Seismic Wave Propagation

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Seismic Wave Propagation
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What are Seismic Waves

• Seismic waves are the vibrations from earthquakes
  that travel through the Earth
• They are the waves of energy suddenly created by
  rock fracture within the earth or an explosion.
• They are the energy that travels through the
  earth and is recorded on seismographs

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History

• Seismology - the Study of Earthquakes and Seismic
  Waves
• Dates back almost 2000 years

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• Around 132 AD, Chinese scientist Chang Heng
  invented the first seismoscope, an instrument
  that could register the occurrence of an
  earthquake.
• Seismographs record a zigzag trace that shows the
  varying amplitude of ground oscillations beneath
  the instrument.
• Sensitive seismographs, which greatly magnify
  these ground motions, can detect strong
  earthquakes from sources anywhere in the world.
• The time, location and magnitude of an earthquake
  can be determined from the data recorded by
  seismograph stations.

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Seismometers and Seismographs

• Seismometers are instruments for detecting ground
  motions
• Seismographs are instruments for recording
  seismic waves from earthquakes.
• Seismometers are based on the principal of an
  inertial mass
• Seismographs amplify, record, and display the
  seismic waves
• Recordings are called seismograms

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Types of Seismic Waves

• Body waves Travel through the earth's interior
• Surface Waves
• Travel along the earth's surface - similar to
  ocean waves

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Seismic Waves
P
Body waves
S
Surface Waves
Love
Ground Roll
Rayleigh
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P-Wave(Body Wave)

• Primary or compressional (P) waves
• The first kind of body wave is the P wave or
  primary wave. This is the fastest kind of seismic
  wave.
• The P wave can move through solid rock and
  fluids, like water or the liquid layers of the
  earth.
• It pushes and pulls the rock it moves through
  just like sound waves push and pull the air.
• Highest velocity (6 km/sec in the crust)

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P-Wave
Particle motion
Deformation propagates
Particle motion consists of alternating
compression and dilation. Particle motion is
parallel to the direction of propagation
(longitudinal). Material returns to its original
shape after wave passes.
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Secondary Wave (S Wave)

• Secondary or shear (S) waves
• The second type of body wave is the S wave or
  secondary wave, which is the second wave you feel
  in an earthquake.
• An S wave is slower than a P wave and can only
  move through solid rock. (3.6 km/sec in the
  crust)
• This wave moves rock up and down, or side-to-side.

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S-Wave
Particle motion
Deformation propagates
Particle motion consists of alternating
transverse motion. Particle motion is
perpendicular to the direction of propagation
(transverse). Transverse particle motion shown
here is vertical but can be in any direction.
Material returns to its original shape after wave
passes.
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L-Wave

• Love Waves
• The first kind of surface wave is called a Love
  wave, named after A.E.H. Love, a British
  mathematician who worked out the mathematical
  model for this kind of wave in 1911.
• It's the fastest surface wave and moves the
  ground from side-to-side.

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L-Wave
Deformation propagates
Particle motion
Particle motion consists of alternating
transverse motions. Particle motion is
horizontal and perpendicular to the direction of
propagation (transverse). Particle motion is
purely horizontal, focus on the Y axis (black
lines) as the wave propagates through it.
Amplitude decreases with depth (yellow lines).
Material returns to its original shape after wave
passes.
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Rayleigh Waves

• Rayleigh Waves
• The other kind of surface wave is the Rayleigh
  wave, named for John William Strutt, Lord
  Rayleigh, who mathematically predicted the
  existence of this kind of wave in 1885.
• A Rayleigh wave rolls along the ground just like
  a wave rolls across a lake or an ocean.
• Because it rolls, it moves the ground up and
  down, and side-to-side in the same direction that
  the wave is moving.
• Most of the shaking felt from an earthquake is
  due to the Rayleigh wave, which can be much
  larger than the other waves.

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Rayleigh Waves
Particle motion
Deformation propagates
Particle motion consists of elliptical motions
(generally retrograde elliptical) in the vertical
plane and parallel to the direction of
propagation. Amplitude decreases with depth.
Material returns to its original shape after wave
passes.
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Seismic Wave Speeds
The bulk modulus (K) of a substance essentially
measures the substance's resistance to uniform
compression. It is defined as the pressure
increase needed to effect a given relative
decrease in volume.
Shear modulus, µ, sometimes referred to as the
modulus of rigidity, is the ratio of shear stress
to the shear strain.
?? shear modulus
? density
K modulus of compressibility (bulk modulus)
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Elastic Materials
Rand quartzite
L
F
stress
DL
F E DL/L (Hookes Law) E Youngs modulus
Young's modulus (E) is a measure of the stiffness
of a given material.
strain
E predicts the amount a wire will extend under
tension, or to predict the load at which a thin
column will buckle under compression
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Seismic Wave Speeds
?? modulus of rigidity
? density
K modulus of compressibility (bulk modulus)
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Seismic Velocities
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Why are seismic waves important?   Some things
seismic waves are good for include       
Mapping the Interior of the Earth       
Monitoring the Compliance of the Comprehensive
Test Ban Treaty        Detection of Contaminated
Aquifers        Finding Prospective Oil and
Natural Gas Locations
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IRIS Earths Interior Structure Poster
Seismic waves through the Earth
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Earths interior structure and seismic raypaths
that are used to determine the Earth structure.