GLY 150: Earthquakes and Volcanoes Spring 2005

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GLY 150 Earthquakes and VolcanoesSpring 2005

• 04/19/2005

Lecture 22b
Nov. 3, 2002 M7.9 Denali Alaska Earthquake
http//www.geolsoc.org.uk/template.cfm?nameDenali
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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
• 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

• 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)

2D or 3D
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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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Local AmplificationSoft Soils

• Sands and gravels, soft muds, and fill amplify
  seismic shaking
• Ideally, structures are placed in bedrock

http//geography.sierra.cc.ca.us/booth/California/
1_lithosphere/earthquakes.htm
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What We Feel In An EarthquakeThe Generation of
Seismic Waves

• In a small earthquake
• Did somebody kick the back of my seat?
• In a big earthquake
• If you are paying attention, you often just feel
  a sudden vertical bump under your feet, then
  nothing until
• Things start shaking from side to side
• As the earthquake continues, there is still side
  to side shacking but you may also feel a rolling
  component

Epicenter (at surface)
Focus (at depth)
Fig. 6.8, Bolt, 1999
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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
39
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