprop.htm
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Distance From FocusDecay In Amplitude With
Distance - Attenuation

• As wave propagates, surrounding material absorbs
  some of its energy
• As a result, the amplitude of the wave gets
  smaller with distance
• Damps high-frequency waves more rapidly than
  low-frequency waves
• All waves lose approximately the same amount of
  energy per cycle (i.e., one complete oscillation)
• Low frequency waves lose less energy over a given
  distance, because the go through fewer cycles,
  than high frequency waves

Short Wavelength
Long Wavelength
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Directivity
ShakeMap from the 1994 Northridge earthquake

• Directed Rupture
• Earthquake ground motions in the direction of
  rupture propagation are often more severe than
  ground motions in other directions from the
  earthquake focus

White arrow gives direction of rupture propagation
http//earthquake.usgs.gov/image_glossary/directiv
ity.html
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Local Amplification Topography

• Focuses earthquake waves
• Concentrates energy
• Traps earthquake waves, increasing the duration
  of shaking

Arrows give wave propagation direction
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Local Amplification Sedimentary Basins
Arrows give wave propagation direction

• Focuses earthquake waves (like a lens)
• Concentrates energy
• Amplifies earthquake waves
• Because wave velocities in sediment are slower,
  incoming waves stack up, thereby increasing wave
  amplitudes
• Traps earthquake waves, increasing the duration
  of shaking

Wavefront
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Local Amplification Sedimentary Basins
Site on solid rock

• Note how amplitude of seismic waves increases as
  seismometers get closer to the basin

Earthquake Epicenter
Basin
Fig. 3.8 Bolt, 1999
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Local AmplificationSoft Soils

• Amplifies earthquake waves
• Because wave velocities in soil are slower,
  incoming waves stack up, thereby increasing wave
  amplitudes
• Traps earthquake waves, increasing the duration
  of shaking

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Local AmplificationSoft Soils

• Amplifies earthquake waves
• Because wave velocities in soil are slower,
  incoming waves stack up, thereby increasing wave
  amplitudes
• Example seismograms
• Sediment vs. Rock

Body wave travel paths thought the Earth
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Seismic WavesWhat You Feel During an Earthquake

• P-waves arrive first. For a small earthquake
  they are generally not felt.
• S-waves arrive next and are usually the strongest
  and most destructive earthquake waves.
• Surface waves (Love and Rayleigh) arrive later
• They travel at the Earths surface so have to
  travel farther in slower materials
• They are slow

Body wave travel paths thought the Earth
http//www.essc.psu.edu/ammon/HTML/Classes/IntroQ
uakes/Notes/waves_and_interior.html
36
Seismic WavesWhat You Feel During an Earthquake

• Since P-wave are longitudinal and generally
  coming from deep in the Earth, you often just
  feel a sudden vertical bump under your feet, then
  nothing until
• Since S-waves are transverse and generally coming
  from deep within the Earth, when they arrive
  things start shaking from side to side
• When the surface ways arrive, Love waves are
  transverse so there is still side to side
  shacking but you may also feel a rolling
  component from the Rayleigh waves

Fig. 1.10a Bolt, 1999
Seismic waves bounce off various reflectors
within the earth
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Seismic WavesWhat You Feel During an Earthquake

• Your distance from the earthquake influences what
  you feel in the earthquake
• Near the earthquake focus
• Wave amplitudes are bigger so you feel more of
  them
• Sharp P and S waves are felt most strongly
• Far from the earthquake focus
• Since the different wave types travel at
  different velocities, the waves are more spread
  out in time (i.e., the shaking might last longer,
  S-P difference larger)
• Rolling surface waves are felt most strongly
• Why?
• Attenuation Waves attenuate (lose energy) with
  increasing distance. High frequency waves
  attenuate more quickly than low frequencies.
• Spherical Spreading Surface waves lose less
  energy with distance than body waves because
  surface waves spread out in 2D and body waves
  spread out in 3D. Thus, surface waves can be
  felt more strongly, further away

