ESS 8 - Earthquakes

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ESS 8 - Earthquakes

• Profs. Vidale Creager

Bolt, 5-17
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What is a wave?

• A wave is a disturbance that travels far through
  a medium while particles of the medium move a
  small amount back and forth and do not experience
  a net translation.
• Examples ripples on a pond, the wave at sports
  events

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Seismic wave radiation

• Radiation - waves that travel outward and carry
  energy
• Examples
• Light energy from space heater
• Travels too fast to see go, 300,000 km/sec
• Water waves from a splash, few m/sec
• Sound waves from a speaker, 300 m/sec
• Seismic waves (motions) are just vibrations of
  the ground, like sound waves are vibrations of
  the air

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Doppler shift
1
2
3
3
2
1
Higher pitch
Lower pitch
For example, approaching siren has a higher pitch
than a receding siren
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Moving wave sources
zebu.uoregon.edu/js/space/ lectures/lec05.html
Doppler shift
Sonic boom
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Breakingsound barrier
http//www.fly.faa.gov/carf/
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Shuttle sonic boom
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The Thrust SST, 1997
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Breaking sound barrieron land! 763 mph, 1997
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Old speed record - Mach 6.7
4520 miles per hour
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X-15 world records

• 4520 miles per hour - Mach 6.7
• 2 km/sec, no longer champ
• 354,000 ft off the ground
• 100 km high
• Flew for just 2 minutes at a time
• Intended to prototype moonships
• NASA vs Air Force turf war, Air Force lost

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X-43A - new record in 2004

• Mach 9.6 (7000 mph), off California coast
• Unpiloted, 12-foot long vehicle
• Ramjet powered (air breathing)
• Burned for just 10 seconds
• 250M research project

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Types of Seismic Waves
Walt on Disneyland train

• P waves
• S waves
• Surface wave train
• Love
• Rayleigh

body waves
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Amplitude
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Wavelength
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Period
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Frequency
Frequency How many waves pass a point in a given
amount of time For sound frequencypitch
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Types of Seismic Waves
Earthquake Station

• Body waves
• Surface waves

Earthquake Station

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Raypathsandwavefronts
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P waves

• Longitudinal - material moves back and forth
  (vibrates) in same direction that wave travels,
  produces compression/dilatation cycle
• Fastest type of wave, so arrives first
• termed Primary wave
• Typical velocities in crust 5 - 7 km/sec
• Travels through solids, fluids, or gas

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P waves
Sun movie
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Another viewof P wave motion
P movie
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Strike-slip P-wave radiation pattern
This is left lateral strike-slip faulting in map
view
N
L
L
N
N
Strong in some directions Lobes Weak in other
directions Nodes
N
L
L
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Different First Motions in different directions
Down
Up
Vertical ground motion
Down
Up
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First direction of motionbeach ball diagrams
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Raypaths bend as seismic waves travel
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First Motions and Rupture modes
Map views
3-D view
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Three primary focal mechanisms
Deformation
Map view of first motions
Strike-slip
Normal
Thrust or reverse
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S waves

• Shearing - material moves back and forth
  perpendicular to the direction the wave travels
  in a twisting motion.
• Slower than P wave, arrives second
• termed Secondary wave
• Typical velocities in crust 3-5 km/sec
• P waves travel 5-7 km/s
• Travels through solids, but not fluids
• because there is no restoring force for the
  perpendicular motions

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S waves
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S wave in a solid
S movie
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Surface Waves

• Travel on surface of Earth
• Two types
• Love waves
• Rayleigh waves
• Travel a bit slower than S waves
• Are the largest amplitude waves
• so the P wave can serve as a warning to take
  cover or shut down critical facilities
• warning ranges from a few to 100 seconds
• Can get 1 s of warning for each 10 km in distance

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More surface waves

• Need a surface to travel along, which is the
  rock-air interface at the Earths surface.
• Motion is strongest near the surface
• Most strongly generated by earthquakes near the
  surface

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Rayleigh waves
Love waves
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Review of Waves
P
S
Love
Rayleigh
Bolt, 1-9
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Where is energy?

• In waves, energy has two forms
• Strain or deformation - like the energy stored by
  deforming a spring - 1/2 kx2
• Motion or vibration - kinetic energy in physics -
  1/2 mv2
• Vibration is the most damaging, but either kind
  of energy can cause damage

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Bouncing ball on a spring
At rest
stretched
moving
compressed
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Water waves are different

• Energy again has two parts
• One part is kinetic energy
• The other part is gravitational
• There is no twisting energy, not much is strain

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Amplitude of seismic waves

• Amplitude is strength of shaking
• Depends on magnitude
• Determines amount of damage
• Amplitude decreases with distance from the
  earthquake
• energy spreading out over larger area
• P wave smallest
• S waves larger
• Surface waves largest

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Because the waves travel at different velocities

• As waves radiate outward from the earthquake,
  through the Earth, they separate into a
  predictable pattern with
• P waves arriving first
• then S waves
• then surface waves

S
P
surface
Time
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P waves, then S waves, then surface waves
At 80, as drawn
10 m
20 m
30-50 m
Body waves
Mikes movie
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Motion has 3 components
Love
Transverse
Rayleigh
Radial
S
Vertical
P
Tromp movie
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Complications for Seismic waves

• Reflection
• Refraction
• Conversion

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Reflection on mirror - simple
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Refraction - Object in water (bending of rays)
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Refraction (water in fishbowl)

• Refraction of light by water
• Because speed of light waves is slower in water
  than in air
• Seismic waves refract too
• And can also switch between P and S

Press, 19-1
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Reflection -complicated
Includes refraction through curved glass
www.nashobawinery.com
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Straw magic
Glass-air refraction
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Light through prism
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Conversions of seismic waves

• P waves and S waves can partially convert to each
  other when they encounter a sharp change in
  seismic velocity
• usually just a small percent of total energy of
  the wave
• The seismic waves quickly get very complicated

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Reflection refraction of waves

• P S waves are reflected, refracted (bent), and
  converted at sharp changes (discontinuities) in
  seismic velocity (as occur between rock layers)

refracted P (most energy)
refracted S
Slow
One incident P or S wave results in up to four
waves
Fast
incident P
reflected P
reflected S
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Simulated reflection
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Waves bounce

Bolt, 1-10
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1992Landers EQ
Peyrat et al., 2000
up
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Simulation of the1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
up
South
North
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Simulation of the 1992 Landers EQ
Peyrat et al., 2000
Produced more energy in the shaking to the north
up
South
North
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Simulation map
Shawn Larsen, LBL Doug Dreger, UCB
Line source Like shock wave
Seismicity map
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Accuratesimulation
Hayward-Calaveras Fault
Right-lateral slip on strand of San Andreas fault
First part of fault breaks
After 15 seconds
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Note strong directivity
Middle of fault breaks
After 30 seconds
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End of fault Breaks Waves keep Going across basin
After 45 seconds
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Fault rupture over Waves keep Going across basin
After 60 seconds
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Fault rupture over Reverbs in basins dying away
After 75 seconds
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Fault rupture over Only middle of basins still
ringing a little bit
After 100 seconds
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Which waves cause damage?
1964 Nigata quake

• S and surface waves are much bigger than P waves,
  and thus cause the most damage.
• Most damaged area is close to fault rupture.
• And damaging quakes rupture for gt several sec
• P, S, and surface waves have not separated, but
  rather arrive almost simultaneously
• So it is hard to isolate each wave in records of
  ground motion from damaged areas
• Waves bounce, and convert between P, S, and
  surface types, complicating identification.

SCEC movie