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Seismic WavesWhat You Feel During an Earthquake
- Other Factors

• Buildings, especially tall ones, may amplify
  shaking
• Often people on the upper floors are the only
  ones to notice small earthquakes
• Buildings can continue to oscillate even after
  the ground stops shaking
• Prolongs the duration shaking for those inside
  the building

Body wave travel paths thought the Earth
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SeismographsIntroduction

• Seismograph instrument that records the time and
  amplitude of ground motions
• Older instruments measured only vertical
  component of deformation
• Newer instruments measure three components of
  motion independently Effectively have three
  instruments in one seismometer
• East-west component North-south component
  Vertical component
• Seismometer senses ground motion

Fig. 3.2 Bolt, 1999
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SeismographsOutput
http//www.mgs.md.gov/esic/seisnet/help.html

• Drum rotates as pen records ground motions
• Provides a continuous record of ground motion in
  a compact fashion
• Today this is done digitally (i.e. paperless
  seismograms)

http//www.mgs.md.gov/esic/seisnet/help.html
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SeismogramsIntroduction

• Seismogram a record of ground motion vs. time
• Because waves oscillate in different directions,
  tend to see different waves on different
  components
• Wave Velocities
• P-waves are fastest (arrive first)
• S-waves are next (arrive second)
• Surface waves are slowest (arrive last)

S-wave Arrival
P-wave Arrival
Surface Wave Arrival
http//www.eas.purdue.edu/braile/edumod/slinky/sl
inky.htm, similar to Fig. 8.4, Bolt, 1999
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SeismogramsTypes of Measurements

• Seismograms generally measure one of the
  following
• Ground Displacement
• Ground Velocity
• Ground Acceleration

http//earthquake.usgs.gov/image_glossary/displace
ment.html
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Locating Earthquakes I

• Seismic waves propagate out from the
  epicenter/focus
• Stations in a seismic network record the first
  arrivals at times which depend on their distance
  from the source

http//earthquake.usgs.gov/faq/images/seismapb.gif
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Locating Earthquakes II

• Recall that P-waves travel faster than S-waves
• The time difference between the P- and S-wave
  arrivals can be used to determine the distance to
  the earthquake epicenter
• As distance from the focus increases, the S-P
  time increases
• Similar to using the time between a lighting
  flash and thunder clap to determine distance to a
  thunderstorm

travel time from focus to seismograph
http//www.usd.edu/esci/figures/BluePlanet.html
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Locating Earthquakes II

• Observation S-P time
• Can determine distances because scientists have
  prior knowledge of S and P wave velocities in the
  Earths
• Velocity Distance/Time
• Distance VelocityTime
• Time Distance/Velocity
• S-P Time (Distance/VS) (Distance/VP)
• Can solve above equation for distance

http//www.tulane.edu/sanelson/geol204/eqcauses.h
tm
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Locating Earthquakes IIITriangulation

• For each seismographic station draw a circle,
  centered at the station, with radius equal to the
  calculated distance
• Need at least 3 stations
• The more stations the better
• The epicenter of the earthquake is at the point
  of intersection of the circles

Fig. 4.9 Pipkin Trent, 2001 similar to Fig.
3.6, Bolt, 1999
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Mercalli vs. Richter Measurements
http//www.wooster.edu/seismic/Images/RICHTER.gif
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Richter Magnitude

• Known/measured quantities
• S-P Time
• Wave Amplitude (corrected for distance from
  focus)
• Insert above information into chart and draw line
  between the two points to determine Richter
  magnitude
• Best known
• magnitude scale

Magnitude
Amplitude
S-P Time
http//earthquake.usgs.gov/image_glossary/richter_
scale.html, same as Box 8-1, Bolt, 1999
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Richter Magnitude What the Numbers Mean

• M 2 Detectable by instruments only
• M 4 People feel faint tremors
• M 5 Structural damage begins to occur
• M 8 Stronger buildings destroyed

http//www.stvincent.ac.uk/Resources/EarthSci/Eart
h/magnitude.html
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Earthquake MeasurementOther Magnitude Scales

• For application to distant earthquakes
• Body Wave Magnitude uses the first 5 seconds of
  a teleseismic (a.k.a. distant) P-wave
• Surface Wave Magnitude derived from the maximum
  amplitude of the Rayleigh wave
• Other magnitude scales also exist
• All give approximately the same values (within
  0.3-0.5 magnitude units)

http//www.essc.psu.edu/ammon/HTML/Classes/IntroQ
uakes/Notes/earthquake_size.html
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Problems With Instrumental Magnitude Measures

• Most magnitude measures underestimate the
  magnitude of extremely large events
• Magnitude scales saturate
• Based on instrumental measurements at a
  particular point in the seismogram
• Rupture lengths so long in large earthquakes that
  the duration of shaking is much longer
• Instrumental measures do not capture all the
  information

http//www.essc.psu.edu/ammon/HTML/Classes/IntroQ
uakes/Notes/earthquake_size.html
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Earthquake MeasurementMoment Magnitude

• Moment Magnitude more accurately describes the
  energy released in large earthquakes
• Calculated from moment of the earthquake.
  Depends on average earthquake slip, fault area,
  and rigidity (resistance to motion) of the
  surrounding rock
• (Note Rupture Area LW)
• Moment RigidityRupture AreaAvg. Displacement
  During EQ
• The most widely used magnitude scale in common
  practice
• Magnitudes are similar to Richter magnitudes
  except for largest earthquakes

http//earthquake.usgs.gov/image_glossary/seismic_
moment.html
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Earthquake MeasurementMoment Magnitude
M 7.3

• Moment magnitude is proportional to slip
  magnitude and rupture area
• As rupture area increases, earthquake magnitude
  increases
• As amount of slip increases, earthquake magnitude
  increases

M 6.9
M 6.9
M 5.6
M 6.7
http//earthquake.usgs.gov/image_glossary/magnitud
e.html
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Earthquake MeasurementEnergy Release

• Earthquake magnitude scales are logarithmic such
  that a unit increase in magnitude results in a
  roughly 32-fold increase in the amount of energy
  released during the event
• e.g., Richter Magnitude
• Unit increase in magnitude 10x increase in
  amplitude of ground shaking
• Unit increase in magnitude 32x increase in
  energy released

Magnitude Approximate Equivalent TNT Energy
4.0 1010 tons
5.0 31800
tons 6.0
1,010,000 tons 7.0
31,800,000 tons 8.0
1,010,000,000 tons 9.0
31,800,000,000 tons
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Earthquake MeasurementHow Big, How Often?

• Descriptor Magnitude Average Annually
• Great 8 and higher
  1
• Major 7 - 7.9
  18
• Strong 6 - 6.9
  120
• Moderate 5 - 5.9
  800
• Light 4 - 4.9
  6,200 (estimated)
• Minor 3 - 3.9
  49,000 (estimated)
• Very Minor 3 about 1,000 per day
• 
  Magnitude 1 - 2 about 8,000 per day

Data from http//wwwneic.cr.usgs.gov/neis/eqlists/
eqstats.html
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Induce Lots of Little Earthquakes?Can This
Eliminate the Big One?

• Compared to a M 7 earthquake, a M 6
  earthquake releases 32 times less energy
• To remove possibility of damage, induced
  earthquakes should be less than M 4 (induced
  earthquakes are typically M
• Major plate boundary earthquakes are typically M
  7-8
• 1 M8 Earthquake 32323232 M4 Earthquakes
• 1,048,576 M4
  Earthquakes
• 12 M4
  Earthquakes per day on San Andreas Fault
• Even a M 6 earthquake would require
• 1 M6 Earthquake 3232 M4 Earthquakes
• 1,024 M4
  Earthquakes
• 4 M4
  Earthquakes per year on San Andreas Fault
• And what about .
• All the other faults within the San Andreas
  System
• Liability insurance if something goes wrong
• Cost

Best Case Scenario
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Earthquake PredictionGoals

• Motivation
• To reduce damage to lives and property
• Prior knowledge useful for choosing sites of
  critical structures (e.g. dams, nuclear reactors)
• We can determine where on Earth earthquakes are
  likely (plate tectonics).
• There is currently no reliable way to predict
  (within days to months) when a major earthquake
  will occur
• Can estimate earthquake times with an accuracy or
  /- 50-150 years (not really practical)
• Objective
• Predict the date
• Predict the location
• Predict the intensity of damage in the quake
• Japan, the former Soviet Union, China, and the
  U.S. have lead the earthquake prediction effort

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Earthquake PredictionThe Prediction Dilemma

• If a prediction is made, it is generally made in
  an area that is know to be seismically active
• By chance alone, the odds of an earthquake are
  not zero
• If a firm prediction is made and nothing happens,
  that must be taken as proof that the methods is
  invalid (at least part of the time)
• If a prediction is made and an earthquake occurs,
  it cannot be taken as proof that the methods used
  to make the prediction were correct. They may
  fail on future occasions
• Must develop body of evidence
• Generally agreed that post-predictions
  (predictions made after the earthquake using data
  from before the earthquake) dont count. Though
  useful, the results may be biased.

59
How Often Do Earthquakes Recur?

• Lithospheric plates are moving steadily at
  well-known velocities
• Since the plates are rigid there is relatively
  little internal deformation
• The plate boundaries are stuck between
  earthquakes
• As the plates continue to move, the force
  eventually becomes so large that the plate
  boundary suddenly becomes unstuck
• This is an earthquake
• Accommodates relative motion between the two
  plates at the plate boundary

Steady Motion
Steady Motion
Stuck Patch
similar to Fig. In This Dynamic Earth
(http//pubs.usgs.gov/publications/text/historical
.html) Plate 13, Bolt, 1999
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Seismic Gaps
Fig. 7.6, Bolt, 1999

• If it has been a long time since the last
  earthquake on a certain segment of the plate
  boundary, than it is a likely place for a future
  earthquake
• The plates move forward interminably
• To accommodate this relative motion, earthquakes
  must occur repeatedly
• The entire plate boundary does not break at one
  time. In each earthquake only a small segment
  does
• Since the plate is moving at uniform velocity,
  each segment must ultimately accommodate the same
  amount of motion
• Each plate boundary segment must keep up with the
  motion of the plate interior

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Seismic GapsLoma Prieta Seismic Gap

• Seismic Gap a part of a fault that is known to
  have earthquakes but that has not had one for a
  long time
• The earthquake makes up the deficit in motion
  instantaneously

http//pubs.usgs.gov/publications/text/tectonics.h
tmlanchor19989073
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Seismic Gaps

• Seismic Gap a part of a fault that is known to
  have earthquakes but that has not had one for a
  long time
• The earthquake makes up the deficit in motion
  instantaneously

Mexico
http//tlacaelel.igeofcu.unam.mx/vladimir/guerrer
o20level/leveling.html
63
Estimating Recurrence Intervals

• Terms
• Long-term fault slip rate (i.e. relative plate
  velocity) often accurately know in well-studied
  areas from plate tectonics
• Time of last earthquake sometimes known
• Earthquake slip a matter of speculation
• Using the seismic gap hypothesis, if we assume an
  average earthquake slip, than we can estimate
  recurrence interval
• Recurrence interval average earthquake slip /
  long-term slip rate
• This hypothesis predicts that earthquakes occur
  at regularly repeating intervals (unfortunately,
  this is not entirely true)